C Implementation-defined behavior
*********************************

A conforming implementation of ISO C is required to document its choice of behavior in each of the areas that are designated ``implementation defined.''  The following lists all such areas, along with the section number from the ISO/IEC 9899:1999 standard.

Translation
===========

  * How a diagnostic is identified (3.10, 5.1.1.3).

  * Whether each nonempty sequence of white-space characters other than new-line is retained or replaced by one space character in translation phase 3 (5.1.1.2).

Environment
===========

The behavior of these points are dependent on the implementation of the C library, and are not defined by GCC itself.

Identifiers
===========

  * Which additional multibyte characters may appear in identifiers and their correspondence to universal character names (6.4.2).

  * The number of significant initial characters in an identifier (5.2.4.1, 6.4.2).

Characters
==========

  * The number of bits in a byte (3.6).

  * The values of the members of the execution character set (5.2.1).

  * The unique value of the member of the execution character set produced for each of the standard alphabetic escape sequences (5.2.2).

  * The value of a char object into which has been stored any character other than a member of the basic execution character set (6.2.5).

  * Which of signed char or unsigned char has the same range, representation, and behavior as ``plain'' char (6.2.5, 6.3.1.1).

  * The mapping of members of the source character set (in character constants and string literals) to members of the execution character set (6.4.4.4, 5.1.1.2).

  * The value of an integer character constant containing more than one character or containing a character or escape sequence that does not map to a single-byte execution character (6.4.4.4).

  * The value of a wide character constant containing more than one multibyte character, or containing a multibyte character or escape sequence not represented in the extended execution character set (6.4.4.4).

  * The current locale used to convert a wide character constant consisting of a single multibyte character that maps to a member of the extended execution character set into a corresponding wide character code (6.4.4.4).

  * The current locale used to convert a wide string literal into corresponding wide character codes (6.4.5).

  * The value of a string literal containing a multibyte character or escape sequence not represented in the execution character set (6.4.5).

Integers
========

  * Any extended integer types that exist in the implementation (6.2.5).

  * Whether signed integer types are represented using sign and magnitude, two's complement, or one's complement, and whether the extraordinary value is a trap representation or an ordinary value (6.2.6.2).

  * The rank of any extended integer type relative to another extended integer type with the same precision (6.3.1.1).

  * The result of, or the signal raised by, converting an integer to a signed integer type when the value cannot be represented in an object of that type (6.3.1.3).

  * The results of some bitwise operations on signed integers (6.5).

Floating point
==============

  * The accuracy of the floating-point operations and of the library functions in <math.h> and <complex.h> that return floating-point results (5.2.4.2.2).

  * The rounding behaviors characterized by non-standard values of FLT_ROUNDS (5.2.4.2.2).

  * The evaluation methods characterized by non-standard negative values of FLT_EVAL_METHOD (5.2.4.2.2).

  * The direction of rounding when an integer is converted to a floating-point number that cannot exactly represent the original value (6.3.1.4).

  * The direction of rounding when a floating-point number is converted to a narrower floating-point number (6.3.1.5).

  * How the nearest representable value or the larger or smaller representable value immediately adjacent to the nearest representable value is chosen for certain floating constants (6.4.4.2).

  * Whether and how floating expressions are contracted when not disallowed by the FP_CONTRACT pragma (6.5).

  * The default state for the FENV_ACCESS pragma (7.6.1).

  * Additional floating-point exceptions, rounding modes, environments, and classifications, and their macro names (7.6, 7.12).

  * The default state for the FP_CONTRACT pragma (7.12.2).

  * Whether the ``inexact'' floating-point exception can be raised when the rounded result actually does equal the mathematical result in an IEC 60559 conformant implementation (F.9).

  * Whether the ``underflow'' (and ``inexact'') floating-point exception can be raised when a result is tiny but not inexact in an IEC 60559 conformant implementation (F.9).

Arrays and pointers
===================

  * The result of converting a pointer to an integer or vice versa (6.3.2.3).

     A cast from pointer to integer discards most-significant bits if the pointer representation is larger than the integer type, sign-extends (Future versions of GCC may zero-extend, or use a target-defined ptr_extend pattern.  Do not rely on sign extension.) if the pointer representation is smaller than the integer type, otherwise the bits are unchanged.

     A cast from integer to pointer discards most-significant bits if the pointer representation is smaller than the integer type, extends according to the signedness of the integer type if the pointer representation is larger than the integer type, otherwise the bits are unchanged.

     When casting from pointer to integer and back again, the resulting pointer must reference the same object as the original pointer, otherwise the behavior is undefined.  That is, one may not use integer arithmetic to avoid the undefined behavior of pointer arithmetic as proscribed in 6.5.6/8.

  * The size of the result of subtracting two pointers to elements of the same array (6.5.6).

Hints
=====

  * The extent to which suggestions made by using the register storage-class specifier are effective (6.7.1).

  * The extent to which suggestions made by using the inline function specifier are effective (6.7.4).

Structures, unions, enumerations, and bit-fields
================================================

  * Whether a ``plain'' int bit-field is treated as a signed int bit-field or as an unsigned int bit-field (6.7.2, 6.7.2.1).

  * Allowable bit-field types other than _Bool, signed int, and unsigned int (6.7.2.1).

  * Whether a bit-field can straddle a storage-unit boundary (6.7.2.1).

  * The order of allocation of bit-fields within a unit (6.7.2.1).

  * The alignment of non-bit-field members of structures (6.7.2.1).

  * The integer type compatible with each enumerated type (6.7.2.2).

Qualifiers
==========

  * What constitutes an access to an object that has volatile-qualified type (6.7.3).

Preprocessing directives
========================

  * How sequences in both forms of header names are mapped to headers or external source file names (6.4.7).

  * Whether the value of a character constant in a constant expression that controls conditional inclusion matches the value of the same character constant in the execution character set (6.10.1).

  * Whether the value of a single-character character constant in a constant expression that controls conditional inclusion may have a negative value (6.10.1).

  * The places that are searched for an included <> delimited header, and how the places are specified or the header is identified (6.10.2).

  * How the named source file is searched for in an included "" delimited header (6.10.2).

  * The method by which preprocessing tokens (possibly resulting from macro expansion) in a #include directive are combined into a header name (6.10.2).

  * The nesting limit for #include processing (6.10.2).

  * Whether the # operator inserts a \ character before the \ character that begins a universal character name in a character constant or string literal (6.10.3.2).

  * The behavior on each recognized non-STDC #pragma directive (6.10.6).

  * The definitions for __DATE__ and __TIME__ when respectively, the date and time of translation are not available (6.10.8).

Library functions
=================

The behavior of these points are dependent on the implementation of the C library, and are not defined by GCC itself.

Architecture
============

  * The values or expressions assigned to the macros specified in the headers <float.h>, <limits.h>, and <stdint.h> (5.2.4.2, 7.18.2, 7.18.3).

  * The number, order, and encoding of bytes in any object (when not explicitly specified in this International Standard) (6.2.6.1).

  * The value of the result of the sizeof operator (6.5.3.4).

Locale-specific behavior
========================

The behavior of these points are dependent on the implementation of the C library, and are not defined by GCC itself.

Extensions to the C Language Family
***********************************

GNU C provides several language features not found in ISO standard C. (The -pedantic option directs GCC to print a warning message if any of these features is used.)  To test for the availability of these features in conditional compilation, check for a predefined macro __GNUC__, which is always defined under GCC.

These extensions are available in C and Objective-C.  Most of them are also available in C++.  See Extensions to the C++ Language, for extensions that apply only to C++.

Some features that are in ISO C99 but not C89 or C++ are also, as extensions, accepted by GCC in C89 mode and in C++.

Statements and Declarations in Expressions
==========================================

A compound statement enclosed in parentheses may appear as an expression in GNU C.  This allows you to use loops, switches, and local variables within an expression.

Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces.  For example:

     ({ int y = foo (); int z;
        if (y > 0) z = y;
        else z = - y;
        z; })

is a valid (though slightly more complex than necessary) expression for the absolute value of foo ().

The last thing in the compound statement should be an expression followed by a semicolon; the value of this subexpression serves as the value of the entire construct.  (If you use some other kind of statement last within the braces, the construct has type void, and thus effectively no value.)

This feature is especially useful in making macro definitions ``safe'' (so that they evaluate each operand exactly once).  For example, the ``maximum'' function is commonly defined as a macro in standard C as follows:

     #define max(a,b) ((a) > (b) ? (a) : (b))

But this definition computes either A or B twice, with bad results if the operand has side effects.  In GNU C, if you know the type of the operands (here let's assume int), you can define the macro safely as follows:

     #define maxint(a,b) \
       ({int _a = (a), _b = (b); _a > _b ? _a : _b; })

Embedded statements are not allowed in constant expressions, such as the value of an enumeration constant, the width of a bit-field, or the initial value of a static variable.

If you don't know the type of the operand, you can still do this, but you must use typeof (see Typeof) or type naming (see Naming Types).

Statement expressions are not supported fully in G++, and their fate there is unclear.  (It is possible that they will become fully supported at some point, or that they will be deprecated, or that the bugs that are present will continue to exist indefinitely.)  Presently, statement expressions do not work well as default arguments.

In addition, there are semantic issues with statement-expressions in C++.  If you try to use statement-expressions instead of inline functions in C++, you may be surprised at the way object destruction is handled.  For example:

     #define foo(a)  ({int b = (a); b + 3; })

does not work the same way as:

     inline int foo(int a) { int b = a; return b + 3; }

In particular, if the expression passed into foo involves the creation of temporaries, the destructors for those temporaries will be run earlier in the case of the macro than in the case of the function.

These considerations mean that it is probably a bad idea to use statement-expressions of this form in header files that are designed to work with C++.  (Note that some versions of the GNU C Library contained header files using statement-expression that lead to precisely this bug.)

Locally Declared Labels
=======================

Each statement expression is a scope in which local labels can be declared.  A local label is simply an identifier; you can jump to it with an ordinary goto statement, but only from within the statement expression it belongs to.

A local label declaration looks like this:

     __label__ LABEL;

or

     __label__ LABEL1, LABEL2, ...;

Local label declarations must come at the beginning of the statement expression, right after the ({, before any ordinary declarations.

The label declaration defines the label name, but does not define the label itself.  You must do this in the usual way, with LABEL:, within the statements of the statement expression.

The local label feature is useful because statement expressions are often used in macros.  If the macro contains nested loops, a goto can be useful for breaking out of them.  However, an ordinary label whose scope is the whole function cannot be used: if the macro can be expanded several times in one function, the label will be multiply defined in that function.  A local label avoids this problem.  For example:

     #define SEARCH(array, target)                     \
     ({                                                \
       __label__ found;                                \
       typeof (target) _SEARCH_target = (target);      \
       typeof (*(array)) *_SEARCH_array = (array);     \
       int i, j;                                       \
       int value;                                      \
       for (i = 0; i < max; i++)                       \
         for (j = 0; j < max; j++)                     \
           if (_SEARCH_array[i][j] == _SEARCH_target)  \
             { value = i; goto found; }                \
       value = -1;                                     \
      found:                                           \
       value;                                          \
     })

Labels as Values
================

You can get the address of a label defined in the current function (or a containing function) with the unary operator &&.  The value has type void *.  This value is a constant and can be used wherever a constant of that type is valid.  For example:

     void *ptr;
     ...
     ptr = &&foo;

To use these values, you need to be able to jump to one.  This is done with the computed goto statement (The analogous feature in Fortran is called an assigned goto, but that name seems inappropriate in C, where one can do more than simply store label addresses in label variables.), goto *EXP;.  For example,

     goto *ptr;

Any expression of type void * is allowed.

One way of using these constants is in initializing a static array that will serve as a jump table:

     static void *array[] = { &&foo, &&bar, &&hack };

Then you can select a label with indexing, like this:

     goto *array[i];

Note that this does not check whether the subscript is in bounds---array indexing in C never does that.

Such an array of label values serves a purpose much like that of the switch statement.  The switch statement is cleaner, so use that rather than an array unless the problem does not fit a switch statement very well.

Another use of label values is in an interpreter for threaded code. The labels within the interpreter function can be stored in the threaded code for super-fast dispatching.

You may not use this mechanism to jump to code in a different function. If you do that, totally unpredictable things will happen.  The best way to avoid this is to store the label address only in automatic variables and never pass it as an argument.

An alternate way to write the above example is

     static const int array[] = { &&foo - &&foo, &&bar - &&foo,
                                  &&hack - &&foo };
     goto *(&&foo + array[i]);

This is more friendly to code living in shared libraries, as it reduces the number of dynamic relocations that are needed, and by consequence, allows the data to be read-only.

Nested Functions
================

A nested function is a function defined inside another function. (Nested functions are not supported for GNU C++.)  The nested function's name is local to the block where it is defined.  For example, here we define a nested function named square, and call it twice:

     foo (double a, double b)
     {
       double square (double z) { return z * z; }

       return square (a) + square (b);
     }

The nested function can access all the variables of the containing function that are visible at the point of its definition.  This is called lexical scoping.  For example, here we show a nested function which uses an inherited variable named offset:

     bar (int *array, int offset, int size)
     {
       int access (int *array, int index)
         { return array[index + offset]; }
       int i;
       ...
       for (i = 0; i < size; i++)
         ... access (array, i) ...
     }

Nested function definitions are permitted within functions in the places where variable definitions are allowed; that is, in any block, before the first statement in the block.

It is possible to call the nested function from outside the scope of its name by storing its address or passing the address to another function:

     hack (int *array, int size)
     {
       void store (int index, int value)
         { array[index] = value; }

       intermediate (store, size);
     }

Here, the function intermediate receives the address of store as an argument.  If intermediate calls store, the arguments given to store are used to store into array. But this technique works only so long as the containing function (hack, in this example) does not exit.

If you try to call the nested function through its address after the containing function has exited, all hell will break loose.  If you try to call it after a containing scope level has exited, and if it refers to some of the variables that are no longer in scope, you may be lucky, but it's not wise to take the risk.  If, however, the nested function does not refer to anything that has gone out of scope, you should be safe.

GCC implements taking the address of a nested function using a technique called trampolines.  A paper describing them is available as

http://people.debian.org/~karlheg/Usenix88-lexic.pdf.

A nested function can jump to a label inherited from a containing function, provided the label was explicitly declared in the containing function (see Local Labels).  Such a jump returns instantly to the containing function, exiting the nested function which did the goto and any intermediate functions as well.  Here is an example:

     bar (int *array, int offset, int size)
     {
       __label__ failure;
       int access (int *array, int index)
         {
           if (index > size)
             goto failure;
           return array[index + offset];
         }
       int i;
       ...
       for (i = 0; i < size; i++)
         ... access (array, i) ...
       ...
       return 0;

      /* Control comes here from access
         if it detects an error.  */
      failure:
       return -1;
     }

A nested function always has internal linkage.  Declaring one with extern is erroneous.  If you need to declare the nested function before its definition, use auto (which is otherwise meaningless for function declarations).

     bar (int *array, int offset, int size)
     {
       __label__ failure;
       auto int access (int *, int);
       ...
       int access (int *array, int index)
         {
           if (index > size)
             goto failure;
           return array[index + offset];
         }
       ...
     }

Constructing Function Calls
===========================

Using the built-in functions described below, you can record the arguments a function received, and call another function with the same arguments, without knowing the number or types of the arguments.

You can also record the return value of that function call, and later return that value, without knowing what data type the function tried to return (as long as your caller expects that data type).

 -- Built-in Function: void * __builtin_apply_args ()
     This built-in function returns a pointer to data describing how to perform a call with the same arguments as were passed to the current function.

     The function saves the arg pointer register, structure value address, and all registers that might be used to pass arguments to a function into a block of memory allocated on the stack.  Then it returns the address of that block.

 -- Built-in Function: void * __builtin_apply (void (*FUNCTION)(), void *ARGUMENTS, size_t SIZE)
     This built-in function invokes FUNCTION with a copy of the parameters described by ARGUMENTS and SIZE.

     The value of ARGUMENTS should be the value returned by __builtin_apply_args.  The argument SIZE specifies the size of the stack argument data, in bytes.

     This function returns a pointer to data describing how to return whatever value was returned by FUNCTION.  The data is saved in a block of memory allocated on the stack.

     It is not always simple to compute the proper value for SIZE.  The value is used by __builtin_apply to compute the amount of data that should be pushed on the stack and copied from the incoming argument area.

 -- Built-in Function: void __builtin_return (void *RESULT)
     This built-in function returns the value described by RESULT from the containing function.  You should specify, for RESULT, a value returned by __builtin_apply.

Naming an Expression's Type
===========================

You can give a name to the type of an expression using a typedef declaration with an initializer.  Here is how to define NAME as a type name for the type of EXP:

     typedef NAME = EXP;

This is useful in conjunction with the statements-within-expressions feature.  Here is how the two together can be used to define a safe ``maximum'' macro that operates on any arithmetic type:

     #define max(a,b) \
       ({typedef _ta = (a), _tb = (b);  \
         _ta _a = (a); _tb _b = (b);     \
         _a > _b ? _a : _b; })

The reason for using names that start with underscores for the local variables is to avoid conflicts with variable names that occur within the expressions that are substituted for a and b.  Eventually we hope to design a new form of declaration syntax that allows you to declare variables whose scopes start only after their initializers; this will be a more reliable way to prevent such conflicts.

Referring to a Type with typeof
===============================

Another way to refer to the type of an expression is with typeof. The syntax of using of this keyword looks like sizeof, but the construct acts semantically like a type name defined with typedef.

There are two ways of writing the argument to typeof: with an expression or with a type.  Here is an example with an expression:

     typeof (x[0](1))

This assumes that x is an array of pointers to functions; the type described is that of the values of the functions.

Here is an example with a typename as the argument:

     typeof (int *)

Here the type described is that of pointers to int.

If you are writing a header file that must work when included in ISO C programs, write __typeof__ instead of typeof. See Alternate Keywords.

A typeof-construct can be used anywhere a typedef name could be used.  For example, you can use it in a declaration, in a cast, or inside of sizeof or typeof.

  * This declares y with the type of what x points to.

          typeof (*x) y;

  * This declares y as an array of such values.

          typeof (*x) y[4];

  * This declares y as an array of pointers to characters:

          typeof (typeof (char *)[4]) y;

     It is equivalent to the following traditional C declaration:

          char *y[4];

     To see the meaning of the declaration using typeof, and why it might be a useful way to write, let's rewrite it with these macros:

          #define pointer(T)  typeof(T *)
          #define array(T, N) typeof(T [N])

     Now the declaration can be rewritten this way:

          array (pointer (char), 4) y;

     Thus, array (pointer (char), 4) is the type of arrays of 4 pointers to char.

Generalized Lvalues
===================

Compound expressions, conditional expressions and casts are allowed as lvalues provided their operands are lvalues.  This means that you can take their addresses or store values into them.

Standard C++ allows compound expressions and conditional expressions as lvalues, and permits casts to reference type, so use of this extension is deprecated for C++ code.

For example, a compound expression can be assigned, provided the last expression in the sequence is an lvalue.  These two expressions are equivalent:

     (a, b) += 5
     a, (b += 5)

Similarly, the address of the compound expression can be taken.  These two expressions are equivalent:

     &(a, b)
     a, &b

A conditional expression is a valid lvalue if its type is not void and the true and false branches are both valid lvalues.  For example, these two expressions are equivalent:

     (a ? b : c) = 5
     (a ? b = 5 : (c = 5))

A cast is a valid lvalue if its operand is an lvalue.  A simple assignment whose left-hand side is a cast works by converting the right-hand side first to the specified type, then to the type of the inner left-hand side expression.  After this is stored, the value is converted back to the specified type to become the value of the assignment.  Thus, if a has type char *, the following two expressions are equivalent:

     (int)a = 5
     (int)(a = (char *)(int)5)

An assignment-with-arithmetic operation such as += applied to a cast performs the arithmetic using the type resulting from the cast, and then continues as in the previous case.  Therefore, these two expressions are equivalent:

     (int)a += 5
     (int)(a = (char *)(int) ((int)a + 5))

You cannot take the address of an lvalue cast, because the use of its address would not work out coherently.  Suppose that &(int)f were permitted, where f has type float.  Then the following statement would try to store an integer bit-pattern where a floating point number belongs:

     *&(int)f = 1;

This is quite different from what (int)f = 1 would do---that would convert 1 to floating point and store it.  Rather than cause this inconsistency, we think it is better to prohibit use of & on a cast.

If you really do want an int * pointer with the address of f, you can simply write (int *)&f.

Conditionals with Omitted Operands
==================================

The middle operand in a conditional expression may be omitted.  Then if the first operand is nonzero, its value is the value of the conditional expression.

Therefore, the expression

     x ? : y

has the value of x if that is nonzero; otherwise, the value of y.

This example is perfectly equivalent to

     x ? x : y

In this simple case, the ability to omit the middle operand is not especially useful.  When it becomes useful is when the first operand does, or may (if it is a macro argument), contain a side effect.  Then repeating the operand in the middle would perform the side effect twice.  Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it.

Double-Word Integers
====================

ISO C99 supports data types for integers that are at least 64 bits wide, and as an extension GCC supports them in C89 mode and in C++. Simply write long long int for a signed integer, or unsigned long long int for an unsigned integer.  To make an integer constant of type long long int, add the suffix LL to the integer.  To make an integer constant of type unsigned long long int, add the suffix ULL to the integer.

You can use these types in arithmetic like any other integer types. Addition, subtraction, and bitwise boolean operations on these types are open-coded on all types of machines.  Multiplication is open-coded if the machine supports fullword-to-doubleword a widening multiply instruction.  Division and shifts are open-coded only on machines that provide special support.  The operations that are not open-coded use special library routines that come with GCC.

There may be pitfalls when you use long long types for function arguments, unless you declare function prototypes.  If a function expects type int for its argument, and you pass a value of type long long int, confusion will result because the caller and the subroutine will disagree about the number of bytes for the argument. Likewise, if the function expects long long int and you pass int.  The best way to avoid such problems is to use prototypes.

Complex Numbers
===============

ISO C99 supports complex floating data types, and as an extension GCC supports them in C89 mode and in C++, and supports complex integer data types which are not part of ISO C99.  You can declare complex types using the keyword _Complex.  As an extension, the older GNU keyword __complex__ is also supported.

For example, _Complex double x; declares x as a variable whose real part and imaginary part are both of type double.  _Complex short int y; declares y to have real and imaginary parts of type short int; this is not likely to be useful, but it shows that the set of complex types is complete.

To write a constant with a complex data type, use the suffix i or j (either one; they are equivalent).  For example, 2.5fi has type _Complex float and 3i has type _Complex int.  Such a constant always has a pure imaginary value, but you can form any complex value you like by adding one to a real constant.  This is a GNU extension; if you have an ISO C99 conforming C library (such as GNU libc), and want to construct complex constants of floating type, you should include <complex.h> and use the macros I or _Complex_I instead.

To extract the real part of a complex-valued expression EXP, write __real__ EXP.  Likewise, use __imag__ to extract the imaginary part.  This is a GNU extension; for values of floating type, you should use the ISO C99 functions crealf, creal, creall, cimagf, cimag and cimagl, declared in <complex.h> and also provided as built-in functions by GCC.

The operator ~ performs complex conjugation when used on a value with a complex type.  This is a GNU extension; for values of floating type, you should use the ISO C99 functions conjf, conj and conjl, declared in <complex.h> and also provided as built-in functions by GCC.

GCC can allocate complex automatic variables in a noncontiguous fashion; it's even possible for the real part to be in a register while the imaginary part is on the stack (or vice-versa).  None of the supported debugging info formats has a way to represent noncontiguous allocation like this, so GCC describes a noncontiguous complex variable as if it were two separate variables of noncomplex type. If the variable's actual name is foo, the two fictitious variables are named foo$real and foo$imag.  You can examine and set these two fictitious variables with your debugger.

A future version of GDB will know how to recognize such pairs and treat them as a single variable with a complex type.

Hex Floats
==========

ISO C99 supports floating-point numbers written not only in the usual decimal notation, such as 1.55e1, but also numbers such as 0x1.fp3 written in hexadecimal format.  As a GNU extension, GCC supports this in C89 mode (except in some cases when strictly conforming) and in C++.  In that format the 0x hex introducer and the p or P exponent field are mandatory.  The exponent is a decimal number that indicates the power of 2 by which the significant part will be multiplied.  Thus 0x1.f is 1 15/16, p3 multiplies it by 8, and the value of 0x1.fp3 is the same as 1.55e1.

Unlike for floating-point numbers in the decimal notation the exponent is always required in the hexadecimal notation.  Otherwise the compiler would not be able to resolve the ambiguity of, e.g., 0x1.f.  This could mean 1.0f or 1.9375 since f is also the extension for floating-point constants of type float.

Arrays of Length Zero
=====================

Zero-length arrays are allowed in GNU C.  They are very useful as the last element of a structure which is really a header for a variable-length object:

     struct line {
       int length;
       char contents[0];
     };

     struct line *thisline = (struct line *)
       malloc (sizeof (struct line) + this_length);
     thisline->length = this_length;

In ISO C89, you would have to give contents a length of 1, which means either you waste space or complicate the argument to malloc.

In ISO C99, you would use a flexible array member, which is slightly different in syntax and semantics:

  * Flexible array members are written as contents[] without the 0.

  * Flexible array members have incomplete type, and so the sizeof operator may not be applied.  As a quirk of the original implementation of zero-length arrays, sizeof evaluates to zero.

  * Flexible array members may only appear as the last member of a struct that is otherwise non-empty.

GCC versions before 3.0 allowed zero-length arrays to be statically initialized, as if they were flexible arrays.  In addition to those cases that were useful, it also allowed initializations in situations that would corrupt later data.  Non-empty initialization of zero-length arrays is now treated like any case where there are more initializer elements than the array holds, in that a suitable warning about "excess elements in array" is given, and the excess elements (all of them, in this case) are ignored.

Instead GCC allows static initialization of flexible array members. This is equivalent to defining a new structure containing the original structure followed by an array of sufficient size to contain the data. I.e. in the following, f1 is constructed as if it were declared like f2.

     struct f1 {
       int x; int y[];
     } f1 = { 1, { 2, 3, 4 } };

     struct f2 {
       struct f1 f1; int data[3];
     } f2 = { { 1 }, { 2, 3, 4 } };

The convenience of this extension is that f1 has the desired type, eliminating the need to consistently refer to f2.f1.

This has symmetry with normal static arrays, in that an array of unknown size is also written with [].

Of course, this extension only makes sense if the extra data comes at the end of a top-level object, as otherwise we would be overwriting data at subsequent offsets.  To avoid undue complication and confusion with initialization of deeply nested arrays, we simply disallow any non-empty initialization except when the structure is the top-level object.  For example:

     struct foo { int x; int y[]; };
     struct bar { struct foo z; };

     struct foo a = { 1, { 2, 3, 4 } };        // Valid.
     struct bar b = { { 1, { 2, 3, 4 } } };    // Invalid.
     struct bar c = { { 1, { } } };            // Valid.
     struct foo d[1] = { { 1 { 2, 3, 4 } } };  // Invalid.

Arrays of Variable Length
=========================

Variable-length automatic arrays are allowed in ISO C99, and as an extension GCC accepts them in C89 mode and in C++.  (However, GCC's implementation of variable-length arrays does not yet conform in detail to the ISO C99 standard.)  These arrays are declared like any other automatic arrays, but with a length that is not a constant expression.  The storage is allocated at the point of declaration and deallocated when the brace-level is exited.  For example:

     FILE *
     concat_fopen (char *s1, char *s2, char *mode)
     {
       char str[strlen (s1) + strlen (s2) + 1];
       strcpy (str, s1);
       strcat (str, s2);
       return fopen (str, mode);
     }

Jumping or breaking out of the scope of the array name deallocates the storage.  Jumping into the scope is not allowed; you get an error message for it.

You can use the function alloca to get an effect much like variable-length arrays.  The function alloca is available in many other C implementations (but not in all).  On the other hand, variable-length arrays are more elegant.

There are other differences between these two methods.  Space allocated with alloca exists until the containing function returns. The space for a variable-length array is deallocated as soon as the array name's scope ends.  (If you use both variable-length arrays and alloca in the same function, deallocation of a variable-length array will also deallocate anything more recently allocated with alloca.)

You can also use variable-length arrays as arguments to functions:

     struct entry
     tester (int len, char data[len][len])
     {
       ...
     }

The length of an array is computed once when the storage is allocated and is remembered for the scope of the array in case you access it with sizeof.

If you want to pass the array first and the length afterward, you can use a forward declaration in the parameter list---another GNU extension.

     struct entry
     tester (int len; char data[len][len], int len)
     {
       ...
     }

The int len before the semicolon is a parameter forward declaration, and it serves the purpose of making the name len known when the declaration of data is parsed.

You can write any number of such parameter forward declarations in the parameter list.  They can be separated by commas or semicolons, but the last one must end with a semicolon, which is followed by the ``real'' parameter declarations.  Each forward declaration must match a ``real'' declaration in parameter name and data type.  ISO C99 does not support parameter forward declarations.

Macros with a Variable Number of Arguments.
===========================================

In the ISO C standard of 1999, a macro can be declared to accept a variable number of arguments much as a function can.  The syntax for defining the macro is similar to that of a function.  Here is an example:

     #define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)

Here ... is a variable argument.  In the invocation of such a macro, it represents the zero or more tokens until the closing parenthesis that ends the invocation, including any commas.  This set of tokens replaces the identifier __VA_ARGS__ in the macro body wherever it appears.  See the CPP manual for more information.

GCC has long supported variadic macros, and used a different syntax that allowed you to give a name to the variable arguments just like any other argument.  Here is an example:

     #define debug(format, args...) fprintf (stderr, format, args)

This is in all ways equivalent to the ISO C example above, but arguably more readable and descriptive.

GNU CPP has two further variadic macro extensions, and permits them to be used with either of the above forms of macro definition.

In standard C, you are not allowed to leave the variable argument out entirely; but you are allowed to pass an empty argument.  For example, this invocation is invalid in ISO C, because there is no comma after the string:

     debug ("A message")

GNU CPP permits you to completely omit the variable arguments in this way.  In the above examples, the compiler would complain, though since the expansion of the macro still has the extra comma after the format string.

To help solve this problem, CPP behaves specially for variable arguments used with the token paste operator, ##.  If instead you write

     #define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)

and if the variable arguments are omitted or empty, the ## operator causes the preprocessor to remove the comma before it.  If you do provide some variable arguments in your macro invocation, GNU CPP does not complain about the paste operation and instead places the variable arguments after the comma.  Just like any other pasted macro argument, these arguments are not macro expanded.

Slightly Looser Rules for Escaped Newlines
==========================================

Recently, the preprocessor has relaxed its treatment of escaped newlines.  Previously, the newline had to immediately follow a backslash.  The current implementation allows whitespace in the form of spaces, horizontal and vertical tabs, and form feeds between the backslash and the subsequent newline.  The preprocessor issues a warning, but treats it as a valid escaped newline and combines the two lines to form a single logical line.  This works within comments and tokens, including multi-line strings, as well as between tokens. Comments are not treated as whitespace for the purposes of this relaxation, since they have not yet been replaced with spaces.

String Literals with Embedded Newlines
======================================

As an extension, GNU CPP permits string literals to cross multiple lines without escaping the embedded newlines.  Each embedded newline is replaced with a single \n character in the resulting string literal, regardless of what form the newline took originally.

CPP currently allows such strings in directives as well (other than the #include family).  This is deprecated and will eventually be removed.

Non-Lvalue Arrays May Have Subscripts
=====================================

In ISO C99, arrays that are not lvalues still decay to pointers, and may be subscripted, although they may not be modified or used after the next sequence point and the unary & operator may not be applied to them.  As an extension, GCC allows such arrays to be subscripted in C89 mode, though otherwise they do not decay to pointers outside C99 mode.  For example, this is valid in GNU C though not valid in C89:

     struct foo {int a[4];};

     struct foo f();

     bar (int index)
     {
       return f().a[index];
     }

Arithmetic on void- and Function-Pointers
=========================================

In GNU C, addition and subtraction operations are supported on pointers to void and on pointers to functions.  This is done by treating the size of a void or of a function as 1.

A consequence of this is that sizeof is also allowed on void and on function types, and returns 1.

The option -Wpointer-arith requests a warning if these extensions are used.

Non-Constant Initializers
=========================

As in standard C++ and ISO C99, the elements of an aggregate initializer for an automatic variable are not required to be constant expressions in GNU C. Here is an example of an initializer with run-time varying elements:

     foo (float f, float g)
     {
       float beat_freqs[2] = { f-g, f+g };
       ...
     }

Compound Literals
=================

ISO C99 supports compound literals.  A compound literal looks like a cast containing an initializer.  Its value is an object of the type specified in the cast, containing the elements specified in the initializer; it is an lvalue.  As an extension, GCC supports compound literals in C89 mode and in C++.

Usually, the specified type is a structure.  Assume that struct foo and structure are declared as shown:

     struct foo {int a; char b[2];} structure;

Here is an example of constructing a struct foo with a compound literal:

     structure = ((struct foo) {x + y, 'a', 0});

This is equivalent to writing the following:

     {
       struct foo temp = {x + y, 'a', 0};
       structure = temp;
     }

You can also construct an array.  If all the elements of the compound literal are (made up of) simple constant expressions, suitable for use in initializers of objects of static storage duration, then the compound literal can be coerced to a pointer to its first element and used in such an initializer, as shown here:

     char **foo = (char *[]) { "x", "y", "z" };

Compound literals for scalar types and union types are is also allowed, but then the compound literal is equivalent to a cast.

As a GNU extension, GCC allows initialization of objects with static storage duration by compound literals (which is not possible in ISO C99, because the initializer is not a constant). It is handled as if the object was initialized only with the bracket enclosed list if compound literal's and object types match. The initializer list of the compound literal must be constant. If the object being initialized has array type of unknown size, the size is determined by compound literal size.

     static struct foo x = (struct foo) {1, 'a', 'b'};
     static int y[] = (int []) {1, 2, 3};
     static int z[] = (int [3]) {1};

The above lines are equivalent to the following:

     static struct foo x = {1, 'a', 'b'};
     static int y[] = {1, 2, 3};
     static int z[] = {1, 0, 0};

Designated Initializers
=======================

Standard C89 requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized.

In ISO C99 you can give the elements in any order, specifying the array indices or structure field names they apply to, and GNU C allows this as an extension in C89 mode as well.  This extension is not implemented in GNU C++.

To specify an array index, write [INDEX] = before the element value.  For example,

     int a[6] = { [4] = 29, [2] = 15 };

is equivalent to

     int a[6] = { 0, 0, 15, 0, 29, 0 };

The index values must be constant expressions, even if the array being initialized is automatic.

An alternative syntax for this which has been obsolete since GCC 2.5 but GCC still accepts is to write [INDEX] before the element value, with no =.

To initialize a range of elements to the same value, write [FIRST ... LAST] = VALUE.  This is a GNU extension.  For example,

     int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };

If the value in it has side-effects, the side-effects will happen only once, not for each initialized field by the range initializer.

Note that the length of the array is the highest value specified plus one.

In a structure initializer, specify the name of a field to initialize with .FIELDNAME = before the element value.  For example, given the following structure,

     struct point { int x, y; };

the following initialization

     struct point p = { .y = yvalue, .x = xvalue };

is equivalent to

     struct point p = { xvalue, yvalue };

Another syntax which has the same meaning, obsolete since GCC 2.5, is FIELDNAME:, as shown here:

     struct point p = { y: yvalue, x: xvalue };

The [INDEX] or .FIELDNAME is known as a designator.  You can also use a designator (or the obsolete colon syntax) when initializing a union, to specify which element of the union should be used.  For example,

     union foo { int i; double d; };

     union foo f = { .d = 4 };

will convert 4 to a double to store it in the union using the second element.  By contrast, casting 4 to type union foo would store it into the union as the integer i, since it is an integer.  (see Cast to Union.)

You can combine this technique of naming elements with ordinary C initialization of successive elements.  Each initializer element that does not have a designator applies to the next consecutive element of the array or structure.  For example,

     int a[6] = { [1] = v1, v2, [4] = v4 };

is equivalent to

     int a[6] = { 0, v1, v2, 0, v4, 0 };

Labeling the elements of an array initializer is especially useful when the indices are characters or belong to an enum type. For example:

     int whitespace[256]
       = { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
           ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };

You can also write a series of .FIELDNAME and [INDEX] designators before an = to specify a nested subobject to initialize; the list is taken relative to the subobject corresponding to the closest surrounding brace pair.  For example, with the struct point declaration above:

     struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };

If the same field is initialized multiple times, it will have value from the last initialization.  If any such overridden initialization has side-effect, it is unspecified whether the side-effect happens or not. Currently, gcc will discard them and issue a warning.

Case Ranges
===========

You can specify a range of consecutive values in a single case label, like this:

     case LOW ... HIGH:

This has the same effect as the proper number of individual case labels, one for each integer value from LOW to HIGH, inclusive.

This feature is especially useful for ranges of ASCII character codes:

     case 'A' ... 'Z':

Be careful: Write spaces around the ..., for otherwise it may be parsed wrong when you use it with integer values.  For example, write this:

     case 1 ... 5:

rather than this:

     case 1...5:

Cast to a Union Type
====================

A cast to union type is similar to other casts, except that the type specified is a union type.  You can specify the type either with union TAG or with a typedef name.  A cast to union is actually a constructor though, not a cast, and hence does not yield an lvalue like normal casts.  (see Compound Literals.)

The types that may be cast to the union type are those of the members of the union.  Thus, given the following union and variables:

     union foo { int i; double d; };
     int x;
     double y;

both x and y can be cast to type union foo.

Using the cast as the right-hand side of an assignment to a variable of union type is equivalent to storing in a member of the union:

     union foo u;
     ...
     u = (union foo) x  ==  u.i = x
     u = (union foo) y  ==  u.d = y

You can also use the union cast as a function argument:

     void hack (union foo);
     ...
     hack ((union foo) x);

Mixed Declarations and Code
===========================

ISO C99 and ISO C++ allow declarations and code to be freely mixed within compound statements.  As an extension, GCC also allows this in C89 mode.  For example, you could do:

     int i;
     ...
     i++;
     int j = i + 2;

Each identifier is visible from where it is declared until the end of the enclosing block.

Declaring Attributes of Functions
=================================

In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully.

The keyword __attribute__ allows you to specify special attributes when making a declaration.  This keyword is followed by an attribute specification inside double parentheses.  The following attributes are currently defined for functions on all targets: noreturn, noinline, always_inline, pure, const, format, format_arg, no_instrument_function, section, constructor, destructor, used, unused, deprecated, weak, malloc, and alias.  Several other attributes are defined for functions on particular target systems.  Other attributes, including section are supported for variables declarations (see Variable Attributes) and for types (see Type Attributes).

You may also specify attributes with __ preceding and following each keyword.  This allows you to use them in header files without being concerned about a possible macro of the same name.  For example, you may use __noreturn__ instead of noreturn.

See Attribute Syntax, for details of the exact syntax for using attributes.

noreturn
     A few standard library functions, such as abort and exit, cannot return.  GCC knows this automatically.  Some programs define their own functions that never return.  You can declare them noreturn to tell the compiler this fact.  For example,

          void fatal () __attribute__ ((noreturn));

          void
          fatal (...)
          {
            ... /* Print error message. */ ...
            exit (1);
          }

     The noreturn keyword tells the compiler to assume that fatal cannot return.  It can then optimize without regard to what would happen if fatal ever did return.  This makes slightly better code.  More importantly, it helps avoid spurious warnings of uninitialized variables.

     Do not assume that registers saved by the calling function are restored before calling the noreturn function.

     It does not make sense for a noreturn function to have a return type other than void.

     The attribute noreturn is not implemented in GCC versions earlier than 2.5.  An alternative way to declare that a function does not return, which works in the current version and in some older versions, is as follows:

          typedef void voidfn ();

          volatile voidfn fatal;

noinline
     This function attribute prevents a function from being considered for inlining.

always_inline
     Generally, functions are not inlined unless optimization is specified. For functions declared inline, this attribute inlines the function even if no optimization level was specified.

pure
     Many functions have no effects except the return value and their return value depends only on the parameters and/or global variables. Such a function can be subject to common subexpression elimination and loop optimization just as an arithmetic operator would be.  These functions should be declared with the attribute pure.  For example,

          int square (int) __attribute__ ((pure));

     says that the hypothetical function square is safe to call fewer times than the program says.

     Some of common examples of pure functions are strlen or memcmp. Interesting non-pure functions are functions with infinite loops or those depending on volatile memory or other system resource, that may change between two consecutive calls (such as feof in a multithreading environment).

     The attribute pure is not implemented in GCC versions earlier than 2.96.

const
     Many functions do not examine any values except their arguments, and have no effects except the return value.  Basically this is just slightly more strict class than the pure attribute above, since function is not allowed to read global memory.

     Note that a function that has pointer arguments and examines the data pointed to must not be declared const.  Likewise, a function that calls a non-const function usually must not be const.  It does not make sense for a const function to return void.

     The attribute const is not implemented in GCC versions earlier than 2.5.  An alternative way to declare that a function has no side effects, which works in the current version and in some older versions, is as follows:

          typedef int intfn ();

          extern const intfn square;

     This approach does not work in GNU C++ from 2.6.0 on, since the language specifies that the const must be attached to the return value.

format (ARCHETYPE, STRING-INDEX, FIRST-TO-CHECK)
     The format attribute specifies that a function takes printf, scanf, strftime or strfmon style arguments which should be type-checked against a format string.  For example, the declaration:

          extern int
          my_printf (void *my_object, const char *my_format, ...)
                __attribute__ ((format (printf, 2, 3)));

     causes the compiler to check the arguments in calls to my_printf for consistency with the printf style format string argument my_format.

     The parameter ARCHETYPE determines how the format string is interpreted, and should be printf, scanf, strftime or strfmon.  (You can also use __printf__, __scanf__, __strftime__ or __strfmon__.)  The parameter STRING-INDEX specifies which argument is the format string argument (starting from 1), while FIRST-TO-CHECK is the number of the first argument to check against the format string.  For functions where the arguments are not available to be checked (such as vprintf), specify the third parameter as zero.  In this case the compiler only checks the format string for consistency.  For strftime formats, the third parameter is required to be zero.

     In the example above, the format string (my_format) is the second argument of the function my_print, and the arguments to check start with the third argument, so the correct parameters for the format attribute are 2 and 3.

     The format attribute allows you to identify your own functions which take format strings as arguments, so that GCC can check the calls to these functions for errors.  The compiler always (unless -ffreestanding is used) checks formats for the standard library functions printf, fprintf, sprintf, scanf, fscanf, sscanf, strftime, vprintf, vfprintf and vsprintf whenever such warnings are requested (using -Wformat), so there is no need to modify the header file stdio.h.  In C99 mode, the functions snprintf, vsnprintf, vscanf, vfscanf and vsscanf are also checked.  Except in strictly conforming C standard modes, the X/Open function strfmon is also checked as are printf_unlocked and fprintf_unlocked. See Options Controlling C Dialect.

format_arg (STRING-INDEX)
     The format_arg attribute specifies that a function takes a format string for a printf, scanf, strftime or strfmon style function and modifies it (for example, to translate it into another language), so the result can be passed to a printf, scanf, strftime or strfmon style function (with the remaining arguments to the format function the same as they would have been for the unmodified string).  For example, the declaration:

          extern char *
          my_dgettext (char *my_domain, const char *my_format)
                __attribute__ ((format_arg (2)));

     causes the compiler to check the arguments in calls to a printf, scanf, strftime or strfmon type function, whose format string argument is a call to the my_dgettext function, for consistency with the format string argument my_format.  If the format_arg attribute had not been specified, all the compiler could tell in such calls to format functions would be that the format string argument is not constant; this would generate a warning when -Wformat-nonliteral is used, but the calls could not be checked without the attribute.

     The parameter STRING-INDEX specifies which argument is the format string argument (starting from 1).

     The format-arg attribute allows you to identify your own functions which modify format strings, so that GCC can check the calls to printf, scanf, strftime or strfmon type function whose operands are a call to one of your own function. The compiler always treats gettext, dgettext, and dcgettext in this manner except when strict ISO C support is requested by -ansi or an appropriate -std option, or -ffreestanding is used.  See Options Controlling C Dialect.

no_instrument_function
     If -finstrument-functions is given, profiling function calls will be generated at entry and exit of most user-compiled functions. Functions with this attribute will not be so instrumented.

section ("SECTION-NAME")
     Normally, the compiler places the code it generates in the text section. Sometimes, however, you need additional sections, or you need certain particular functions to appear in special sections.  The section attribute specifies that a function lives in a particular section. For example, the declaration:

          extern void foobar (void) __attribute__ ((section ("bar")));

     puts the function foobar in the bar section.

     Some file formats do not support arbitrary sections so the section attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead.

constructor
destructor
     The constructor attribute causes the function to be called automatically before execution enters main ().  Similarly, the destructor attribute causes the function to be called automatically after main () has completed or exit () has been called.  Functions with these attributes are useful for initializing data that will be used implicitly during the execution of the program.

     These attributes are not currently implemented for Objective-C.

unused
     This attribute, attached to a function, means that the function is meant to be possibly unused.  GCC will not produce a warning for this function.  GNU C++ does not currently support this attribute as definitions without parameters are valid in C++.

used
     This attribute, attached to a function, means that code must be emitted for the function even if it appears that the function is not referenced. This is useful, for example, when the function is referenced only in inline assembly.

deprecated
     The deprecated attribute results in a warning if the function is used anywhere in the source file.  This is useful when identifying functions that are expected to be removed in a future version of a program.  The warning also includes the location of the declaration of the deprecated function, to enable users to easily find further information about why the function is deprecated, or what they should do instead.  Note that the warnings only occurs for uses:

          int old_fn () __attribute__ ((deprecated));
          int old_fn ();
          int (*fn_ptr)() = old_fn;

     results in a warning on line 3 but not line 2.

     The deprecated attribute can also be used for variables and types (see Variable Attributes, see Type Attributes.)

weak
     The weak attribute causes the declaration to be emitted as a weak symbol rather than a global.  This is primarily useful in defining library functions which can be overridden in user code, though it can also be used with non-function declarations.  Weak symbols are supported for ELF targets, and also for a.out targets when using the GNU assembler and linker.

malloc
     The malloc attribute is used to tell the compiler that a function may be treated as if it were the malloc function.  The compiler assumes that calls to malloc result in a pointers that cannot alias anything. This will often improve optimization.

alias ("TARGET")
     The alias attribute causes the declaration to be emitted as an alias for another symbol, which must be specified.  For instance,

          void __f () { /* Do something. */; }
          void f () __attribute__ ((weak, alias ("__f")));

     declares f to be a weak alias for __f.  In C++, the mangled name for the target must be used.

     Not all target machines support this attribute.

visibility ("VISIBILITY_TYPE")
     The visibility attribute on ELF targets causes the declaration to be emitted with hidden, protected or internal visibility.

          void __attribute__ ((visibility ("protected")))
          f () { /* Do something. */; }
          int i __attribute__ ((visibility ("hidden")));

     See the ELF gABI for complete details, but the short story is

     hidden
          Hidden visibility indicates that the symbol will not be placed into the dynamic symbol table, so no other module (executable or shared library) can reference it directly.

     protected
          Protected visibility indicates that the symbol will be placed in the dynamic symbol table, but that references within the defining module will bind to the local symbol.  That is, the symbol cannot be overridden by another module.

     internal
          Internal visibility is like hidden visibility, but with additional processor specific semantics.  Unless otherwise specified by the psABI, gcc defines internal visibility to mean that the function is never called from another module.  Note that hidden symbols, while then cannot be referenced directly by other modules, can be referenced indirectly via function pointers.  By indicating that a symbol cannot be called from outside the module, gcc may for instance omit the load of a PIC register since it is known that the calling function loaded the correct value.

     Not all ELF targets support this attribute.

regparm (NUMBER)
     On the Intel 386, the regparm attribute causes the compiler to pass up to NUMBER integer arguments in registers EAX, EDX, and ECX instead of on the stack.  Functions that take a variable number of arguments will continue to be passed all of their arguments on the stack.

stdcall
     On the Intel 386, the stdcall attribute causes the compiler to assume that the called function will pop off the stack space used to pass arguments, unless it takes a variable number of arguments.

     The PowerPC compiler for Windows NT currently ignores the stdcall attribute.

cdecl
     On the Intel 386, the cdecl attribute causes the compiler to assume that the calling function will pop off the stack space used to pass arguments.  This is useful to override the effects of the -mrtd switch.

     The PowerPC compiler for Windows NT currently ignores the cdecl attribute.

longcall
     On the RS/6000 and PowerPC, the longcall attribute causes the compiler to always call the function via a pointer, so that functions which reside further than 64 megabytes (67,108,864 bytes) from the current location can be called.

long_call/short_call
     This attribute allows to specify how to call a particular function on ARM.  Both attributes override the -mlong-calls (see ARM Options) command line switch and #pragma long_calls settings.  The long_call attribute causes the compiler to always call the function by first loading its address into a register and then using the contents of that register.   The short_call attribute always places the offset to the function from the call site into the BL instruction directly.

dllimport
     On the PowerPC running Windows NT, the dllimport attribute causes the compiler to call the function via a global pointer to the function pointer that is set up by the Windows NT dll library.  The pointer name is formed by combining __imp_ and the function name.

dllexport
     On the PowerPC running Windows NT, the dllexport attribute causes the compiler to provide a global pointer to the function pointer, so that it can be called with the dllimport attribute.  The pointer name is formed by combining __imp_ and the function name.

exception (EXCEPT-FUNC [, EXCEPT-ARG])
     On the PowerPC running Windows NT, the exception attribute causes the compiler to modify the structured exception table entry it emits for the declared function.  The string or identifier EXCEPT-FUNC is placed in the third entry of the structured exception table.  It represents a function, which is called by the exception handling mechanism if an exception occurs.  If it was specified, the string or identifier EXCEPT-ARG is placed in the fourth entry of the structured exception table.

function_vector
     Use this attribute on the H8/300 and H8/300H to indicate that the specified function should be called through the function vector.  Calling a function through the function vector will reduce code size, however; the function vector has a limited size (maximum 128 entries on the H8/300 and 64 entries on the H8/300H) and shares space with the interrupt vector.

     You must use GAS and GLD from GNU binutils version 2.7 or later for this attribute to work correctly.

interrupt
     Use this attribute on the ARM, AVR, M32R/D and Xstormy16 ports to indicate that the specified function is an interrupt handler.  The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present.

     Note, interrupt handlers for the H8/300, H8/300H and SH processors can be specified via the interrupt_handler attribute.

     Note, on the AVR interrupts will be enabled inside the function.

     Note, for the ARM you can specify the kind of interrupt to be handled by adding an optional parameter to the interrupt attribute like this:

          void f () __attribute__ ((interrupt ("IRQ")));

     Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF.

interrupt_handler
     Use this attribute on the H8/300, H8/300H and SH to indicate that the specified function is an interrupt handler.  The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present.

sp_switch
     Use this attribute on the SH to indicate an interrupt_handler function should switch to an alternate stack.  It expects a string argument that names a global variable holding the address of the alternate stack.

          void *alt_stack;
          void f () __attribute__ ((interrupt_handler,
                                    sp_switch ("alt_stack")));

trap_exit
     Use this attribute on the SH for an interrupt_handle to return using trapa instead of rte.  This attribute expects an integer argument specifying the trap number to be used.

eightbit_data
     Use this attribute on the H8/300 and H8/300H to indicate that the specified variable should be placed into the eight bit data section. The compiler will generate more efficient code for certain operations on data in the eight bit data area.  Note the eight bit data area is limited to 256 bytes of data.

     You must use GAS and GLD from GNU binutils version 2.7 or later for this attribute to work correctly.

tiny_data
     Use this attribute on the H8/300H to indicate that the specified variable should be placed into the tiny data section. The compiler will generate more efficient code for loads and stores on data in the tiny data section.  Note the tiny data area is limited to slightly under 32kbytes of data.

signal
     Use this attribute on the AVR to indicate that the specified function is an signal handler.  The compiler will generate function entry and exit sequences suitable for use in an signal handler when this attribute is present.  Interrupts will be disabled inside function.

naked
     Use this attribute on the ARM or AVR ports to indicate that the specified function do not need prologue/epilogue sequences generated by the compiler.  It is up to the programmer to provide these sequences.

model (MODEL-NAME)
     Use this attribute on the M32R/D to set the addressability of an object, and the code generated for a function. The identifier MODEL-NAME is one of small, medium, or large, representing each of the code models.

     Small model objects live in the lower 16MB of memory (so that their addresses can be loaded with the ld24 instruction), and are callable with the bl instruction.

     Medium model objects may live anywhere in the 32-bit address space (the compiler will generate seth/add3 instructions to load their addresses), and are callable with the bl instruction.

     Large model objects may live anywhere in the 32-bit address space (the compiler will generate seth/add3 instructions to load their addresses), and may not be reachable with the bl instruction (the compiler will generate the much slower seth/add3/jl instruction sequence).

You can specify multiple attributes in a declaration by separating them by commas within the double parentheses or by immediately following an attribute declaration with another attribute declaration.

Some people object to the __attribute__ feature, suggesting that ISO C's #pragma should be used instead.  At the time __attribute__ was designed, there were two reasons for not doing this.

 1. It is impossible to generate #pragma commands from a macro.

 2. There is no telling what the same #pragma might mean in another compiler.

These two reasons applied to almost any application that might have been proposed for #pragma.  It was basically a mistake to use #pragma for anything.

The ISO C99 standard includes _Pragma, which now allows pragmas to be generated from macros.  In addition, a #pragma GCC namespace is now in use for GCC-specific pragmas.  However, it has been found convenient to use __attribute__ to achieve a natural attachment of attributes to their corresponding declarations, whereas #pragma GCC is of use for constructs that do not naturally form part of the grammar.  See Miscellaneous Preprocessing Directives (The C Preprocessor).

Attribute Syntax
================

This section describes the syntax with which __attribute__ may be used, and the constructs to which attribute specifiers bind, for the C language.  Some details may vary for C++ and Objective-C.  Because of infelicities in the grammar for attributes, some forms described here may not be successfully parsed in all cases.

There are some problems with the semantics of attributes in C++.  For example, there are no manglings for attributes, although they may affect code generation, so problems may arise when attributed types are used in conjunction with templates or overloading.  Similarly, typeid does not distinguish between types with different attributes.  Support for attributes in C++ may be restricted in future to attributes on declarations only, but not on nested declarators.

See Function Attributes, for details of the semantics of attributes applying to functions.  See Variable Attributes, for details of the semantics of attributes applying to variables.  See Type Attributes, for details of the semantics of attributes applying to structure, union and enumerated types.

An attribute specifier is of the form __attribute__ ((ATTRIBUTE-LIST)).  An attribute list is a possibly empty comma-separated sequence of attributes, where each attribute is one of the following:

  * Empty.  Empty attributes are ignored.

  * A word (which may be an identifier such as unused, or a reserved word such as const).

  * A word, followed by, in parentheses, parameters for the attribute. These parameters take one of the following forms:

       * An identifier.  For example, mode attributes use this form.

       * An identifier followed by a comma and a non-empty comma-separated list of expressions.  For example, format attributes use this form.

       * A possibly empty comma-separated list of expressions.  For example, format_arg attributes use this form with the list being a single integer constant expression, and alias attributes use this form with the list being a single string constant.

An attribute specifier list is a sequence of one or more attribute specifiers, not separated by any other tokens.

An attribute specifier list may appear after the colon following a label, other than a case or default label.  The only attribute it makes sense to use after a label is unused.  This feature is intended for code generated by programs which contains labels that may be unused but which is compiled with -Wall.  It would not normally be appropriate to use in it human-written code, though it could be useful in cases where the code that jumps to the label is contained within an #ifdef conditional.

An attribute specifier list may appear as part of a struct, union or enum specifier.  It may go either immediately after the struct, union or enum keyword, or after the closing brace.  It is ignored if the content of the structure, union or enumerated type is not defined in the specifier in which the attribute specifier list is used---that is, in usages such as struct __attribute__((foo)) bar with no following opening brace. Where attribute specifiers follow the closing brace, they are considered to relate to the structure, union or enumerated type defined, not to any enclosing declaration the type specifier appears in, and the type defined is not complete until after the attribute specifiers.

Otherwise, an attribute specifier appears as part of a declaration, counting declarations of unnamed parameters and type names, and relates to that declaration (which may be nested in another declaration, for example in the case of a parameter declaration), or to a particular declarator within a declaration.  Where an attribute specifier is applied to a parameter declared as a function or an array, it should apply to the function or array rather than the pointer to which the parameter is implicitly converted, but this is not yet correctly implemented.

Any list of specifiers and qualifiers at the start of a declaration may contain attribute specifiers, whether or not such a list may in that context contain storage class specifiers.  (Some attributes, however, are essentially in the nature of storage class specifiers, and only make sense where storage class specifiers may be used; for example, section.)  There is one necessary limitation to this syntax: the first old-style parameter declaration in a function definition cannot begin with an attribute specifier, because such an attribute applies to the function instead by syntax described below (which, however, is not yet implemented in this case).  In some other cases, attribute specifiers are permitted by this grammar but not yet supported by the compiler.  All attribute specifiers in this place relate to the declaration as a whole.  In the obsolescent usage where a type of int is implied by the absence of type specifiers, such a list of specifiers and qualifiers may be an attribute specifier list with no other specifiers or qualifiers.

An attribute specifier list may appear immediately before a declarator (other than the first) in a comma-separated list of declarators in a declaration of more than one identifier using a single list of specifiers and qualifiers.  Such attribute specifiers apply only to the identifier before whose declarator they appear.  For example, in

     __attribute__((noreturn)) void d0 (void),
         __attribute__((format(printf, 1, 2))) d1 (const char *, ...),
          d2 (void)

the noreturn attribute applies to all the functions declared; the format attribute only applies to d1.

An attribute specifier list may appear immediately before the comma, = or semicolon terminating the declaration of an identifier other than a function definition.  At present, such attribute specifiers apply to the declared object or function, but in future they may attach to the outermost adjacent declarator.  In simple cases there is no difference, but, for example, in

     void (****f)(void) __attribute__((noreturn));

at present the noreturn attribute applies to f, which causes a warning since f is not a function, but in future it may apply to the function ****f.  The precise semantics of what attributes in such cases will apply to are not yet specified.  Where an assembler name for an object or function is specified (see Asm Labels), at present the attribute must follow the asm specification; in future, attributes before the asm specification may apply to the adjacent declarator, and those after it to the declared object or function.

An attribute specifier list may, in future, be permitted to appear after the declarator in a function definition (before any old-style parameter declarations or the function body).

Attribute specifiers may be mixed with type qualifiers appearing inside the [] of a parameter array declarator, in the C99 construct by which such qualifiers are applied to the pointer to which the array is implicitly converted.  Such attribute specifiers apply to the pointer, not to the array, but at present this is not implemented and they are ignored.

An attribute specifier list may appear at the start of a nested declarator.  At present, there are some limitations in this usage: the attributes correctly apply to the declarator, but for most individual attributes the semantics this implies are not implemented. When attribute specifiers follow the * of a pointer declarator, they may be mixed with any type qualifiers present. The following describes the formal semantics of this syntax.  It will make the most sense if you are familiar with the formal specification of declarators in the ISO C standard.

Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration T D1, where T contains declaration specifiers that specify a type TYPE (such as int) and D1 is a declarator that contains an identifier IDENT.  The type specified for IDENT for derived declarators whose type does not include an attribute specifier is as in the ISO C standard.

If D1 has the form ( ATTRIBUTE-SPECIFIER-LIST D ), and the declaration T D specifies the type ``DERIVED-DECLARATOR-TYPE-LIST TYPE'' for IDENT, then T D1 specifies the type ``DERIVED-DECLARATOR-TYPE-LIST ATTRIBUTE-SPECIFIER-LIST TYPE'' for IDENT.

If D1 has the form * TYPE-QUALIFIER-AND-ATTRIBUTE-SPECIFIER-LIST D, and the declaration T D specifies the type ``DERIVED-DECLARATOR-TYPE-LIST TYPE'' for IDENT, then T D1 specifies the type ``DERIVED-DECLARATOR-TYPE-LIST TYPE-QUALIFIER-AND-ATTRIBUTE-SPECIFIER-LIST TYPE'' for IDENT.

For example,

     void (__attribute__((noreturn)) ****f) (void);

specifies the type ``pointer to pointer to pointer to pointer to non-returning function returning void''.  As another example,

     char *__attribute__((aligned(8))) *f;

specifies the type ``pointer to 8-byte-aligned pointer to char''. Note again that this does not work with most attributes; for example, the usage of aligned and noreturn attributes given above is not yet supported.

For compatibility with existing code written for compiler versions that did not implement attributes on nested declarators, some laxity is allowed in the placing of attributes.  If an attribute that only applies to types is applied to a declaration, it will be treated as applying to the type of that declaration.  If an attribute that only applies to declarations is applied to the type of a declaration, it will be treated as applying to that declaration; and, for compatibility with code placing the attributes immediately before the identifier declared, such an attribute applied to a function return type will be treated as applying to the function type, and such an attribute applied to an array element type will be treated as applying to the array type.  If an attribute that only applies to function types is applied to a pointer-to-function type, it will be treated as applying to the pointer target type; if such an attribute is applied to a function return type that is not a pointer-to-function type, it will be treated as applying to the function type.

Prototypes and Old-Style Function Definitions
=============================================

GNU C extends ISO C to allow a function prototype to override a later old-style non-prototype definition.  Consider the following example:

     /* Use prototypes unless the compiler is old-fashioned.  */
     #ifdef __STDC__
     #define P(x) x
     #else
     #define P(x) ()
     #endif

     /* Prototype function declaration.  */
     int isroot P((uid_t));

     /* Old-style function definition.  */
     int
     isroot (x)   /* ??? lossage here ??? */
          uid_t x;
     {
       return x == 0;
     }

Suppose the type uid_t happens to be short.  ISO C does not allow this example, because subword arguments in old-style non-prototype definitions are promoted.  Therefore in this example the function definition's argument is really an int, which does not match the prototype argument type of short.

This restriction of ISO C makes it hard to write code that is portable to traditional C compilers, because the programmer does not know whether the uid_t type is short, int, or long.  Therefore, in cases like these GNU C allows a prototype to override a later old-style definition.  More precisely, in GNU C, a function prototype argument type overrides the argument type specified by a later old-style definition if the former type is the same as the latter type before promotion.  Thus in GNU C the above example is equivalent to the following:

     int isroot (uid_t);

     int
     isroot (uid_t x)
     {
       return x == 0;
     }

GNU C++ does not support old-style function definitions, so this extension is irrelevant.

C++ Style Comments
==================

In GNU C, you may use C++ style comments, which start with // and continue until the end of the line.  Many other C implementations allow such comments, and they are included in the 1999 C standard.  However, C++ style comments are not recognized if you specify an -std option specifying a version of ISO C before C99, or -ansi (equivalent to -std=c89).

Dollar Signs in Identifier Names
================================

In GNU C, you may normally use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. However, dollar signs in identifiers are not supported on a few target machines, typically because the target assembler does not allow them.

The Character ESC in Constants
==============================

You can use the sequence \e in a string or character constant to stand for the ASCII character ESC.

Inquiring on Alignment of Types or Variables
============================================

The keyword __alignof__ allows you to inquire about how an object is aligned, or the minimum alignment usually required by a type.  Its syntax is just like sizeof.

For example, if the target machine requires a double value to be aligned on an 8-byte boundary, then __alignof__ (double) is 8. This is true on many RISC machines.  On more traditional machine designs, __alignof__ (double) is 4 or even 2.

Some machines never actually require alignment; they allow reference to any data type even at an odd addresses.  For these machines, __alignof__ reports the recommended alignment of a type.

If the operand of __alignof__ is an lvalue rather than a type, its value is the required alignment for its type, taking into account any minimum alignment specified with GCC's __attribute__ extension (see Variable Attributes).  For example, after this declaration:

     struct foo { int x; char y; } foo1;

the value of __alignof__ (foo1.y) is 1, even though its actual alignment is probably 2 or 4, the same as __alignof__ (int).

It is an error to ask for the alignment of an incomplete type.

Specifying Attributes of Variables
==================================

The keyword __attribute__ allows you to specify special attributes of variables or structure fields.  This keyword is followed by an attribute specification inside double parentheses.  Ten attributes are currently defined for variables: aligned, mode, nocommon, packed, section, transparent_union, unused, deprecated, vector_size, and weak.  Some other attributes are defined for variables on particular target systems.  Other attributes are available for functions (see Function Attributes) and for types (see Type Attributes).  Other front ends might define more attributes (see Extensions to the C++ Language).

You may also specify attributes with __ preceding and following each keyword.  This allows you to use them in header files without being concerned about a possible macro of the same name.  For example, you may use __aligned__ instead of aligned.

See Attribute Syntax, for details of the exact syntax for using attributes.

aligned (ALIGNMENT)
     This attribute specifies a minimum alignment for the variable or structure field, measured in bytes.  For example, the declaration:

          int x __attribute__ ((aligned (16))) = 0;

     causes the compiler to allocate the global variable x on a 16-byte boundary.  On a 68040, this could be used in conjunction with an asm expression to access the move16 instruction which requires 16-byte aligned operands.

     You can also specify the alignment of structure fields.  For example, to create a double-word aligned int pair, you could write:

          struct foo { int x[2] __attribute__ ((aligned (8))); };

     This is an alternative to creating a union with a double member that forces the union to be double-word aligned.

     It is not possible to specify the alignment of functions; the alignment of functions is determined by the machine's requirements and cannot be changed.  You cannot specify alignment for a typedef name because such a name is just an alias, not a distinct type.

     As in the preceding examples, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given variable or structure field.  Alternatively, you can leave out the alignment factor and just ask the compiler to align a variable or field to the maximum useful alignment for the target machine you are compiling for.  For example, you could write:

          short array[3] __attribute__ ((aligned));

     Whenever you leave out the alignment factor in an aligned attribute specification, the compiler automatically sets the alignment for the declared variable or field to the largest alignment which is ever used for any data type on the target machine you are compiling for.  Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables or fields that you have aligned this way.

     The aligned attribute can only increase the alignment; but you can decrease it by specifying packed as well.  See below.

     Note that the effectiveness of aligned attributes may be limited by inherent limitations in your linker.  On many systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment.  (For some linkers, the maximum supported alignment may be very very small.)  If your linker is only able to align variables up to a maximum of 8 byte alignment, then specifying aligned(16) in an __attribute__ will still only provide you with 8 byte alignment.  See your linker documentation for further information.

mode (MODE)
     This attribute specifies the data type for the declaration---whichever type corresponds to the mode MODE.  This in effect lets you request an integer or floating point type according to its width.

     You may also specify a mode of byte or __byte__ to indicate the mode corresponding to a one-byte integer, word or __word__ for the mode of a one-word integer, and pointer or __pointer__ for the mode used to represent pointers.

nocommon
     This attribute specifies requests GCC not to place a variable ``common'' but instead to allocate space for it directly.  If you specify the -fno-common flag, GCC will do this for all variables.

     Specifying the nocommon attribute for a variable provides an initialization of zeros.  A variable may only be initialized in one source file.

packed
     The packed attribute specifies that a variable or structure field should have the smallest possible alignment---one byte for a variable, and one bit for a field, unless you specify a larger value with the aligned attribute.

     Here is a structure in which the field x is packed, so that it immediately follows a:

          struct foo
          {
            char a;
            int x[2] __attribute__ ((packed));
          };

section ("SECTION-NAME")
     Normally, the compiler places the objects it generates in sections like data and bss.  Sometimes, however, you need additional sections, or you need certain particular variables to appear in special sections, for example to map to special hardware.  The section attribute specifies that a variable (or function) lives in a particular section.  For example, this small program uses several specific section names:

          struct duart a __attribute__ ((section ("DUART_A"))) = { 0 };
          struct duart b __attribute__ ((section ("DUART_B"))) = { 0 };
          char stack[10000] __attribute__ ((section ("STACK"))) = { 0 };
          int init_data __attribute__ ((section ("INITDATA"))) = 0;

          main()
          {
            /* Initialize stack pointer */
            init_sp (stack + sizeof (stack));

            /* Initialize initialized data */
            memcpy (&init_data, &data, &edata - &data);

            /* Turn on the serial ports */
            init_duart (&a);
            init_duart (&b);
          }

     Use the section attribute with an initialized definition of a global variable, as shown in the example.  GCC issues a warning and otherwise ignores the section attribute in uninitialized variable declarations.

     You may only use the section attribute with a fully initialized global definition because of the way linkers work.  The linker requires each object be defined once, with the exception that uninitialized variables tentatively go in the common (or bss) section and can be multiply ``defined''.  You can force a variable to be initialized with the -fno-common flag or the nocommon attribute.

     Some file formats do not support arbitrary sections so the section attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead.

shared
     On Windows NT, in addition to putting variable definitions in a named section, the section can also be shared among all running copies of an executable or DLL.  For example, this small program defines shared data by putting it in a named section shared and marking the section shareable:

          int foo __attribute__((section ("shared"), shared)) = 0;

          int
          main()
          {
            /* Read and write foo.  All running
               copies see the same value.  */
            return 0;
          }

     You may only use the shared attribute along with section attribute with a fully initialized global definition because of the way linkers work.  See section attribute for more information.

     The shared attribute is only available on Windows NT.

transparent_union
     This attribute, attached to a function parameter which is a union, means that the corresponding argument may have the type of any union member, but the argument is passed as if its type were that of the first union member.  For more details see Type Attributes.  You can also use this attribute on a typedef for a union data type; then it applies to all function parameters with that type.

unused
     This attribute, attached to a variable, means that the variable is meant to be possibly unused.  GCC will not produce a warning for this variable.

deprecated
     The deprecated attribute results in a warning if the variable is used anywhere in the source file.  This is useful when identifying variables that are expected to be removed in a future version of a program.  The warning also includes the location of the declaration of the deprecated variable, to enable users to easily find further information about why the variable is deprecated, or what they should do instead.  Note that the warnings only occurs for uses:

          extern int old_var __attribute__ ((deprecated));
          extern int old_var;
          int new_fn () { return old_var; }

     results in a warning on line 3 but not line 2.

     The deprecated attribute can also be used for functions and types (see Function Attributes, see Type Attributes.)

vector_size (BYTES)
     This attribute specifies the vector size for the variable, measured in bytes.  For example, the declaration:

          int foo __attribute__ ((vector_size (16)));

     causes the compiler to set the mode for foo, to be 16 bytes, divided into int sized units.  Assuming a 32-bit int (a vector of 4 units of 4 bytes), the corresponding mode of foo will be V4SI.

     This attribute is only applicable to integral and float scalars, although arrays, pointers, and function return values are allowed in conjunction with this construct.

     Aggregates with this attribute are invalid, even if they are of the same size as a corresponding scalar.  For example, the declaration:

          struct S { int a; };
          struct S  __attribute__ ((vector_size (16))) foo;

     is invalid even if the size of the structure is the same as the size of the int.

weak
     The weak attribute is described in see Function Attributes.

model (MODEL-NAME)
     Use this attribute on the M32R/D to set the addressability of an object. The identifier MODEL-NAME is one of small, medium, or large, representing each of the code models.

     Small model objects live in the lower 16MB of memory (so that their addresses can be loaded with the ld24 instruction).

     Medium and large model objects may live anywhere in the 32-bit address space (the compiler will generate seth/add3 instructions to load their addresses).

To specify multiple attributes, separate them by commas within the double parentheses: for example, __attribute__ ((aligned (16), packed)).

Specifying Attributes of Types
==============================

The keyword __attribute__ allows you to specify special attributes of struct and union types when you define such types.  This keyword is followed by an attribute specification inside double parentheses.  Five attributes are currently defined for types: aligned, packed, transparent_union, unused, and deprecated.  Other attributes are defined for functions (see Function Attributes) and for variables (see Variable Attributes).

You may also specify any one of these attributes with __ preceding and following its keyword.  This allows you to use these attributes in header files without being concerned about a possible macro of the same name.  For example, you may use __aligned__ instead of aligned.

You may specify the aligned and transparent_union attributes either in a typedef declaration or just past the closing curly brace of a complete enum, struct or union type definition and the packed attribute only past the closing brace of a definition.

You may also specify attributes between the enum, struct or union tag and the name of the type rather than after the closing brace.

See Attribute Syntax, for details of the exact syntax for using attributes.

aligned (ALIGNMENT)
     This attribute specifies a minimum alignment (in bytes) for variables of the specified type.  For example, the declarations:

          struct S { short f[3]; } __attribute__ ((aligned (8)));
          typedef int more_aligned_int __attribute__ ((aligned (8)));

     force the compiler to insure (as far as it can) that each variable whose type is struct S or more_aligned_int will be allocated and aligned at least on a 8-byte boundary.  On a Sparc, having all variables of type struct S aligned to 8-byte boundaries allows the compiler to use the ldd and std (doubleword load and store) instructions when copying one variable of type struct S to another, thus improving run-time efficiency.

     Note that the alignment of any given struct or union type is required by the ISO C standard to be at least a perfect multiple of the lowest common multiple of the alignments of all of the members of the struct or union in question.  This means that you can effectively adjust the alignment of a struct or union type by attaching an aligned attribute to any one of the members of such a type, but the notation illustrated in the example above is a more obvious, intuitive, and readable way to request the compiler to adjust the alignment of an entire struct or union type.

     As in the preceding example, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given struct or union type.  Alternatively, you can leave out the alignment factor and just ask the compiler to align a type to the maximum useful alignment for the target machine you are compiling for.  For example, you could write:

          struct S { short f[3]; } __attribute__ ((aligned));

     Whenever you leave out the alignment factor in an aligned attribute specification, the compiler automatically sets the alignment for the type to the largest alignment which is ever used for any data type on the target machine you are compiling for.  Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables which have types that you have aligned this way.

     In the example above, if the size of each short is 2 bytes, then the size of the entire struct S type is 6 bytes.  The smallest power of two which is greater than or equal to that is 8, so the compiler sets the alignment for the entire struct S type to 8 bytes.

     Note that although you can ask the compiler to select a time-efficient alignment for a given type and then declare only individual stand-alone objects of that type, the compiler's ability to select a time-efficient alignment is primarily useful only when you plan to create arrays of variables having the relevant (efficiently aligned) type.  If you declare or use arrays of variables of an efficiently-aligned type, then it is likely that your program will also be doing pointer arithmetic (or subscripting, which amounts to the same thing) on pointers to the relevant type, and the code that the compiler generates for these pointer arithmetic operations will often be more efficient for efficiently-aligned types than for other types.

     The aligned attribute can only increase the alignment; but you can decrease it by specifying packed as well.  See below.

     Note that the effectiveness of aligned attributes may be limited by inherent limitations in your linker.  On many systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment.  (For some linkers, the maximum supported alignment may be very very small.)  If your linker is only able to align variables up to a maximum of 8 byte alignment, then specifying aligned(16) in an __attribute__ will still only provide you with 8 byte alignment.  See your linker documentation for further information.

packed
     This attribute, attached to an enum, struct, or union type definition, specified that the minimum required memory be used to represent the type.

     Specifying this attribute for struct and union types is equivalent to specifying the packed attribute on each of the structure or union members.  Specifying the -fshort-enums flag on the line is equivalent to specifying the packed attribute on all enum definitions.

     You may only specify this attribute after a closing curly brace on an enum definition, not in a typedef declaration, unless that declaration also contains the definition of the enum.

transparent_union
     This attribute, attached to a union type definition, indicates that any function parameter having that union type causes calls to that function to be treated in a special way.

     First, the argument corresponding to a transparent union type can be of any type in the union; no cast is required.  Also, if the union contains a pointer type, the corresponding argument can be a null pointer constant or a void pointer expression; and if the union contains a void pointer type, the corresponding argument can be any pointer expression. If the union member type is a pointer, qualifiers like const on the referenced type must be respected, just as with normal pointer conversions.

     Second, the argument is passed to the function using the calling conventions of first member of the transparent union, not the calling conventions of the union itself.  All members of the union must have the same machine representation; this is necessary for this argument passing to work properly.

     Transparent unions are designed for library functions that have multiple interfaces for compatibility reasons.  For example, suppose the wait function must accept either a value of type int * to comply with Posix, or a value of type union wait * to comply with the 4.1BSD interface.  If wait's parameter were void *, wait would accept both kinds of arguments, but it would also accept any other pointer type and this would make argument type checking less useful.  Instead, <sys/wait.h> might define the interface as follows:

          typedef union
            {
              int *__ip;
              union wait *__up;
            } wait_status_ptr_t __attribute__ ((__transparent_union__));

          pid_t wait (wait_status_ptr_t);

     This interface allows either int * or union wait * arguments to be passed, using the int * calling convention. The program can call wait with arguments of either type:

          int w1 () { int w; return wait (&w); }
          int w2 () { union wait w; return wait (&w); }

     With this interface, wait's implementation might look like this:

          pid_t wait (wait_status_ptr_t p)
          {
            return waitpid (-1, p.__ip, 0);
          }

unused
     When attached to a type (including a union or a struct), this attribute means that variables of that type are meant to appear possibly unused.  GCC will not produce a warning for any variables of that type, even if the variable appears to do nothing.  This is often the case with lock or thread classes, which are usually defined and then not referenced, but contain constructors and destructors that have nontrivial bookkeeping functions.

deprecated
     The deprecated attribute results in a warning if the type is used anywhere in the source file.  This is useful when identifying types that are expected to be removed in a future version of a program. If possible, the warning also includes the location of the declaration of the deprecated type, to enable users to easily find further information about why the type is deprecated, or what they should do instead.  Note that the warnings only occur for uses and then only if the type is being applied to an identifier that itself is not being declared as deprecated.

          typedef int T1 __attribute__ ((deprecated));
          T1 x;
          typedef T1 T2;
          T2 y;
          typedef T1 T3 __attribute__ ((deprecated));
          T3 z __attribute__ ((deprecated));

     results in a warning on line 2 and 3 but not lines 4, 5, or 6.  No warning is issued for line 4 because T2 is not explicitly deprecated.  Line 5 has no warning because T3 is explicitly deprecated.  Similarly for line 6.

     The deprecated attribute can also be used for functions and variables (see Function Attributes, see Variable Attributes.)

To specify multiple attributes, separate them by commas within the double parentheses: for example, __attribute__ ((aligned (16), packed)).

An Inline Function is As Fast As a Macro
========================================

By declaring a function inline, you can direct GCC to integrate that function's code into the code for its callers.  This makes execution faster by eliminating the function-call overhead; in addition, if any of the actual argument values are constant, their known values may permit simplifications at compile time so that not all of the inline function's code needs to be included.  The effect on code size is less predictable; object code may be larger or smaller with function inlining, depending on the particular case.  Inlining of functions is an optimization and it really ``works'' only in optimizing compilation.  If you don't use -O, no function is really inline.

Inline functions are included in the ISO C99 standard, but there are currently substantial differences between what GCC implements and what the ISO C99 standard requires.

To declare a function inline, use the inline keyword in its declaration, like this:

     inline int
     inc (int *a)
     {
       (*a)++;
     }

(If you are writing a header file to be included in ISO C programs, write __inline__ instead of inline.  See Alternate Keywords.) You can also make all ``simple enough'' functions inline with the option -finline-functions.

Note that certain usages in a function definition can make it unsuitable for inline substitution.  Among these usages are: use of varargs, use of alloca, use of variable sized data types (see Variable Length), use of computed goto (see Labels as Values), use of nonlocal goto, and nested functions (see Nested Functions).  Using -Winline will warn when a function marked inline could not be substituted, and will give the reason for the failure.

Note that in C and Objective-C, unlike C++, the inline keyword does not affect the linkage of the function.

GCC automatically inlines member functions defined within the class body of C++ programs even if they are not explicitly declared inline.  (You can override this with -fno-default-inline; see Options Controlling C++ Dialect.)

When a function is both inline and static, if all calls to the function are integrated into the caller, and the function's address is never used, then the function's own assembler code is never referenced. In this case, GCC does not actually output assembler code for the function, unless you specify the option -fkeep-inline-functions. Some calls cannot be integrated for various reasons (in particular, calls that precede the function's definition cannot be integrated, and neither can recursive calls within the definition).  If there is a nonintegrated call, then the function is compiled to assembler code as usual.  The function must also be compiled as usual if the program refers to its address, because that can't be inlined.

When an inline function is not static, then the compiler must assume that there may be calls from other source files; since a global symbol can be defined only once in any program, the function must not be defined in the other source files, so the calls therein cannot be integrated. Therefore, a non-static inline function is always compiled on its own in the usual fashion.

If you specify both inline and extern in the function definition, then the definition is used only for inlining.  In no case is the function compiled on its own, not even if you refer to its address explicitly.  Such an address becomes an external reference, as if you had only declared the function, and had not defined it.

This combination of inline and extern has almost the effect of a macro.  The way to use it is to put a function definition in a header file with these keywords, and put another copy of the definition (lacking inline and extern) in a library file. The definition in the header file will cause most calls to the function to be inlined.  If any uses of the function remain, they will refer to the single copy in the library.

For future compatibility with when GCC implements ISO C99 semantics for inline functions, it is best to use static inline only.  (The existing semantics will remain available when -std=gnu89 is specified, but eventually the default will be -std=gnu99 and that will implement the C99 semantics, though it does not do so yet.)

GCC does not inline any functions when not optimizing unless you specify the always_inline attribute for the function, like this:

     /* Prototype.  */
     inline void foo (const char) __attribute__((always_inline));

Assembler Instructions with C Expression Operands
=================================================

In an assembler instruction using asm, you can specify the operands of the instruction using C expressions.  This means you need not guess which registers or memory locations will contain the data you want to use.

You must specify an assembler instruction template much like what appears in a machine description, plus an operand constraint string for each operand.

For example, here is how to use the 68881's fsinx instruction:

     asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));

Here angle is the C expression for the input operand while result is that of the output operand.  Each has "f" as its operand constraint, saying that a floating point register is required. The = in =f indicates that the operand is an output; all output operands' constraints must use =.  The constraints use the same language used in the machine description (see Constraints).

Each operand is described by an operand-constraint string followed by the C expression in parentheses.  A colon separates the assembler template from the first output operand and another separates the last output operand from the first input, if any.  Commas separate the operands within each group.  The total number of operands is currently limited to 30; this limitation may be lifted in some future version of GCC.

If there are no output operands but there are input operands, you must place two consecutive colons surrounding the place where the output operands would go.

As of GCC version 3.1, it is also possible to specify input and output operands using symbolic names which can be referenced within the assembler code.  These names are specified inside square brackets preceding the constraint string, and can be referenced inside the assembler code using %[NAME] instead of a percentage sign followed by the operand number.  Using named operands the above example could look like:

     asm ("fsinx %[angle],%[output]"
          : [output] "=f" (result)
          : [angle] "f" (angle));

Note that the symbolic operand names have no relation whatsoever to other C identifiers.  You may use any name you like, even those of existing C symbols, but must ensure that no two operands within the same assembler construct use the same symbolic name.

Output operand expressions must be lvalues; the compiler can check this. The input operands need not be lvalues.  The compiler cannot check whether the operands have data types that are reasonable for the instruction being executed.  It does not parse the assembler instruction template and does not know what it means or even whether it is valid assembler input.  The extended asm feature is most often used for machine instructions the compiler itself does not know exist.  If the output expression cannot be directly addressed (for example, it is a bit-field), your constraint must allow a register.  In that case, GCC will use the register as the output of the asm, and then store that register into the output.

The ordinary output operands must be write-only; GCC will assume that the values in these operands before the instruction are dead and need not be generated.  Extended asm supports input-output or read-write operands.  Use the constraint character + to indicate such an operand and list it with the output operands.

When the constraints for the read-write operand (or the operand in which only some of the bits are to be changed) allows a register, you may, as an alternative, logically split its function into two separate operands, one input operand and one write-only output operand.  The connection between them is expressed by constraints which say they need to be in the same location when the instruction executes.  You can use the same C expression for both operands, or different expressions.  For example, here we write the (fictitious) combine instruction with bar as its read-only source operand and foo as its read-write destination:

     asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));

The constraint "0" for operand 1 says that it must occupy the same location as operand 0.  A number in constraint is allowed only in an input operand and it must refer to an output operand.

Only a number in the constraint can guarantee that one operand will be in the same place as another.  The mere fact that foo is the value of both operands is not enough to guarantee that they will be in the same place in the generated assembler code.  The following would not work reliably:

     asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));

Various optimizations or reloading could cause operands 0 and 1 to be in different registers; GCC knows no reason not to do so.  For example, the compiler might find a copy of the value of foo in one register and use it for operand 1, but generate the output operand 0 in a different register (copying it afterward to foo's own address).  Of course, since the register for operand 1 is not even mentioned in the assembler code, the result will not work, but GCC can't tell that.

As of GCC version 3.1, one may write [NAME] instead of the operand number for a matching constraint.  For example:

     asm ("cmoveq %1,%2,%[result]"
          : [result] "=r"(result)
          : "r" (test), "r"(new), "[result]"(old));

Some instructions clobber specific hard registers.  To describe this, write a third colon after the input operands, followed by the names of the clobbered hard registers (given as strings).  Here is a realistic example for the VAX:

     asm volatile ("movc3 %0,%1,%2"
                   : /* no outputs */
                   : "g" (from), "g" (to), "g" (count)
                   : "r0", "r1", "r2", "r3", "r4", "r5");

You may not write a clobber description in a way that overlaps with an input or output operand.  For example, you may not have an operand describing a register class with one member if you mention that register in the clobber list.  There is no way for you to specify that an input operand is modified without also specifying it as an output operand.  Note that if all the output operands you specify are for this purpose (and hence unused), you will then also need to specify volatile for the asm construct, as described below, to prevent GCC from deleting the asm statement as unused.

If you refer to a particular hardware register from the assembler code, you will probably have to list the register after the third colon to tell the compiler the register's value is modified.  In some assemblers, the register names begin with %; to produce one % in the assembler code, you must write %% in the input.

If your assembler instruction can alter the condition code register, add cc to the list of clobbered registers.  GCC on some machines represents the condition codes as a specific hardware register; cc serves to name this register.  On other machines, the condition code is handled differently, and specifying cc has no effect.  But it is valid no matter what the machine.

If your assembler instruction modifies memory in an unpredictable fashion, add memory to the list of clobbered registers.  This will cause GCC to not keep memory values cached in registers across the assembler instruction.  You will also want to add the volatile keyword if the memory affected is not listed in the inputs or outputs of the asm, as the memory clobber does not count as a side-effect of the asm.

You can put multiple assembler instructions together in a single asm template, separated by the characters normally used in assembly code for the system.  A combination that works in most places is a newline to break the line, plus a tab character to move to the instruction field (written as \n\t).  Sometimes semicolons can be used, if the assembler allows semicolons as a line-breaking character.  Note that some assembler dialects use semicolons to start a comment. The input operands are guaranteed not to use any of the clobbered registers, and neither will the output operands' addresses, so you can read and write the clobbered registers as many times as you like.  Here is an example of multiple instructions in a template; it assumes the subroutine _foo accepts arguments in registers 9 and 10:

     asm ("movl %0,r9\n\tmovl %1,r10\n\tcall _foo"
          : /* no outputs */
          : "g" (from), "g" (to)
          : "r9", "r10");

Unless an output operand has the & constraint modifier, GCC may allocate it in the same register as an unrelated input operand, on the assumption the inputs are consumed before the outputs are produced. This assumption may be false if the assembler code actually consists of more than one instruction.  In such a case, use & for each output operand that may not overlap an input.  See Modifiers.

If you want to test the condition code produced by an assembler instruction, you must include a branch and a label in the asm construct, as follows:

     asm ("clr %0\n\tfrob %1\n\tbeq 0f\n\tmov #1,%0\n0:"
          : "g" (result)
          : "g" (input));

This assumes your assembler supports local labels, as the GNU assembler and most Unix assemblers do.

Speaking of labels, jumps from one asm to another are not supported.  The compiler's optimizers do not know about these jumps, and therefore they cannot take account of them when deciding how to optimize.

Usually the most convenient way to use these asm instructions is to encapsulate them in macros that look like functions.  For example,

     #define sin(x)       \
     ({ double __value, __arg = (x);   \
        asm ("fsinx %1,%0": "=f" (__value): "f" (__arg));  \
        __value; })

Here the variable __arg is used to make sure that the instruction operates on a proper double value, and to accept only those arguments x which can convert automatically to a double.

Another way to make sure the instruction operates on the correct data type is to use a cast in the asm.  This is different from using a variable __arg in that it converts more different types.  For example, if the desired type were int, casting the argument to int would accept a pointer with no complaint, while assigning the argument to an int variable named __arg would warn about using a pointer unless the caller explicitly casts it.

If an asm has output operands, GCC assumes for optimization purposes the instruction has no side effects except to change the output operands.  This does not mean instructions with a side effect cannot be used, but you must be careful, because the compiler may eliminate them if the output operands aren't used, or move them out of loops, or replace two with one if they constitute a common subexpression.  Also, if your instruction does have a side effect on a variable that otherwise appears not to change, the old value of the variable may be reused later if it happens to be found in a register.

You can prevent an asm instruction from being deleted, moved significantly, or combined, by writing the keyword volatile after the asm.  For example:

     #define get_and_set_priority(new)              \
     ({ int __old;                                  \
        asm volatile ("get_and_set_priority %0, %1" \
                      : "=g" (__old) : "g" (new));  \
        __old; })

If you write an asm instruction with no outputs, GCC will know the instruction has side-effects and will not delete the instruction or move it outside of loops.

The volatile keyword indicates that the instruction has important side-effects.  GCC will not delete a volatile asm if it is reachable.  (The instruction can still be deleted if GCC can prove that control-flow will never reach the location of the instruction.)  In addition, GCC will not reschedule instructions across a volatile asm instruction.  For example:

     *(volatile int *)addr = foo;
     asm volatile ("eieio" : : );

Assume addr contains the address of a memory mapped device register.  The PowerPC eieio instruction (Enforce In-order Execution of I/O) tells the CPU to make sure that the store to that device register happens before it issues any other I/O.

Note that even a volatile asm instruction can be moved in ways that appear insignificant to the compiler, such as across jump instructions.  You can't expect a sequence of volatile asm instructions to remain perfectly consecutive.  If you want consecutive output, use a single asm.  Also, GCC will perform some optimizations across a volatile asm instruction; GCC does not ``forget everything'' when it encounters a volatile asm instruction the way some other compilers do.

An asm instruction without any operands or clobbers (an ``old style'' asm) will be treated identically to a volatile asm instruction.

It is a natural idea to look for a way to give access to the condition code left by the assembler instruction.  However, when we attempted to implement this, we found no way to make it work reliably.  The problem is that output operands might need reloading, which would result in additional following ``store'' instructions.  On most machines, these instructions would alter the condition code before there was time to test it.  This problem doesn't arise for ordinary ``test'' and ``compare'' instructions because they don't have any output operands.

For reasons similar to those described above, it is not possible to give an assembler instruction access to the condition code left by previous instructions.

If you are writing a header file that should be includable in ISO C programs, write __asm__ instead of asm.  See Alternate Keywords.

i386 floating point asm operands
--------------------------------

There are several rules on the usage of stack-like regs in asm_operands insns.  These rules apply only to the operands that are stack-like regs:

 1. Given a set of input regs that die in an asm_operands, it is necessary to know which are implicitly popped by the asm, and which must be explicitly popped by gcc.

     An input reg that is implicitly popped by the asm must be explicitly clobbered, unless it is constrained to match an output operand.

 2. For any input reg that is implicitly popped by an asm, it is necessary to know how to adjust the stack to compensate for the pop. If any non-popped input is closer to the top of the reg-stack than the implicitly popped reg, it would not be possible to know what the stack looked like---it's not clear how the rest of the stack ``slides up''.

     All implicitly popped input regs must be closer to the top of the reg-stack than any input that is not implicitly popped.

     It is possible that if an input dies in an insn, reload might use the input reg for an output reload.  Consider this example:

          asm ("foo" : "=t" (a) : "f" (b));

     This asm says that input B is not popped by the asm, and that the asm pushes a result onto the reg-stack, i.e., the stack is one deeper after the asm than it was before.  But, it is possible that reload will think that it can use the same reg for both the input and the output, if input B dies in this insn.

     If any input operand uses the f constraint, all output reg constraints must use the & earlyclobber.

     The asm above would be written as

          asm ("foo" : "=&t" (a) : "f" (b));

 3. Some operands need to be in particular places on the stack.  All output operands fall in this category---there is no other way to know which regs the outputs appear in unless the user indicates this in the constraints.

     Output operands must specifically indicate which reg an output appears in after an asm.  =f is not allowed: the operand constraints must select a class with a single reg.

 4. Output operands may not be ``inserted'' between existing stack regs. Since no 387 opcode uses a read/write operand, all output operands are dead before the asm_operands, and are pushed by the asm_operands. It makes no sense to push anywhere but the top of the reg-stack.

     Output operands must start at the top of the reg-stack: output operands may not ``skip'' a reg.

 5. Some asm statements may need extra stack space for internal calculations.  This can be guaranteed by clobbering stack registers unrelated to the inputs and outputs.

Here are a couple of reasonable asms to want to write.  This asm takes one input, which is internally popped, and produces two outputs.

     asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp));

This asm takes two inputs, which are popped by the fyl2xp1 opcode, and replaces them with one output.  The user must code the st(1) clobber for reg-stack.c to know that fyl2xp1 pops both inputs.

     asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)");

     [file md.texi not included]

Controlling Names Used in Assembler Code
========================================

You can specify the name to be used in the assembler code for a C function or variable by writing the asm (or __asm__) keyword after the declarator as follows:

     int foo asm ("myfoo") = 2;

This specifies that the name to be used for the variable foo in the assembler code should be myfoo rather than the usual _foo.

On systems where an underscore is normally prepended to the name of a C function or variable, this feature allows you to define names for the linker that do not start with an underscore.

It does not make sense to use this feature with a non-static local variable since such variables do not have assembler names.  If you are trying to put the variable in a particular register, see Explicit Reg Vars.  GCC presently accepts such code with a warning, but will probably be changed to issue an error, rather than a warning, in the future.

You cannot use asm in this way in a function definition; but you can get the same effect by writing a declaration for the function before its definition and putting asm there, like this:

     extern func () asm ("FUNC");

     func (x, y)
          int x, y;
     ...

It is up to you to make sure that the assembler names you choose do not conflict with any other assembler symbols.  Also, you must not use a register name; that would produce completely invalid assembler code.  GCC does not as yet have the ability to store static variables in registers. Perhaps that will be added.

Variables in Specified Registers
================================

GNU C allows you to put a few global variables into specified hardware registers.  You can also specify the register in which an ordinary register variable should be allocated.

  * Global register variables reserve registers throughout the program. This may be useful in programs such as programming language interpreters which have a couple of global variables that are accessed very often.

  * Local register variables in specific registers do not reserve the registers.  The compiler's data flow analysis is capable of determining where the specified registers contain live values, and where they are available for other uses.  Stores into local register variables may be deleted when they appear to be dead according to dataflow analysis.  References to local register variables may be deleted or moved or simplified.

     These local variables are sometimes convenient for use with the extended asm feature (see Extended Asm), if you want to write one output of the assembler instruction directly into a particular register. (This will work provided the register you specify fits the constraints specified for that operand in the asm.)

Defining Global Register Variables
----------------------------------

You can define a global register variable in GNU C like this:

     register int *foo asm ("a5");

Here a5 is the name of the register which should be used.  Choose a register which is normally saved and restored by function calls on your machine, so that library routines will not clobber it.

Naturally the register name is cpu-dependent, so you would need to conditionalize your program according to cpu type.  The register a5 would be a good choice on a 68000 for a variable of pointer type.  On machines with register windows, be sure to choose a ``global'' register that is not affected magically by the function call mechanism.

In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals.  For example, some 68000 operating systems call this register %a5.

Eventually there may be a way of asking the compiler to choose a register automatically, but first we need to figure out how it should choose and how to enable you to guide the choice.  No solution is evident.

Defining a global register variable in a certain register reserves that register entirely for this use, at least within the current compilation. The register will not be allocated for any other purpose in the functions in the current compilation.  The register will not be saved and restored by these functions.  Stores into this register are never deleted even if they would appear to be dead, but references may be deleted or moved or simplified.

It is not safe to access the global register variables from signal handlers, or from more than one thread of control, because the system library routines may temporarily use the register for other things (unless you recompile them specially for the task at hand).

It is not safe for one function that uses a global register variable to call another such function foo by way of a third function lose that was compiled without knowledge of this variable (i.e. in a different source file in which the variable wasn't declared).  This is because lose might save the register and put some other value there. For example, you can't expect a global register variable to be available in the comparison-function that you pass to qsort, since qsort might have put something else in that register.  (If you are prepared to recompile qsort with the same global register variable, you can solve this problem.)

If you want to recompile qsort or other source files which do not actually use your global register variable, so that they will not use that register for any other purpose, then it suffices to specify the compiler option -ffixed-REG.  You need not actually add a global register declaration to their source code.

A function which can alter the value of a global register variable cannot safely be called from a function compiled without this variable, because it could clobber the value the caller expects to find there on return. Therefore, the function which is the entry point into the part of the program that uses the global register variable must explicitly save and restore the value which belongs to its caller.

On most machines, longjmp will restore to each global register variable the value it had at the time of the setjmp.  On some machines, however, longjmp will not change the value of global register variables.  To be portable, the function that called setjmp should make other arrangements to save the values of the global register variables, and to restore them in a longjmp.  This way, the same thing will happen regardless of what longjmp does.

All global register variable declarations must precede all function definitions.  If such a declaration could appear after function definitions, the declaration would be too late to prevent the register from being used for other purposes in the preceding functions.

Global register variables may not have initial values, because an executable file has no means to supply initial contents for a register.

On the Sparc, there are reports that g3 ... g7 are suitable registers, but certain library functions, such as getwd, as well as the subroutines for division and remainder, modify g3 and g4.  g1 and g2 are local temporaries.

On the 68000, a2 ... a5 should be suitable, as should d2 ... d7. Of course, it will not do to use more than a few of those.

Specifying Registers for Local Variables
----------------------------------------

You can define a local register variable with a specified register like this:

     register int *foo asm ("a5");

Here a5 is the name of the register which should be used.  Note that this is the same syntax used for defining global register variables, but for a local variable it would appear within a function.

Naturally the register name is cpu-dependent, but this is not a problem, since specific registers are most often useful with explicit assembler instructions (see Extended Asm).  Both of these things generally require that you conditionalize your program according to cpu type.

In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals.  For example, some 68000 operating systems call this register %a5.

Defining such a register variable does not reserve the register; it remains available for other uses in places where flow control determines the variable's value is not live.  However, these registers are made unavailable for use in the reload pass; excessive use of this feature leaves the compiler too few available registers to compile certain functions.

This option does not guarantee that GCC will generate code that has this variable in the register you specify at all times.  You may not code an explicit reference to this register in an asm statement and assume it will always refer to this variable.

Stores into local register variables may be deleted when they appear to be dead according to dataflow analysis.  References to local register variables may be deleted or moved or simplified.

Alternate Keywords
==================

-ansi and the various -std options disable certain keywords.  This causes trouble when you want to use GNU C extensions, or a general-purpose header file that should be usable by all programs, including ISO C programs.  The keywords asm, typeof and inline are not available in programs compiled with -ansi or -std (although inline can be used in a program compiled with -std=c99).  The ISO C99 keyword restrict is only available when -std=gnu99 (which will eventually be the default) or -std=c99 (or the equivalent -std=iso9899:1999) is used.

The way to solve these problems is to put __ at the beginning and end of each problematical keyword.  For example, use __asm__ instead of asm, and __inline__ instead of inline.

Other C compilers won't accept these alternative keywords; if you want to compile with another compiler, you can define the alternate keywords as macros to replace them with the customary keywords.  It looks like this:

     #ifndef __GNUC__
     #define __asm__ asm
     #endif

-pedantic and other options cause warnings for many GNU C extensions. You can prevent such warnings within one expression by writing __extension__ before the expression.  __extension__ has no effect aside from this.

Incomplete enum Types
=====================

You can define an enum tag without specifying its possible values. This results in an incomplete type, much like what you get if you write struct foo without describing the elements.  A later declaration which does specify the possible values completes the type.

You can't allocate variables or storage using the type while it is incomplete.  However, you can work with pointers to that type.

This extension may not be very useful, but it makes the handling of enum more consistent with the way struct and union are handled.

This extension is not supported by GNU C++.

Function Names as Strings
=========================

GCC predefines two magic identifiers to hold the name of the current function.  The identifier __FUNCTION__ holds the name of the function as it appears in the source.  The identifier __PRETTY_FUNCTION__ holds the name of the function pretty printed in a language specific fashion.

These names are always the same in a C function, but in a C++ function they may be different.  For example, this program:

     extern "C" {
     extern int printf (char *, ...);
     }

     class a {
      public:
       sub (int i)
         {
           printf ("__FUNCTION__ = %s\n", __FUNCTION__);
           printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
         }
     };

     int
     main (void)
     {
       a ax;
       ax.sub (0);
       return 0;
     }

gives this output:

     __FUNCTION__ = sub
     __PRETTY_FUNCTION__ = int  a::sub (int)

The compiler automagically replaces the identifiers with a string literal containing the appropriate name.  Thus, they are neither preprocessor macros, like __FILE__ and __LINE__, nor variables.  This means that they catenate with other string literals, and that they can be used to initialize char arrays.  For example

     char here[] = "Function " __FUNCTION__ " in " __FILE__;

On the other hand, #ifdef __FUNCTION__ does not have any special meaning inside a function, since the preprocessor does not do anything special with the identifier __FUNCTION__.

Note that these semantics are deprecated, and that GCC 3.2 will handle __FUNCTION__ and __PRETTY_FUNCTION__ the same way as __func__.  __func__ is defined by the ISO standard C99:

     The identifier __func__ is implicitly declared by the translator
     as if, immediately following the opening brace of each function
     definition, the declaration

          static const char __func__[] = "function-name";

     appeared, where function-name is the name of the lexically-enclosing
     function.  This name is the unadorned name of the function.

By this definition, __func__ is a variable, not a string literal. In particular, __func__ does not catenate with other string literals.

In C++, __FUNCTION__ and __PRETTY_FUNCTION__ are variables, declared in the same way as __func__.

Getting the Return or Frame Address of a Function
=================================================

These functions may be used to get information about the callers of a function.

 -- Built-in Function: void * __builtin_return_address (unsigned int LEVEL)
     This function returns the return address of the current function, or of one of its callers.  The LEVEL argument is number of frames to scan up the call stack.  A value of 0 yields the return address of the current function, a value of 1 yields the return address of the caller of the current function, and so forth.

     The LEVEL argument must be a constant integer.

     On some machines it may be impossible to determine the return address of any function other than the current one; in such cases, or when the top of the stack has been reached, this function will return 0 or a random value. In addition, __builtin_frame_address may be used to determine if the top of the stack has been reached.

     This function should only be used with a nonzero argument for debugging purposes.

 -- Built-in Function: void * __builtin_frame_address (unsigned int LEVEL)
     This function is similar to __builtin_return_address, but it returns the address of the function frame rather than the return address of the function.  Calling __builtin_frame_address with a value of 0 yields the frame address of the current function, a value of 1 yields the frame address of the caller of the current function, and so forth.

     The frame is the area on the stack which holds local variables and saved registers.  The frame address is normally the address of the first word pushed on to the stack by the function.  However, the exact definition depends upon the processor and the calling convention.  If the processor has a dedicated frame pointer register, and the function has a frame, then __builtin_frame_address will return the value of the frame pointer register.

     On some machines it may be impossible to determine the frame address of any function other than the current one; in such cases, or when the top of the stack has been reached, this function will return 0 if the first frame pointer is properly initialized by the startup code.

     This function should only be used with a nonzero argument for debugging purposes.

Using vector instructions through built-in functions
====================================================

On some targets, the instruction set contains SIMD vector instructions that operate on multiple values contained in one large register at the same time. For example, on the i386 the MMX, 3Dnow! and SSE extensions can be used this way.

The first step in using these extensions is to provide the necessary data types.  This should be done using an appropriate typedef:

     typedef int v4si __attribute__ ((mode(V4SI)));

The base type int is effectively ignored by the compiler, the actual properties of the new type v4si are defined by the __attribute__.  It defines the machine mode to be used; for vector types these have the form VNB; N should be the number of elements in the vector, and B should be the base mode of the individual elements.  The following can be used as base modes:

QI
     An integer that is as wide as the smallest addressable unit, usually 8 bits.

HI
     An integer, twice as wide as a QI mode integer, usually 16 bits.

SI
     An integer, four times as wide as a QI mode integer, usually 32 bits.

DI
     An integer, eight times as wide as a QI mode integer, usually 64 bits.

SF
     A floating point value, as wide as a SI mode integer, usually 32 bits.

DF
     A floating point value, as wide as a DI mode integer, usually 64 bits.

Not all base types or combinations are always valid; which modes can be used is determined by the target machine.  For example, if targetting the i386 MMX extensions, only V8QI, V4HI and V2SI are allowed modes.

There are no V1xx vector modes - they would be identical to the corresponding base mode.

There is no distinction between signed and unsigned vector modes.  This distinction is made by the operations that perform on the vectors, not by the data type.

The types defined in this manner are somewhat special, they cannot be used with most normal C operations (i.e., a vector addition can not be represented by a normal addition of two vector type variables).  You can declare only variables and use them in function calls and returns, as well as in assignments and some casts.  It is possible to cast from one vector type to another, provided they are of the same size (in fact, you can also cast vectors to and from other datatypes of the same size).

A port that supports vector operations provides a set of built-in functions that can be used to operate on vectors.  For example, a function to add two vectors and multiply the result by a third could look like this:

     v4si f (v4si a, v4si b, v4si c)
     {
       v4si tmp = __builtin_addv4si (a, b);
       return __builtin_mulv4si (tmp, c);
     }

Other built-in functions provided by GCC
========================================

GCC provides a large number of built-in functions other than the ones mentioned above.  Some of these are for internal use in the processing of exceptions or variable-length argument lists and will not be documented here because they may change from time to time; we do not recommend general use of these functions.

The remaining functions are provided for optimization purposes.

GCC includes built-in versions of many of the functions in the standard C library.  The versions prefixed with __builtin_ will always be treated as having the same meaning as the C library function even if you specify the -fno-builtin option. (see C Dialect Options) Many of these functions are only optimized in certain cases; if they are not optimized in a particular case, a call to the library function will be emitted.

The functions abort, exit, _Exit and _exit are recognized and presumed not to return, but otherwise are not built in.  _exit is not recognized in strict ISO C mode (-ansi, -std=c89 or -std=c99).  _Exit is not recognized in strict C89 mode (-ansi or -std=c89).

Outside strict ISO C mode, the functions alloca, bcmp, bzero, index, rindex, ffs, fputs_unlocked, printf_unlocked and fprintf_unlocked may be handled as built-in functions.  All these functions have corresponding versions prefixed with __builtin_, which may be used even in strict C89 mode.

The ISO C99 functions conj, conjf, conjl, creal, crealf, creall, cimag, cimagf, cimagl, llabs and imaxabs are handled as built-in functions except in strict ISO C89 mode.  There are also built-in versions of the ISO C99 functions cosf, cosl, fabsf, fabsl, sinf, sinl, sqrtf, and sqrtl, that are recognized in any mode since ISO C89 reserves these names for the purpose to which ISO C99 puts them.  All these functions have corresponding versions prefixed with __builtin_.

The ISO C89 functions abs, cos, fabs, fprintf, fputs, labs, memcmp, memcpy, memset, printf, sin, sqrt, strcat, strchr, strcmp, strcpy, strcspn, strlen, strncat, strncmp, strncpy, strpbrk, strrchr, strspn, and strstr are all recognized as built-in functions unless -fno-builtin is specified (or -fno-builtin-FUNCTION is specified for an individual function).  All of these functions have corresponding versions prefixed with __builtin_.

GCC provides built-in versions of the ISO C99 floating point comparison macros that avoid raising exceptions for unordered operands.  They have the same names as the standard macros ( isgreater, isgreaterequal, isless, islessequal, islessgreater, and isunordered) , with __builtin_ prefixed.  We intend for a library implementor to be able to simply #define each standard macro to its built-in equivalent.

 -- Built-in Function: int __builtin_types_compatible_p (TYPE1, TYPE2)

     You can use the built-in function __builtin_types_compatible_p to determine whether two types are the same.

     This built-in function returns 1 if the unqualified versions of the types TYPE1 and TYPE2 (which are types, not expressions) are compatible, 0 otherwise.  The result of this built-in function can be used in integer constant expressions.

     This built-in function ignores top level qualifiers (e.g., const, volatile).  For example, int is equivalent to const int.

     The type int[] and int[5] are compatible.  On the other hand, int and char * are not compatible, even if the size of their types, on the particular architecture are the same.  Also, the amount of pointer indirection is taken into account when determining similarity.  Consequently, short * is not similar to short **.  Furthermore, two types that are typedefed are considered compatible if their underlying types are compatible.

     An enum type is considered to be compatible with another enum type.  For example, enum {foo, bar} is similar to enum {hot, dog}.

     You would typically use this function in code whose execution varies depending on the arguments' types.  For example:

          #define foo(x)                                                  \
            ({                                                           \
              typeof (x) tmp;                                             \
              if (__builtin_types_compatible_p (typeof (x), long double)) \
                tmp = foo_long_double (tmp);                              \
              else if (__builtin_types_compatible_p (typeof (x), double)) \
                tmp = foo_double (tmp);                                   \
              else if (__builtin_types_compatible_p (typeof (x), float))  \
                tmp = foo_float (tmp);                                    \
              else                                                        \
                abort ();                                                 \
              tmp;                                                        \
            })

     Note: This construct is only available for C.

 -- Built-in Function: TYPE __builtin_choose_expr (CONST_EXP, EXP1, EXP2)

     You can use the built-in function __builtin_choose_expr to evaluate code depending on the value of a constant expression.  This built-in function returns EXP1 if CONST_EXP, which is a constant expression that must be able to be determined at compile time, is nonzero.  Otherwise it returns 0.

     This built-in function is analogous to the ? : operator in C, except that the expression returned has its type unaltered by promotion rules.  Also, the built-in function does not evaluate the expression that was not chosen.  For example, if CONST_EXP evaluates to true, EXP2 is not evaluated even if it has side-effects.

     This built-in function can return an lvalue if the chosen argument is an lvalue.

     If EXP1 is returned, the return type is the same as EXP1's type.  Similarly, if EXP2 is returned, its return type is the same as EXP2.

     Example:

          #define foo(x)                                                               \
            __builtin_choose_expr (__builtin_types_compatible_p (typeof (x), double),  \
              foo_double (x),                                                          \
              __builtin_choose_expr (__builtin_types_compatible_p (typeof (x), float), \
                foo_float (x),                                                         \
                /* The void expression results in a compile-time error             \
                   when assigning the result to something.  */                     \
                (void)0))

     Note: This construct is only available for C.  Furthermore, the unused expression (EXP1 or EXP2 depending on the value of CONST_EXP) may still generate syntax errors.  This may change in future revisions.

 -- Built-in Function: int __builtin_constant_p (EXP)
     You can use the built-in function __builtin_constant_p to determine if a value is known to be constant at compile-time and hence that GCC can perform constant-folding on expressions involving that value.  The argument of the function is the value to test.  The function returns the integer 1 if the argument is known to be a compile-time constant and 0 if it is not known to be a compile-time constant.  A return of 0 does not indicate that the value is not a constant, but merely that GCC cannot prove it is a constant with the specified value of the -O option.

     You would typically use this function in an embedded application where memory was a critical resource.  If you have some complex calculation, you may want it to be folded if it involves constants, but need to call a function if it does not.  For example:

          #define Scale_Value(X)      \
            (__builtin_constant_p (X) \
            ? ((X) * SCALE + OFFSET) : Scale (X))

     You may use this built-in function in either a macro or an inline function.  However, if you use it in an inlined function and pass an argument of the function as the argument to the built-in, GCC will never return 1 when you call the inline function with a string constant or compound literal (see Compound Literals) and will not return 1 when you pass a constant numeric value to the inline function unless you specify the -O option.

     You may also use __builtin_constant_p in initializers for static data.  For instance, you can write

          static const int table[] = {
             __builtin_constant_p (EXPRESSION) ? (EXPRESSION) : -1,
             /* ... */
          };

     This is an acceptable initializer even if EXPRESSION is not a constant expression.  GCC must be more conservative about evaluating the built-in in this case, because it has no opportunity to perform optimization.

     Previous versions of GCC did not accept this built-in in data initializers.  The earliest version where it is completely safe is 3.0.1.

 -- Built-in Function: long __builtin_expect (long EXP, long C)
     You may use __builtin_expect to provide the compiler with branch prediction information.  In general, you should prefer to use actual profile feedback for this (-fprofile-arcs), as programmers are notoriously bad at predicting how their programs actually perform.  However, there are applications in which this data is hard to collect.

     The return value is the value of EXP, which should be an integral expression.  The value of C must be a compile-time constant.  The semantics of the built-in are that it is expected that EXP == C.  For example:

          if (__builtin_expect (x, 0))
            foo ();

     would indicate that we do not expect to call foo, since we expect x to be zero.  Since you are limited to integral expressions for EXP, you should use constructions such as

          if (__builtin_expect (ptr != NULL, 1))
            error ();

     when testing pointer or floating-point values.

 -- Built-in Function: void __builtin_prefetch (const void *ADDR, ...)
     This function is used to minimize cache-miss latency by moving data into a cache before it is accessed. You can insert calls to __builtin_prefetch into code for which you know addresses of data in memory that is likely to be accessed soon. If the target supports them, data prefetch instructions will be generated. If the prefetch is done early enough before the access then the data will be in the cache by the time it is accessed.

     The value of ADDR is the address of the memory to prefetch. There are two optional arguments, RW and LOCALITY. The value of RW is a compile-time constant one or zero; one means that the prefetch is preparing for a write to the memory address and zero, the default, means that the prefetch is preparing for a read. The value LOCALITY must be a compile-time constant integer between zero and three.  A value of zero means that the data has no temporal locality, so it need not be left in the cache after the access.  A value of three means that the data has a high degree of temporal locality and should be left in all levels of cache possible.  Values of one and two mean, respectively, a low or moderate degree of temporal locality.  The default is three.

          for (i = 0; i < n; i++)
            {
              a[i] = a[i] + b[i];
              __builtin_prefetch (&a[i+j], 1, 1);
              __builtin_prefetch (&b[i+j], 0, 1);
              /* ... */
            }

     Data prefetch does not generate faults if ADDR is invalid, but the address expression itself must be valid.  For example, a prefetch of p->next will not fault if p->next is not a valid address, but evaluation will fault if p is not a valid address.

     If the target does not support data prefetch, the address expression is evaluated if it includes side effects but no other code is generated and GCC does not issue a warning.

Built-in Functions Specific to Particular Target Machines
=========================================================

On some target machines, GCC supports many built-in functions specific to those machines.  Generally these generate calls to specific machine instructions, but allow the compiler to schedule those calls.

X86 Built-in Functions
----------------------

These built-in functions are available for the i386 and x86-64 family of computers, depending on the command-line switches used.

The following machine modes are available for use with MMX built-in functions (see Vector Extensions): V2SI for a vector of two 32-bit integers, V4HI for a vector of four 16-bit integers, and V8QI for a vector of eight 8-bit integers.  Some of the built-in functions operate on MMX registers as a whole 64-bit entity, these use DI as their mode.

If 3Dnow extensions are enabled, V2SF is used as a mode for a vector of two 32-bit floating point values.

If SSE extensions are enabled, V4SF is used for a vector of four 32-bit floating point values.  Some instructions use a vector of four 32-bit integers, these use V4SI.  Finally, some instructions operate on an entire vector register, interpreting it as a 128-bit integer, these use mode TI.

The following built-in functions are made available by -mmmx. All of them generate the machine instruction that is part of the name.

     v8qi __builtin_ia32_paddb (v8qi, v8qi)
     v4hi __builtin_ia32_paddw (v4hi, v4hi)
     v2si __builtin_ia32_paddd (v2si, v2si)
     v8qi __builtin_ia32_psubb (v8qi, v8qi)
     v4hi __builtin_ia32_psubw (v4hi, v4hi)
     v2si __builtin_ia32_psubd (v2si, v2si)
     v8qi __builtin_ia32_paddsb (v8qi, v8qi)
     v4hi __builtin_ia32_paddsw (v4hi, v4hi)
     v8qi __builtin_ia32_psubsb (v8qi, v8qi)
     v4hi __builtin_ia32_psubsw (v4hi, v4hi)
     v8qi __builtin_ia32_paddusb (v8qi, v8qi)
     v4hi __builtin_ia32_paddusw (v4hi, v4hi)
     v8qi __builtin_ia32_psubusb (v8qi, v8qi)
     v4hi __builtin_ia32_psubusw (v4hi, v4hi)
     v4hi __builtin_ia32_pmullw (v4hi, v4hi)
     v4hi __builtin_ia32_pmulhw (v4hi, v4hi)
     di __builtin_ia32_pand (di, di)
     di __builtin_ia32_pandn (di,di)
     di __builtin_ia32_por (di, di)
     di __builtin_ia32_pxor (di, di)
     v8qi __builtin_ia32_pcmpeqb (v8qi, v8qi)
     v4hi __builtin_ia32_pcmpeqw (v4hi, v4hi)
     v2si __builtin_ia32_pcmpeqd (v2si, v2si)
     v8qi __builtin_ia32_pcmpgtb (v8qi, v8qi)
     v4hi __builtin_ia32_pcmpgtw (v4hi, v4hi)
     v2si __builtin_ia32_pcmpgtd (v2si, v2si)
     v8qi __builtin_ia32_punpckhbw (v8qi, v8qi)
     v4hi __builtin_ia32_punpckhwd (v4hi, v4hi)
     v2si __builtin_ia32_punpckhdq (v2si, v2si)
     v8qi __builtin_ia32_punpcklbw (v8qi, v8qi)
     v4hi __builtin_ia32_punpcklwd (v4hi, v4hi)
     v2si __builtin_ia32_punpckldq (v2si, v2si)
     v8qi __builtin_ia32_packsswb (v4hi, v4hi)
     v4hi __builtin_ia32_packssdw (v2si, v2si)
     v8qi __builtin_ia32_packuswb (v4hi, v4hi)

The following built-in functions are made available either with -msse, or with a combination of -m3dnow and -march=athlon.  All of them generate the machine instruction that is part of the name.

     v4hi __builtin_ia32_pmulhuw (v4hi, v4hi)
     v8qi __builtin_ia32_pavgb (v8qi, v8qi)
     v4hi __builtin_ia32_pavgw (v4hi, v4hi)
     v4hi __builtin_ia32_psadbw (v8qi, v8qi)
     v8qi __builtin_ia32_pmaxub (v8qi, v8qi)
     v4hi __builtin_ia32_pmaxsw (v4hi, v4hi)
     v8qi __builtin_ia32_pminub (v8qi, v8qi)
     v4hi __builtin_ia32_pminsw (v4hi, v4hi)
     int __builtin_ia32_pextrw (v4hi, int)
     v4hi __builtin_ia32_pinsrw (v4hi, int, int)
     int __builtin_ia32_pmovmskb (v8qi)
     void __builtin_ia32_maskmovq (v8qi, v8qi, char *)
     void __builtin_ia32_movntq (di *, di)
     void __builtin_ia32_sfence (void)

The following built-in functions are available when -msse is used. All of them generate the machine instruction that is part of the name.

     int __builtin_ia32_comieq (v4sf, v4sf)
     int __builtin_ia32_comineq (v4sf, v4sf)
     int __builtin_ia32_comilt (v4sf, v4sf)
     int __builtin_ia32_comile (v4sf, v4sf)
     int __builtin_ia32_comigt (v4sf, v4sf)
     int __builtin_ia32_comige (v4sf, v4sf)
     int __builtin_ia32_ucomieq (v4sf, v4sf)
     int __builtin_ia32_ucomineq (v4sf, v4sf)
     int __builtin_ia32_ucomilt (v4sf, v4sf)
     int __builtin_ia32_ucomile (v4sf, v4sf)
     int __builtin_ia32_ucomigt (v4sf, v4sf)
     int __builtin_ia32_ucomige (v4sf, v4sf)
     v4sf __builtin_ia32_addps (v4sf, v4sf)
     v4sf __builtin_ia32_subps (v4sf, v4sf)
     v4sf __builtin_ia32_mulps (v4sf, v4sf)
     v4sf __builtin_ia32_divps (v4sf, v4sf)
     v4sf __builtin_ia32_addss (v4sf, v4sf)
     v4sf __builtin_ia32_subss (v4sf, v4sf)
     v4sf __builtin_ia32_mulss (v4sf, v4sf)
     v4sf __builtin_ia32_divss (v4sf, v4sf)
     v4si __builtin_ia32_cmpeqps (v4sf, v4sf)
     v4si __builtin_ia32_cmpltps (v4sf, v4sf)
     v4si __builtin_ia32_cmpleps (v4sf, v4sf)
     v4si __builtin_ia32_cmpgtps (v4sf, v4sf)
     v4si __builtin_ia32_cmpgeps (v4sf, v4sf)
     v4si __builtin_ia32_cmpunordps (v4sf, v4sf)
     v4si __builtin_ia32_cmpneqps (v4sf, v4sf)
     v4si __builtin_ia32_cmpnltps (v4sf, v4sf)
     v4si __builtin_ia32_cmpnleps (v4sf, v4sf)
     v4si __builtin_ia32_cmpngtps (v4sf, v4sf)
     v4si __builtin_ia32_cmpngeps (v4sf, v4sf)
     v4si __builtin_ia32_cmpordps (v4sf, v4sf)
     v4si __builtin_ia32_cmpeqss (v4sf, v4sf)
     v4si __builtin_ia32_cmpltss (v4sf, v4sf)
     v4si __builtin_ia32_cmpless (v4sf, v4sf)
     v4si __builtin_ia32_cmpgtss (v4sf, v4sf)
     v4si __builtin_ia32_cmpgess (v4sf, v4sf)
     v4si __builtin_ia32_cmpunordss (v4sf, v4sf)
     v4si __builtin_ia32_cmpneqss (v4sf, v4sf)
     v4si __builtin_ia32_cmpnlts (v4sf, v4sf)
     v4si __builtin_ia32_cmpnless (v4sf, v4sf)
     v4si __builtin_ia32_cmpngtss (v4sf, v4sf)
     v4si __builtin_ia32_cmpngess (v4sf, v4sf)
     v4si __builtin_ia32_cmpordss (v4sf, v4sf)
     v4sf __builtin_ia32_maxps (v4sf, v4sf)
     v4sf __builtin_ia32_maxss (v4sf, v4sf)
     v4sf __builtin_ia32_minps (v4sf, v4sf)
     v4sf __builtin_ia32_minss (v4sf, v4sf)
     v4sf __builtin_ia32_andps (v4sf, v4sf)
     v4sf __builtin_ia32_andnps (v4sf, v4sf)
     v4sf __builtin_ia32_orps (v4sf, v4sf)
     v4sf __builtin_ia32_xorps (v4sf, v4sf)
     v4sf __builtin_ia32_movss (v4sf, v4sf)
     v4sf __builtin_ia32_movhlps (v4sf, v4sf)
     v4sf __builtin_ia32_movlhps (v4sf, v4sf)
     v4sf __builtin_ia32_unpckhps (v4sf, v4sf)
     v4sf __builtin_ia32_unpcklps (v4sf, v4sf)
     v4sf __builtin_ia32_cvtpi2ps (v4sf, v2si)
     v4sf __builtin_ia32_cvtsi2ss (v4sf, int)
     v2si __builtin_ia32_cvtps2pi (v4sf)
     int __builtin_ia32_cvtss2si (v4sf)
     v2si __builtin_ia32_cvttps2pi (v4sf)
     int __builtin_ia32_cvttss2si (v4sf)
     v4sf __builtin_ia32_rcpps (v4sf)
     v4sf __builtin_ia32_rsqrtps (v4sf)
     v4sf __builtin_ia32_sqrtps (v4sf)
     v4sf __builtin_ia32_rcpss (v4sf)
     v4sf __builtin_ia32_rsqrtss (v4sf)
     v4sf __builtin_ia32_sqrtss (v4sf)
     v4sf __builtin_ia32_shufps (v4sf, v4sf, int)
     void __builtin_ia32_movntps (float *, v4sf)
     int __builtin_ia32_movmskps (v4sf)

The following built-in functions are available when -msse is used.

v4sf __builtin_ia32_loadaps (float *)
     Generates the movaps machine instruction as a load from memory.

void __builtin_ia32_storeaps (float *, v4sf)
     Generates the movaps machine instruction as a store to memory.

v4sf __builtin_ia32_loadups (float *)
     Generates the movups machine instruction as a load from memory.

void __builtin_ia32_storeups (float *, v4sf)
     Generates the movups machine instruction as a store to memory.

v4sf __builtin_ia32_loadsss (float *)
     Generates the movss machine instruction as a load from memory.

void __builtin_ia32_storess (float *, v4sf)
     Generates the movss machine instruction as a store to memory.

v4sf __builtin_ia32_loadhps (v4sf, v2si *)
     Generates the movhps machine instruction as a load from memory.

v4sf __builtin_ia32_loadlps (v4sf, v2si *)
     Generates the movlps machine instruction as a load from memory

void __builtin_ia32_storehps (v4sf, v2si *)
     Generates the movhps machine instruction as a store to memory.

void __builtin_ia32_storelps (v4sf, v2si *)
     Generates the movlps machine instruction as a store to memory.

The following built-in functions are available when -m3dnow is used. All of them generate the machine instruction that is part of the name.

     void __builtin_ia32_femms (void)
     v8qi __builtin_ia32_pavgusb (v8qi, v8qi)
     v2si __builtin_ia32_pf2id (v2sf)
     v2sf __builtin_ia32_pfacc (v2sf, v2sf)
     v2sf __builtin_ia32_pfadd (v2sf, v2sf)
     v2si __builtin_ia32_pfcmpeq (v2sf, v2sf)
     v2si __builtin_ia32_pfcmpge (v2sf, v2sf)
     v2si __builtin_ia32_pfcmpgt (v2sf, v2sf)
     v2sf __builtin_ia32_pfmax (v2sf, v2sf)
     v2sf __builtin_ia32_pfmin (v2sf, v2sf)
     v2sf __builtin_ia32_pfmul (v2sf, v2sf)
     v2sf __builtin_ia32_pfrcp (v2sf)
     v2sf __builtin_ia32_pfrcpit1 (v2sf, v2sf)
     v2sf __builtin_ia32_pfrcpit2 (v2sf, v2sf)
     v2sf __builtin_ia32_pfrsqrt (v2sf)
     v2sf __builtin_ia32_pfrsqrtit1 (v2sf, v2sf)
     v2sf __builtin_ia32_pfsub (v2sf, v2sf)
     v2sf __builtin_ia32_pfsubr (v2sf, v2sf)
     v2sf __builtin_ia32_pi2fd (v2si)
     v4hi __builtin_ia32_pmulhrw (v4hi, v4hi)

The following built-in functions are available when both -m3dnow and -march=athlon are used.  All of them generate the machine instruction that is part of the name.

     v2si __builtin_ia32_pf2iw (v2sf)
     v2sf __builtin_ia32_pfnacc (v2sf, v2sf)
     v2sf __builtin_ia32_pfpnacc (v2sf, v2sf)
     v2sf __builtin_ia32_pi2fw (v2si)
     v2sf __builtin_ia32_pswapdsf (v2sf)
     v2si __builtin_ia32_pswapdsi (v2si)

PowerPC AltiVec Built-in Functions
----------------------------------

These built-in functions are available for the PowerPC family of computers, depending on the command-line switches used.

The following machine modes are available for use with AltiVec built-in functions (see Vector Extensions): V4SI for a vector of four 32-bit integers, V4SF for a vector of four 32-bit floating point numbers, V8HI for a vector of eight 16-bit integers, and V16QI for a vector of sixteen 8-bit integers.

The following functions are made available by including <altivec.h> and using -maltivec and -mabi=altivec.  The functions implement the functionality described in Motorola's AltiVec Programming Interface Manual.

Note: Only the <altivec.h> interface is supported. Internally, GCC uses built-in functions to achieve the functionality in the aforementioned header file, but they are not supported and are subject to change without notice.

     vector signed char vec_abs (vector signed char, vector signed char);
     vector signed short vec_abs (vector signed short, vector signed short);
     vector signed int vec_abs (vector signed int, vector signed int);
     vector signed float vec_abs (vector signed float, vector signed float);

     vector signed char vec_abss (vector signed char, vector signed char);
     vector signed short vec_abss (vector signed short, vector signed short);

     vector signed char vec_add (vector signed char, vector signed char);
     vector unsigned char vec_add (vector signed char, vector unsigned char);

     vector unsigned char vec_add (vector unsigned char, vector signed char);

     vector unsigned char vec_add (vector unsigned char,
                                   vector unsigned char);
     vector signed short vec_add (vector signed short, vector signed short);
     vector unsigned short vec_add (vector signed short,
                                    vector unsigned short);
     vector unsigned short vec_add (vector unsigned short,
                                    vector signed short);
     vector unsigned short vec_add (vector unsigned short,
                                    vector unsigned short);
     vector signed int vec_add (vector signed int, vector signed int);
     vector unsigned int vec_add (vector signed int, vector unsigned int);
     vector unsigned int vec_add (vector unsigned int, vector signed int);
     vector unsigned int vec_add (vector unsigned int, vector unsigned int);
     vector float vec_add (vector float, vector float);

     vector unsigned int vec_addc (vector unsigned int, vector unsigned int);

     vector unsigned char vec_adds (vector signed char,
                                    vector unsigned char);
     vector unsigned char vec_adds (vector unsigned char,
                                    vector signed char);
     vector unsigned char vec_adds (vector unsigned char,
                                    vector unsigned char);
     vector signed char vec_adds (vector signed char, vector signed char);
     vector unsigned short vec_adds (vector signed short,
                                     vector unsigned short);
     vector unsigned short vec_adds (vector unsigned short,
                                     vector signed short);
     vector unsigned short vec_adds (vector unsigned short,
                                     vector unsigned short);
     vector signed short vec_adds (vector signed short, vector signed short);

     vector unsigned int vec_adds (vector signed int, vector unsigned int);
     vector unsigned int vec_adds (vector unsigned int, vector signed int);
     vector unsigned int vec_adds (vector unsigned int, vector unsigned int);

     vector signed int vec_adds (vector signed int, vector signed int);

     vector float vec_and (vector float, vector float);
     vector float vec_and (vector float, vector signed int);
     vector float vec_and (vector signed int, vector float);
     vector signed int vec_and (vector signed int, vector signed int);
     vector unsigned int vec_and (vector signed int, vector unsigned int);
     vector unsigned int vec_and (vector unsigned int, vector signed int);
     vector unsigned int vec_and (vector unsigned int, vector unsigned int);
     vector signed short vec_and (vector signed short, vector signed short);
     vector unsigned short vec_and (vector signed short,
                                    vector unsigned short);
     vector unsigned short vec_and (vector unsigned short,
                                    vector signed short);
     vector unsigned short vec_and (vector unsigned short,
                                    vector unsigned short);
     vector signed char vec_and (vector signed char, vector signed char);
     vector unsigned char vec_and (vector signed char, vector unsigned char);

     vector unsigned char vec_and (vector unsigned char, vector signed char);

     vector unsigned char vec_and (vector unsigned char,
                                   vector unsigned char);

     vector float vec_andc (vector float, vector float);
     vector float vec_andc (vector float, vector signed int);
     vector float vec_andc (vector signed int, vector float);
     vector signed int vec_andc (vector signed int, vector signed int);
     vector unsigned int vec_andc (vector signed int, vector unsigned int);
     vector unsigned int vec_andc (vector unsigned int, vector signed int);
     vector unsigned int vec_andc (vector unsigned int, vector unsigned int);

     vector signed short vec_andc (vector signed short, vector signed short);

     vector unsigned short vec_andc (vector signed short,
                                     vector unsigned short);
     vector unsigned short vec_andc (vector unsigned short,
                                     vector signed short);
     vector unsigned short vec_andc (vector unsigned short,
                                     vector unsigned short);
     vector signed char vec_andc (vector signed char, vector signed char);
     vector unsigned char vec_andc (vector signed char,
                                    vector unsigned char);
     vector unsigned char vec_andc (vector unsigned char,
                                    vector signed char);
     vector unsigned char vec_andc (vector unsigned char,
                                    vector unsigned char);

     vector unsigned char vec_avg (vector unsigned char,
                                   vector unsigned char);
     vector signed char vec_avg (vector signed char, vector signed char);
     vector unsigned short vec_avg (vector unsigned short,
                                    vector unsigned short);
     vector signed short vec_avg (vector signed short, vector signed short);
     vector unsigned int vec_avg (vector unsigned int, vector unsigned int);
     vector signed int vec_avg (vector signed int, vector signed int);

     vector float vec_ceil (vector float);

     vector signed int vec_cmpb (vector float, vector float);

     vector signed char vec_cmpeq (vector signed char, vector signed char);
     vector signed char vec_cmpeq (vector unsigned char,
                                   vector unsigned char);
     vector signed short vec_cmpeq (vector signed short,
                                    vector signed short);
     vector signed short vec_cmpeq (vector unsigned short,
                                    vector unsigned short);
     vector signed int vec_cmpeq (vector signed int, vector signed int);
     vector signed int vec_cmpeq (vector unsigned int, vector unsigned int);
     vector signed int vec_cmpeq (vector float, vector float);

     vector signed int vec_cmpge (vector float, vector float);

     vector signed char vec_cmpgt (vector unsigned char,
                                   vector unsigned char);
     vector signed char vec_cmpgt (vector signed char, vector signed char);
     vector signed short vec_cmpgt (vector unsigned short,
                                    vector unsigned short);
     vector signed short vec_cmpgt (vector signed short,
                                    vector signed short);
     vector signed int vec_cmpgt (vector unsigned int, vector unsigned int);
     vector signed int vec_cmpgt (vector signed int, vector signed int);
     vector signed int vec_cmpgt (vector float, vector float);

     vector signed int vec_cmple (vector float, vector float);

     vector signed char vec_cmplt (vector unsigned char,
                                   vector unsigned char);
     vector signed char vec_cmplt (vector signed char, vector signed char);
     vector signed short vec_cmplt (vector unsigned short,
                                    vector unsigned short);
     vector signed short vec_cmplt (vector signed short,
                                    vector signed short);
     vector signed int vec_cmplt (vector unsigned int, vector unsigned int);
     vector signed int vec_cmplt (vector signed int, vector signed int);
     vector signed int vec_cmplt (vector float, vector float);

     vector float vec_ctf (vector unsigned int, const char);
     vector float vec_ctf (vector signed int, const char);

     vector signed int vec_cts (vector float, const char);

     vector unsigned int vec_ctu (vector float, const char);

     void vec_dss (const char);

     void vec_dssall (void);

     void vec_dst (void *, int, const char);

     void vec_dstst (void *, int, const char);

     void vec_dststt (void *, int, const char);

     void vec_dstt (void *, int, const char);

     vector float vec_expte (vector float, vector float);

     vector float vec_floor (vector float, vector float);

     vector float vec_ld (int, vector float *);
     vector float vec_ld (int, float *):
     vector signed int vec_ld (int, int *);
     vector signed int vec_ld (int, vector signed int *);
     vector unsigned int vec_ld (int, vector unsigned int *);
     vector unsigned int vec_ld (int, unsigned int *);
     vector signed short vec_ld (int, short *, vector signed short *);
     vector unsigned short vec_ld (int, unsigned short *,
                                   vector unsigned short *);
     vector signed char vec_ld (int, signed char *);
     vector signed char vec_ld (int, vector signed char *);
     vector unsigned char vec_ld (int, unsigned char *);
     vector unsigned char vec_ld (int, vector unsigned char *);

     vector signed char vec_lde (int, signed char *);
     vector unsigned char vec_lde (int, unsigned char *);
     vector signed short vec_lde (int, short *);
     vector unsigned short vec_lde (int, unsigned short *);
     vector float vec_lde (int, float *);
     vector signed int vec_lde (int, int *);
     vector unsigned int vec_lde (int, unsigned int *);

     void float vec_ldl (int, float *);
     void float vec_ldl (int, vector float *);
     void signed int vec_ldl (int, vector signed int *);
     void signed int vec_ldl (int, int *);
     void unsigned int vec_ldl (int, unsigned int *);
     void unsigned int vec_ldl (int, vector unsigned int *);
     void signed short vec_ldl (int, vector signed short *);
     void signed short vec_ldl (int, short *);
     void unsigned short vec_ldl (int, vector unsigned short *);
     void unsigned short vec_ldl (int, unsigned short *);
     void signed char vec_ldl (int, vector signed char *);
     void signed char vec_ldl (int, signed char *);
     void unsigned char vec_ldl (int, vector unsigned char *);
     void unsigned char vec_ldl (int, unsigned char *);

     vector float vec_loge (vector float);

     vector unsigned char vec_lvsl (int, void *, int *);

     vector unsigned char vec_lvsr (int, void *, int *);

     vector float vec_madd (vector float, vector float, vector float);

     vector signed short vec_madds (vector signed short, vector signed short,
                                    vector signed short);

     vector unsigned char vec_max (vector signed char, vector unsigned char);

     vector unsigned char vec_max (vector unsigned char, vector signed char);

     vector unsigned char vec_max (vector unsigned char,
                                   vector unsigned char);
     vector signed char vec_max (vector signed char, vector signed char);
     vector unsigned short vec_max (vector signed short,
                                    vector unsigned short);
     vector unsigned short vec_max (vector unsigned short,
                                    vector signed short);
     vector unsigned short vec_max (vector unsigned short,
                                    vector unsigned short);
     vector signed short vec_max (vector signed short, vector signed short);
     vector unsigned int vec_max (vector signed int, vector unsigned int);
     vector unsigned int vec_max (vector unsigned int, vector signed int);
     vector unsigned int vec_max (vector unsigned int, vector unsigned int);
     vector signed int vec_max (vector signed int, vector signed int);
     vector float vec_max (vector float, vector float);

     vector signed char vec_mergeh (vector signed char, vector signed char);
     vector unsigned char vec_mergeh (vector unsigned char,
                                      vector unsigned char);
     vector signed short vec_mergeh (vector signed short,
                                     vector signed short);
     vector unsigned short vec_mergeh (vector unsigned short,
                                       vector unsigned short);
     vector float vec_mergeh (vector float, vector float);
     vector signed int vec_mergeh (vector signed int, vector signed int);
     vector unsigned int vec_mergeh (vector unsigned int,
                                     vector unsigned int);

     vector signed char vec_mergel (vector signed char, vector signed char);
     vector unsigned char vec_mergel (vector unsigned char,
                                      vector unsigned char);
     vector signed short vec_mergel (vector signed short,
                                     vector signed short);
     vector unsigned short vec_mergel (vector unsigned short,
                                       vector unsigned short);
     vector float vec_mergel (vector float, vector float);
     vector signed int vec_mergel (vector signed int, vector signed int);
     vector unsigned int vec_mergel (vector unsigned int,
                                     vector unsigned int);

     vector unsigned short vec_mfvscr (void);

     vector unsigned char vec_min (vector signed char, vector unsigned char);

     vector unsigned char vec_min (vector unsigned char, vector signed char);

     vector unsigned char vec_min (vector unsigned char,
                                   vector unsigned char);
     vector signed char vec_min (vector signed char, vector signed char);
     vector unsigned short vec_min (vector signed short,
                                    vector unsigned short);
     vector unsigned short vec_min (vector unsigned short,
                                    vector signed short);
     vector unsigned short vec_min (vector unsigned short,
                                    vector unsigned short);
     vector signed short vec_min (vector signed short, vector signed short);
     vector unsigned int vec_min (vector signed int, vector unsigned int);
     vector unsigned int vec_min (vector unsigned int, vector signed int);
     vector unsigned int vec_min (vector unsigned int, vector unsigned int);
     vector signed int vec_min (vector signed int, vector signed int);
     vector float vec_min (vector float, vector float);

     vector signed short vec_mladd (vector signed short, vector signed short,
                                    vector signed short);
     vector signed short vec_mladd (vector signed short,
                                    vector unsigned short,
                                    vector unsigned short);
     vector signed short vec_mladd (vector unsigned short,
                                    vector signed short,
                                    vector signed short);
     vector unsigned short vec_mladd (vector unsigned short,
                                      vector unsigned short,
                                      vector unsigned short);

     vector signed short vec_mradds (vector signed short,
                                     vector signed short,
                                     vector signed short);

     vector unsigned int vec_msum (vector unsigned char,
                                   vector unsigned char,
                                   vector unsigned int);
     vector signed int vec_msum (vector signed char, vector unsigned char,
                                 vector signed int);
     vector unsigned int vec_msum (vector unsigned short,
                                   vector unsigned short,
                                   vector unsigned int);
     vector signed int vec_msum (vector signed short, vector signed short,
                                 vector signed int);

     vector unsigned int vec_msums (vector unsigned short,
                                    vector unsigned short,
                                    vector unsigned int);
     vector signed int vec_msums (vector signed short, vector signed short,
                                  vector signed int);

     void vec_mtvscr (vector signed int);
     void vec_mtvscr (vector unsigned int);
     void vec_mtvscr (vector signed short);
     void vec_mtvscr (vector unsigned short);
     void vec_mtvscr (vector signed char);
     void vec_mtvscr (vector unsigned char);

     vector unsigned short vec_mule (vector unsigned char,
                                     vector unsigned char);
     vector signed short vec_mule (vector signed char, vector signed char);
     vector unsigned int vec_mule (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_mule (vector signed short, vector signed short);

     vector unsigned short vec_mulo (vector unsigned char,
                                     vector unsigned char);
     vector signed short vec_mulo (vector signed char, vector signed char);
     vector unsigned int vec_mulo (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_mulo (vector signed short, vector signed short);

     vector float vec_nmsub (vector float, vector float, vector float);

     vector float vec_nor (vector float, vector float);
     vector signed int vec_nor (vector signed int, vector signed int);
     vector unsigned int vec_nor (vector unsigned int, vector unsigned int);
     vector signed short vec_nor (vector signed short, vector signed short);
     vector unsigned short vec_nor (vector unsigned short,
                                    vector unsigned short);
     vector signed char vec_nor (vector signed char, vector signed char);
     vector unsigned char vec_nor (vector unsigned char,
                                   vector unsigned char);

     vector float vec_or (vector float, vector float);
     vector float vec_or (vector float, vector signed int);
     vector float vec_or (vector signed int, vector float);
     vector signed int vec_or (vector signed int, vector signed int);
     vector unsigned int vec_or (vector signed int, vector unsigned int);
     vector unsigned int vec_or (vector unsigned int, vector signed int);
     vector unsigned int vec_or (vector unsigned int, vector unsigned int);
     vector signed short vec_or (vector signed short, vector signed short);
     vector unsigned short vec_or (vector signed short,
                                   vector unsigned short);
     vector unsigned short vec_or (vector unsigned short,
                                   vector signed short);
     vector unsigned short vec_or (vector unsigned short,
                                   vector unsigned short);
     vector signed char vec_or (vector signed char, vector signed char);
     vector unsigned char vec_or (vector signed char, vector unsigned char);
     vector unsigned char vec_or (vector unsigned char, vector signed char);
     vector unsigned char vec_or (vector unsigned char,
                                  vector unsigned char);

     vector signed char vec_pack (vector signed short, vector signed short);
     vector unsigned char vec_pack (vector unsigned short,
                                    vector unsigned short);
     vector signed short vec_pack (vector signed int, vector signed int);
     vector unsigned short vec_pack (vector unsigned int,
                                     vector unsigned int);

     vector signed short vec_packpx (vector unsigned int,
                                     vector unsigned int);

     vector unsigned char vec_packs (vector unsigned short,
                                     vector unsigned short);
     vector signed char vec_packs (vector signed short, vector signed short);

     vector unsigned short vec_packs (vector unsigned int,
                                      vector unsigned int);
     vector signed short vec_packs (vector signed int, vector signed int);

     vector unsigned char vec_packsu (vector unsigned short,
                                      vector unsigned short);
     vector unsigned char vec_packsu (vector signed short,
                                      vector signed short);
     vector unsigned short vec_packsu (vector unsigned int,
                                       vector unsigned int);
     vector unsigned short vec_packsu (vector signed int, vector signed int);

     vector float vec_perm (vector float, vector float,
                            vector unsigned char);
     vector signed int vec_perm (vector signed int, vector signed int,
                                 vector unsigned char);
     vector unsigned int vec_perm (vector unsigned int, vector unsigned int,
                                   vector unsigned char);
     vector signed short vec_perm (vector signed short, vector signed short,
                                   vector unsigned char);
     vector unsigned short vec_perm (vector unsigned short,
                                     vector unsigned short,
                                     vector unsigned char);
     vector signed char vec_perm (vector signed char, vector signed char,
                                  vector unsigned char);
     vector unsigned char vec_perm (vector unsigned char,
                                    vector unsigned char,
                                    vector unsigned char);

     vector float vec_re (vector float);

     vector signed char vec_rl (vector signed char, vector unsigned char);
     vector unsigned char vec_rl (vector unsigned char,
                                  vector unsigned char);
     vector signed short vec_rl (vector signed short, vector unsigned short);

     vector unsigned short vec_rl (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_rl (vector signed int, vector unsigned int);
     vector unsigned int vec_rl (vector unsigned int, vector unsigned int);

     vector float vec_round (vector float);

     vector float vec_rsqrte (vector float);

     vector float vec_sel (vector float, vector float, vector signed int);
     vector float vec_sel (vector float, vector float, vector unsigned int);
     vector signed int vec_sel (vector signed int, vector signed int,
                                vector signed int);
     vector signed int vec_sel (vector signed int, vector signed int,
                                vector unsigned int);
     vector unsigned int vec_sel (vector unsigned int, vector unsigned int,
                                  vector signed int);
     vector unsigned int vec_sel (vector unsigned int, vector unsigned int,
                                  vector unsigned int);
     vector signed short vec_sel (vector signed short, vector signed short,
                                  vector signed short);
     vector signed short vec_sel (vector signed short, vector signed short,
                                  vector unsigned short);
     vector unsigned short vec_sel (vector unsigned short,
                                    vector unsigned short,
                                    vector signed short);
     vector unsigned short vec_sel (vector unsigned short,
                                    vector unsigned short,
                                    vector unsigned short);
     vector signed char vec_sel (vector signed char, vector signed char,
                                 vector signed char);
     vector signed char vec_sel (vector signed char, vector signed char,
                                 vector unsigned char);
     vector unsigned char vec_sel (vector unsigned char,
                                   vector unsigned char,
                                   vector signed char);
     vector unsigned char vec_sel (vector unsigned char,
                                   vector unsigned char,
                                   vector unsigned char);

     vector signed char vec_sl (vector signed char, vector unsigned char);
     vector unsigned char vec_sl (vector unsigned char,
                                  vector unsigned char);
     vector signed short vec_sl (vector signed short, vector unsigned short);

     vector unsigned short vec_sl (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_sl (vector signed int, vector unsigned int);
     vector unsigned int vec_sl (vector unsigned int, vector unsigned int);

     vector float vec_sld (vector float, vector float, const char);
     vector signed int vec_sld (vector signed int, vector signed int,
                                const char);
     vector unsigned int vec_sld (vector unsigned int, vector unsigned int,
                                  const char);
     vector signed short vec_sld (vector signed short, vector signed short,
                                  const char);
     vector unsigned short vec_sld (vector unsigned short,
                                    vector unsigned short, const char);
     vector signed char vec_sld (vector signed char, vector signed char,
                                 const char);
     vector unsigned char vec_sld (vector unsigned char,
                                   vector unsigned char,
                                   const char);

     vector signed int vec_sll (vector signed int, vector unsigned int);
     vector signed int vec_sll (vector signed int, vector unsigned short);
     vector signed int vec_sll (vector signed int, vector unsigned char);
     vector unsigned int vec_sll (vector unsigned int, vector unsigned int);
     vector unsigned int vec_sll (vector unsigned int,
                                  vector unsigned short);
     vector unsigned int vec_sll (vector unsigned int, vector unsigned char);

     vector signed short vec_sll (vector signed short, vector unsigned int);
     vector signed short vec_sll (vector signed short,
                                  vector unsigned short);
     vector signed short vec_sll (vector signed short, vector unsigned char);

     vector unsigned short vec_sll (vector unsigned short,
                                    vector unsigned int);
     vector unsigned short vec_sll (vector unsigned short,
                                    vector unsigned short);
     vector unsigned short vec_sll (vector unsigned short,
                                    vector unsigned char);
     vector signed char vec_sll (vector signed char, vector unsigned int);
     vector signed char vec_sll (vector signed char, vector unsigned short);
     vector signed char vec_sll (vector signed char, vector unsigned char);
     vector unsigned char vec_sll (vector unsigned char,
                                   vector unsigned int);
     vector unsigned char vec_sll (vector unsigned char,
                                   vector unsigned short);
     vector unsigned char vec_sll (vector unsigned char,
                                   vector unsigned char);

     vector float vec_slo (vector float, vector signed char);
     vector float vec_slo (vector float, vector unsigned char);
     vector signed int vec_slo (vector signed int, vector signed char);
     vector signed int vec_slo (vector signed int, vector unsigned char);
     vector unsigned int vec_slo (vector unsigned int, vector signed char);
     vector unsigned int vec_slo (vector unsigned int, vector unsigned char);

     vector signed short vec_slo (vector signed short, vector signed char);
     vector signed short vec_slo (vector signed short, vector unsigned char);

     vector unsigned short vec_slo (vector unsigned short,
                                    vector signed char);
     vector unsigned short vec_slo (vector unsigned short,
                                    vector unsigned char);
     vector signed char vec_slo (vector signed char, vector signed char);
     vector signed char vec_slo (vector signed char, vector unsigned char);
     vector unsigned char vec_slo (vector unsigned char, vector signed char);

     vector unsigned char vec_slo (vector unsigned char,
                                   vector unsigned char);

     vector signed char vec_splat (vector signed char, const char);
     vector unsigned char vec_splat (vector unsigned char, const char);
     vector signed short vec_splat (vector signed short, const char);
     vector unsigned short vec_splat (vector unsigned short, const char);
     vector float vec_splat (vector float, const char);
     vector signed int vec_splat (vector signed int, const char);
     vector unsigned int vec_splat (vector unsigned int, const char);

     vector signed char vec_splat_s8 (const char);

     vector signed short vec_splat_s16 (const char);

     vector signed int vec_splat_s32 (const char);

     vector unsigned char vec_splat_u8 (const char);

     vector unsigned short vec_splat_u16 (const char);

     vector unsigned int vec_splat_u32 (const char);

     vector signed char vec_sr (vector signed char, vector unsigned char);
     vector unsigned char vec_sr (vector unsigned char,
                                  vector unsigned char);
     vector signed short vec_sr (vector signed short, vector unsigned short);

     vector unsigned short vec_sr (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_sr (vector signed int, vector unsigned int);
     vector unsigned int vec_sr (vector unsigned int, vector unsigned int);

     vector signed char vec_sra (vector signed char, vector unsigned char);
     vector unsigned char vec_sra (vector unsigned char,
                                   vector unsigned char);
     vector signed short vec_sra (vector signed short,
                                  vector unsigned short);
     vector unsigned short vec_sra (vector unsigned short,
                                    vector unsigned short);
     vector signed int vec_sra (vector signed int, vector unsigned int);
     vector unsigned int vec_sra (vector unsigned int, vector unsigned int);

     vector signed int vec_srl (vector signed int, vector unsigned int);
     vector signed int vec_srl (vector signed int, vector unsigned short);
     vector signed int vec_srl (vector signed int, vector unsigned char);
     vector unsigned int vec_srl (vector unsigned int, vector unsigned int);
     vector unsigned int vec_srl (vector unsigned int,
                                  vector unsigned short);
     vector unsigned int vec_srl (vector unsigned int, vector unsigned char);

     vector signed short vec_srl (vector signed short, vector unsigned int);
     vector signed short vec_srl (vector signed short,
                                  vector unsigned short);
     vector signed short vec_srl (vector signed short, vector unsigned char);

     vector unsigned short vec_srl (vector unsigned short,
                                    vector unsigned int);
     vector unsigned short vec_srl (vector unsigned short,
                                    vector unsigned short);
     vector unsigned short vec_srl (vector unsigned short,
                                    vector unsigned char);
     vector signed char vec_srl (vector signed char, vector unsigned int);
     vector signed char vec_srl (vector signed char, vector unsigned short);
     vector signed char vec_srl (vector signed char, vector unsigned char);
     vector unsigned char vec_srl (vector unsigned char,
                                   vector unsigned int);
     vector unsigned char vec_srl (vector unsigned char,
                                   vector unsigned short);
     vector unsigned char vec_srl (vector unsigned char,
                                   vector unsigned char);

     vector float vec_sro (vector float, vector signed char);
     vector float vec_sro (vector float, vector unsigned char);
     vector signed int vec_sro (vector signed int, vector signed char);
     vector signed int vec_sro (vector signed int, vector unsigned char);
     vector unsigned int vec_sro (vector unsigned int, vector signed char);
     vector unsigned int vec_sro (vector unsigned int, vector unsigned char);

     vector signed short vec_sro (vector signed short, vector signed char);
     vector signed short vec_sro (vector signed short, vector unsigned char);

     vector unsigned short vec_sro (vector unsigned short,
                                    vector signed char);
     vector unsigned short vec_sro (vector unsigned short,
                                    vector unsigned char);
     vector signed char vec_sro (vector signed char, vector signed char);
     vector signed char vec_sro (vector signed char, vector unsigned char);
     vector unsigned char vec_sro (vector unsigned char, vector signed char);

     vector unsigned char vec_sro (vector unsigned char,
                                   vector unsigned char);

     void vec_st (vector float, int, float *);
     void vec_st (vector float, int, vector float *);
     void vec_st (vector signed int, int, int *);
     void vec_st (vector signed int, int, unsigned int *);
     void vec_st (vector unsigned int, int, unsigned int *);
     void vec_st (vector unsigned int, int, vector unsigned int *);
     void vec_st (vector signed short, int, short *);
     void vec_st (vector signed short, int, vector unsigned short *);
     void vec_st (vector signed short, int, vector signed short *);
     void vec_st (vector unsigned short, int, unsigned short *);
     void vec_st (vector unsigned short, int, vector unsigned short *);
     void vec_st (vector signed char, int, signed char *);
     void vec_st (vector signed char, int, unsigned char *);
     void vec_st (vector signed char, int, vector signed char *);
     void vec_st (vector unsigned char, int, unsigned char *);
     void vec_st (vector unsigned char, int, vector unsigned char *);

     void vec_ste (vector signed char, int, unsigned char *);
     void vec_ste (vector signed char, int, signed char *);
     void vec_ste (vector unsigned char, int, unsigned char *);
     void vec_ste (vector signed short, int, short *);
     void vec_ste (vector signed short, int, unsigned short *);
     void vec_ste (vector unsigned short, int, void *);
     void vec_ste (vector signed int, int, unsigned int *);
     void vec_ste (vector signed int, int, int *);
     void vec_ste (vector unsigned int, int, unsigned int *);
     void vec_ste (vector float, int, float *);

     void vec_stl (vector float, int, vector float *);
     void vec_stl (vector float, int, float *);
     void vec_stl (vector signed int, int, vector signed int *);
     void vec_stl (vector signed int, int, int *);
     void vec_stl (vector signed int, int, unsigned int *);
     void vec_stl (vector unsigned int, int, vector unsigned int *);
     void vec_stl (vector unsigned int, int, unsigned int *);
     void vec_stl (vector signed short, int, short *);
     void vec_stl (vector signed short, int, unsigned short *);
     void vec_stl (vector signed short, int, vector signed short *);
     void vec_stl (vector unsigned short, int, unsigned short *);
     void vec_stl (vector unsigned short, int, vector signed short *);
     void vec_stl (vector signed char, int, signed char *);
     void vec_stl (vector signed char, int, unsigned char *);
     void vec_stl (vector signed char, int, vector signed char *);
     void vec_stl (vector unsigned char, int, unsigned char *);
     void vec_stl (vector unsigned char, int, vector unsigned char *);

     vector signed char vec_sub (vector signed char, vector signed char);
     vector unsigned char vec_sub (vector signed char, vector unsigned char);

     vector unsigned char vec_sub (vector unsigned char, vector signed char);

     vector unsigned char vec_sub (vector unsigned char,
                                   vector unsigned char);
     vector signed short vec_sub (vector signed short, vector signed short);
     vector unsigned short vec_sub (vector signed short,
                                    vector unsigned short);
     vector unsigned short vec_sub (vector unsigned short,
                                    vector signed short);
     vector unsigned short vec_sub (vector unsigned short,
                                    vector unsigned short);
     vector signed int vec_sub (vector signed int, vector signed int);
     vector unsigned int vec_sub (vector signed int, vector unsigned int);
     vector unsigned int vec_sub (vector unsigned int, vector signed int);
     vector unsigned int vec_sub (vector unsigned int, vector unsigned int);
     vector float vec_sub (vector float, vector float);

     vector unsigned int vec_subc (vector unsigned int, vector unsigned int);

     vector unsigned char vec_subs (vector signed char,
                                    vector unsigned char);
     vector unsigned char vec_subs (vector unsigned char,
                                    vector signed char);
     vector unsigned char vec_subs (vector unsigned char,
                                    vector unsigned char);
     vector signed char vec_subs (vector signed char, vector signed char);
     vector unsigned short vec_subs (vector signed short,
                                     vector unsigned short);
     vector unsigned short vec_subs (vector unsigned short,
                                     vector signed short);
     vector unsigned short vec_subs (vector unsigned short,
                                     vector unsigned short);
     vector signed short vec_subs (vector signed short, vector signed short);

     vector unsigned int vec_subs (vector signed int, vector unsigned int);
     vector unsigned int vec_subs (vector unsigned int, vector signed int);
     vector unsigned int vec_subs (vector unsigned int, vector unsigned int);

     vector signed int vec_subs (vector signed int, vector signed int);

     vector unsigned int vec_sum4s (vector unsigned char,
                                    vector unsigned int);
     vector signed int vec_sum4s (vector signed char, vector signed int);
     vector signed int vec_sum4s (vector signed short, vector signed int);

     vector signed int vec_sum2s (vector signed int, vector signed int);

     vector signed int vec_sums (vector signed int, vector signed int);

     vector float vec_trunc (vector float);

     vector signed short vec_unpackh (vector signed char);
     vector unsigned int vec_unpackh (vector signed short);
     vector signed int vec_unpackh (vector signed short);

     vector signed short vec_unpackl (vector signed char);
     vector unsigned int vec_unpackl (vector signed short);
     vector signed int vec_unpackl (vector signed short);

     vector float vec_xor (vector float, vector float);
     vector float vec_xor (vector float, vector signed int);
     vector float vec_xor (vector signed int, vector float);
     vector signed int vec_xor (vector signed int, vector signed int);
     vector unsigned int vec_xor (vector signed int, vector unsigned int);
     vector unsigned int vec_xor (vector unsigned int, vector signed int);
     vector unsigned int vec_xor (vector unsigned int, vector unsigned int);
     vector signed short vec_xor (vector signed short, vector signed short);
     vector unsigned short vec_xor (vector signed short,
                                    vector unsigned short);
     vector unsigned short vec_xor (vector unsigned short,
                                    vector signed short);
     vector unsigned short vec_xor (vector unsigned short,
                                    vector unsigned short);
     vector signed char vec_xor (vector signed char, vector signed char);
     vector unsigned char vec_xor (vector signed char, vector unsigned char);

     vector unsigned char vec_xor (vector unsigned char, vector signed char);

     vector unsigned char vec_xor (vector unsigned char,
                                   vector unsigned char);

     vector signed int vec_all_eq (vector signed char, vector unsigned char);

     vector signed int vec_all_eq (vector signed char, vector signed char);
     vector signed int vec_all_eq (vector unsigned char, vector signed char);

     vector signed int vec_all_eq (vector unsigned char,
                                   vector unsigned char);
     vector signed int vec_all_eq (vector signed short,
                                   vector unsigned short);
     vector signed int vec_all_eq (vector signed short, vector signed short);

     vector signed int vec_all_eq (vector unsigned short,
                                   vector signed short);
     vector signed int vec_all_eq (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_all_eq (vector signed int, vector unsigned int);
     vector signed int vec_all_eq (vector signed int, vector signed int);
     vector signed int vec_all_eq (vector unsigned int, vector signed int);
     vector signed int vec_all_eq (vector unsigned int, vector unsigned int);

     vector signed int vec_all_eq (vector float, vector float);

     vector signed int vec_all_ge (vector signed char, vector unsigned char);

     vector signed int vec_all_ge (vector unsigned char, vector signed char);

     vector signed int vec_all_ge (vector unsigned char,
                                   vector unsigned char);
     vector signed int vec_all_ge (vector signed char, vector signed char);
     vector signed int vec_all_ge (vector signed short,
                                   vector unsigned short);
     vector signed int vec_all_ge (vector unsigned short,
                                   vector signed short);
     vector signed int vec_all_ge (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_all_ge (vector signed short, vector signed short);

     vector signed int vec_all_ge (vector signed int, vector unsigned int);
     vector signed int vec_all_ge (vector unsigned int, vector signed int);
     vector signed int vec_all_ge (vector unsigned int, vector unsigned int);

     vector signed int vec_all_ge (vector signed int, vector signed int);
     vector signed int vec_all_ge (vector float, vector float);

     vector signed int vec_all_gt (vector signed char, vector unsigned char);

     vector signed int vec_all_gt (vector unsigned char, vector signed char);

     vector signed int vec_all_gt (vector unsigned char,
                                   vector unsigned char);
     vector signed int vec_all_gt (vector signed char, vector signed char);
     vector signed int vec_all_gt (vector signed short,
                                   vector unsigned short);
     vector signed int vec_all_gt (vector unsigned short,
                                   vector signed short);
     vector signed int vec_all_gt (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_all_gt (vector signed short, vector signed short);

     vector signed int vec_all_gt (vector signed int, vector unsigned int);
     vector signed int vec_all_gt (vector unsigned int, vector signed int);
     vector signed int vec_all_gt (vector unsigned int, vector unsigned int);

     vector signed int vec_all_gt (vector signed int, vector signed int);
     vector signed int vec_all_gt (vector float, vector float);

     vector signed int vec_all_in (vector float, vector float);

     vector signed int vec_all_le (vector signed char, vector unsigned char);

     vector signed int vec_all_le (vector unsigned char, vector signed char);

     vector signed int vec_all_le (vector unsigned char,
                                   vector unsigned char);
     vector signed int vec_all_le (vector signed char, vector signed char);
     vector signed int vec_all_le (vector signed short,
                                   vector unsigned short);
     vector signed int vec_all_le (vector unsigned short,
                                   vector signed short);
     vector signed int vec_all_le (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_all_le (vector signed short, vector signed short);

     vector signed int vec_all_le (vector signed int, vector unsigned int);
     vector signed int vec_all_le (vector unsigned int, vector signed int);
     vector signed int vec_all_le (vector unsigned int, vector unsigned int);

     vector signed int vec_all_le (vector signed int, vector signed int);
     vector signed int vec_all_le (vector float, vector float);

     vector signed int vec_all_lt (vector signed char, vector unsigned char);

     vector signed int vec_all_lt (vector unsigned char, vector signed char);

     vector signed int vec_all_lt (vector unsigned char,
                                   vector unsigned char);
     vector signed int vec_all_lt (vector signed char, vector signed char);
     vector signed int vec_all_lt (vector signed short,
                                   vector unsigned short);
     vector signed int vec_all_lt (vector unsigned short,
                                   vector signed short);
     vector signed int vec_all_lt (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_all_lt (vector signed short, vector signed short);

     vector signed int vec_all_lt (vector signed int, vector unsigned int);
     vector signed int vec_all_lt (vector unsigned int, vector signed int);
     vector signed int vec_all_lt (vector unsigned int, vector unsigned int);

     vector signed int vec_all_lt (vector signed int, vector signed int);
     vector signed int vec_all_lt (vector float, vector float);

     vector signed int vec_all_nan (vector float);

     vector signed int vec_all_ne (vector signed char, vector unsigned char);

     vector signed int vec_all_ne (vector signed char, vector signed char);
     vector signed int vec_all_ne (vector unsigned char, vector signed char);

     vector signed int vec_all_ne (vector unsigned char,
                                   vector unsigned char);
     vector signed int vec_all_ne (vector signed short,
                                   vector unsigned short);
     vector signed int vec_all_ne (vector signed short, vector signed short);

     vector signed int vec_all_ne (vector unsigned short,
                                   vector signed short);
     vector signed int vec_all_ne (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_all_ne (vector signed int, vector unsigned int);
     vector signed int vec_all_ne (vector signed int, vector signed int);
     vector signed int vec_all_ne (vector unsigned int, vector signed int);
     vector signed int vec_all_ne (vector unsigned int, vector unsigned int);

     vector signed int vec_all_ne (vector float, vector float);

     vector signed int vec_all_nge (vector float, vector float);

     vector signed int vec_all_ngt (vector float, vector float);

     vector signed int vec_all_nle (vector float, vector float);

     vector signed int vec_all_nlt (vector float, vector float);

     vector signed int vec_all_numeric (vector float);

     vector signed int vec_any_eq (vector signed char, vector unsigned char);

     vector signed int vec_any_eq (vector signed char, vector signed char);
     vector signed int vec_any_eq (vector unsigned char, vector signed char);

     vector signed int vec_any_eq (vector unsigned char,
                                   vector unsigned char);
     vector signed int vec_any_eq (vector signed short,
                                   vector unsigned short);
     vector signed int vec_any_eq (vector signed short, vector signed short);

     vector signed int vec_any_eq (vector unsigned short,
                                   vector signed short);
     vector signed int vec_any_eq (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_any_eq (vector signed int, vector unsigned int);
     vector signed int vec_any_eq (vector signed int, vector signed int);
     vector signed int vec_any_eq (vector unsigned int, vector signed int);
     vector signed int vec_any_eq (vector unsigned int, vector unsigned int);

     vector signed int vec_any_eq (vector float, vector float);

     vector signed int vec_any_ge (vector signed char, vector unsigned char);

     vector signed int vec_any_ge (vector unsigned char, vector signed char);

     vector signed int vec_any_ge (vector unsigned char,
                                   vector unsigned char);
     vector signed int vec_any_ge (vector signed char, vector signed char);
     vector signed int vec_any_ge (vector signed short,
                                   vector unsigned short);
     vector signed int vec_any_ge (vector unsigned short,
                                   vector signed short);
     vector signed int vec_any_ge (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_any_ge (vector signed short, vector signed short);

     vector signed int vec_any_ge (vector signed int, vector unsigned int);
     vector signed int vec_any_ge (vector unsigned int, vector signed int);
     vector signed int vec_any_ge (vector unsigned int, vector unsigned int);

     vector signed int vec_any_ge (vector signed int, vector signed int);
     vector signed int vec_any_ge (vector float, vector float);

     vector signed int vec_any_gt (vector signed char, vector unsigned char);

     vector signed int vec_any_gt (vector unsigned char, vector signed char);

     vector signed int vec_any_gt (vector unsigned char,
                                   vector unsigned char);
     vector signed int vec_any_gt (vector signed char, vector signed char);
     vector signed int vec_any_gt (vector signed short,
                                   vector unsigned short);
     vector signed int vec_any_gt (vector unsigned short,
                                   vector signed short);
     vector signed int vec_any_gt (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_any_gt (vector signed short, vector signed short);

     vector signed int vec_any_gt (vector signed int, vector unsigned int);
     vector signed int vec_any_gt (vector unsigned int, vector signed int);
     vector signed int vec_any_gt (vector unsigned int, vector unsigned int);

     vector signed int vec_any_gt (vector signed int, vector signed int);
     vector signed int vec_any_gt (vector float, vector float);

     vector signed int vec_any_le (vector signed char, vector unsigned char);

     vector signed int vec_any_le (vector unsigned char, vector signed char);

     vector signed int vec_any_le (vector unsigned char,
                                   vector unsigned char);
     vector signed int vec_any_le (vector signed char, vector signed char);
     vector signed int vec_any_le (vector signed short,
                                   vector unsigned short);
     vector signed int vec_any_le (vector unsigned short,
                                   vector signed short);
     vector signed int vec_any_le (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_any_le (vector signed short, vector signed short);

     vector signed int vec_any_le (vector signed int, vector unsigned int);
     vector signed int vec_any_le (vector unsigned int, vector signed int);
     vector signed int vec_any_le (vector unsigned int, vector unsigned int);

     vector signed int vec_any_le (vector signed int, vector signed int);
     vector signed int vec_any_le (vector float, vector float);

     vector signed int vec_any_lt (vector signed char, vector unsigned char);

     vector signed int vec_any_lt (vector unsigned char, vector signed char);

     vector signed int vec_any_lt (vector unsigned char,
                                   vector unsigned char);
     vector signed int vec_any_lt (vector signed char, vector signed char);
     vector signed int vec_any_lt (vector signed short,
                                   vector unsigned short);
     vector signed int vec_any_lt (vector unsigned short,
                                   vector signed short);
     vector signed int vec_any_lt (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_any_lt (vector signed short, vector signed short);

     vector signed int vec_any_lt (vector signed int, vector unsigned int);
     vector signed int vec_any_lt (vector unsigned int, vector signed int);
     vector signed int vec_any_lt (vector unsigned int, vector unsigned int);

     vector signed int vec_any_lt (vector signed int, vector signed int);
     vector signed int vec_any_lt (vector float, vector float);

     vector signed int vec_any_nan (vector float);

     vector signed int vec_any_ne (vector signed char, vector unsigned char);

     vector signed int vec_any_ne (vector signed char, vector signed char);
     vector signed int vec_any_ne (vector unsigned char, vector signed char);

     vector signed int vec_any_ne (vector unsigned char,
                                   vector unsigned char);
     vector signed int vec_any_ne (vector signed short,
                                   vector unsigned short);
     vector signed int vec_any_ne (vector signed short, vector signed short);

     vector signed int vec_any_ne (vector unsigned short,
                                   vector signed short);
     vector signed int vec_any_ne (vector unsigned short,
                                   vector unsigned short);
     vector signed int vec_any_ne (vector signed int, vector unsigned int);
     vector signed int vec_any_ne (vector signed int, vector signed int);
     vector signed int vec_any_ne (vector unsigned int, vector signed int);
     vector signed int vec_any_ne (vector unsigned int, vector unsigned int);

     vector signed int vec_any_ne (vector float, vector float);

     vector signed int vec_any_nge (vector float, vector float);

     vector signed int vec_any_ngt (vector float, vector float);

     vector signed int vec_any_nle (vector float, vector float);

     vector signed int vec_any_nlt (vector float, vector float);

     vector signed int vec_any_numeric (vector float);

     vector signed int vec_any_out (vector float, vector float);

Pragmas Accepted by GCC
=======================

GCC supports several types of pragmas, primarily in order to compile code originally written for other compilers.  Note that in general we do not recommend the use of pragmas; see Function Attributes, for further explanation.

ARM Pragmas
-----------

The ARM target defines pragmas for controlling the default addition of long_call and short_call attributes to functions. See Function Attributes, for information about the effects of these attributes.

long_calls
     Set all subsequent functions to have the long_call attribute.

no_long_calls
     Set all subsequent functions to have the short_call attribute.

long_calls_off
     Do not affect the long_call or short_call attributes of subsequent functions.

Darwin Pragmas
--------------

The following pragmas are available for all architectures running the Darwin operating system.  These are useful for compatibility with other MacOS compilers.

mark TOKENS...
     This pragma is accepted, but has no effect.

options align=ALIGNMENT
     This pragma sets the alignment of fields in structures.  The values of ALIGNMENT may be mac68k, to emulate m68k alignment, or power, to emulate PowerPC alignment.  Uses of this pragma nest properly; to restore the previous setting, use reset for the ALIGNMENT.

segment TOKENS...
     This pragma is accepted, but has no effect.

unused (VAR [, VAR]...)
     This pragma declares variables to be possibly unused.  GCC will not produce warnings for the listed variables.  The effect is similar to that of the unused attribute, except that this pragma may appear anywhere within the variables' scopes.

Unnamed struct/union fields within structs/unions.
==================================================

For compatibility with other compilers, GCC allows you to define a structure or union that contains, as fields, structures and unions without names.  For example:

     struct {
       int a;
       union {
         int b;
         float c;
       };
       int d;
     } foo;

In this example, the user would be able to access members of the unnamed union with code like foo.b.  Note that only unnamed structs and unions are allowed, you may not have, for example, an unnamed int.

You must never create such structures that cause ambiguous field definitions. For example, this structure:

     struct {
       int a;
       struct {
         int a;
       };
     } foo;

It is ambiguous which a is being referred to with foo.a. Such constructs are not supported and must be avoided.  In the future, such constructs may be detected and treated as compilation errors.

Extensions to the C++ Language
******************************

The GNU compiler provides these extensions to the C++ language (and you can also use most of the C language extensions in your C++ programs).  If you want to write code that checks whether these features are available, you can test for the GNU compiler the same way as for C programs: check for a predefined macro __GNUC__.  You can also use __GNUG__ to test specifically for GNU C++ (see Standard Predefined Macros (The C Preprocessor)).

Minimum and Maximum Operators in C++
====================================

It is very convenient to have operators which return the ``minimum'' or the ``maximum'' of two arguments.  In GNU C++ (but not in GNU C),

A <? B
     is the minimum, returning the smaller of the numeric values A and B;

A >? B
     is the maximum, returning the larger of the numeric values A and B.

These operations are not primitive in ordinary C++, since you can use a macro to return the minimum of two things in C++, as in the following example.

     #define MIN(X,Y) ((X) < (Y) ? : (X) : (Y))

You might then use int min = MIN (i, j); to set MIN to the minimum value of variables I and J.

However, side effects in X or Y may cause unintended behavior.  For example, MIN (i++, j++) will fail, incrementing the smaller counter twice.  A GNU C extension allows you to write safe macros that avoid this kind of problem (see Naming an Expression's Type).  However, writing MIN and MAX as macros also forces you to use function-call notation for a fundamental arithmetic operation.  Using GNU C++ extensions, you can write int min = i <? j; instead.

Since <? and >? are built into the compiler, they properly handle expressions with side-effects;  int min = i++ <? j++; works correctly.

When is a Volatile Object Accessed?
===================================

Both the C and C++ standard have the concept of volatile objects.  These are normally accessed by pointers and used for accessing hardware.  The standards encourage compilers to refrain from optimizations concerning accesses to volatile objects that it might perform on non-volatile objects.  The C standard leaves it implementation defined as to what constitutes a volatile access.  The C++ standard omits to specify this, except to say that C++ should behave in a similar manner to C with respect to volatiles, where possible.  The minimum either standard specifies is that at a sequence point all previous accesses to volatile objects have stabilized and no subsequent accesses have occurred.  Thus an implementation is free to reorder and combine volatile accesses which occur between sequence points, but cannot do so for accesses across a sequence point.  The use of volatiles does not allow you to violate the restriction on updating objects multiple times within a sequence point.

In most expressions, it is intuitively obvious what is a read and what is a write.  For instance

     volatile int *dst = SOMEVALUE;
     volatile int *src = SOMEOTHERVALUE;
     *dst = *src;

will cause a read of the volatile object pointed to by SRC and stores the value into the volatile object pointed to by DST.  There is no guarantee that these reads and writes are atomic, especially for objects larger than int.

Less obvious expressions are where something which looks like an access is used in a void context.  An example would be,

     volatile int *src = SOMEVALUE;
     *src;

With C, such expressions are rvalues, and as rvalues cause a read of the object, GCC interprets this as a read of the volatile being pointed to.  The C++ standard specifies that such expressions do not undergo lvalue to rvalue conversion, and that the type of the dereferenced object may be incomplete.  The C++ standard does not specify explicitly that it is this lvalue to rvalue conversion which is responsible for causing an access.  However, there is reason to believe that it is, because otherwise certain simple expressions become undefined.  However, because it would surprise most programmers, G++ treats dereferencing a pointer to volatile object of complete type in a void context as a read of the object.  When the object has incomplete type, G++ issues a warning.

     struct S;
     struct T {int m;};
     volatile S *ptr1 = SOMEVALUE;
     volatile T *ptr2 = SOMEVALUE;
     *ptr1;
     *ptr2;

In this example, a warning is issued for *ptr1, and *ptr2 causes a read of the object pointed to.  If you wish to force an error on the first case, you must force a conversion to rvalue with, for instance a static cast, static_cast<S>(*ptr1).

When using a reference to volatile, G++ does not treat equivalent expressions as accesses to volatiles, but instead issues a warning that no volatile is accessed.  The rationale for this is that otherwise it becomes difficult to determine where volatile access occur, and not possible to ignore the return value from functions returning volatile references.  Again, if you wish to force a read, cast the reference to an rvalue.

Restricting Pointer Aliasing
============================

As with gcc, g++ understands the C99 feature of restricted pointers, specified with the __restrict__, or __restrict type qualifier.  Because you cannot compile C++ by specifying the -std=c99 language flag, restrict is not a keyword in C++.

In addition to allowing restricted pointers, you can specify restricted references, which indicate that the reference is not aliased in the local context.

     void fn (int *__restrict__ rptr, int &__restrict__ rref)
     {
       ...
     }

In the body of fn, RPTR points to an unaliased integer and RREF refers to a (different) unaliased integer.

You may also specify whether a member function's THIS pointer is unaliased by using __restrict__ as a member function qualifier.

     void T::fn () __restrict__
     {
       ...
     }

Within the body of T::fn, THIS will have the effective definition T *__restrict__ const this.  Notice that the interpretation of a __restrict__ member function qualifier is different to that of const or volatile qualifier, in that it is applied to the pointer rather than the object.  This is consistent with other compilers which implement restricted pointers.

As with all outermost parameter qualifiers, __restrict__ is ignored in function definition matching.  This means you only need to specify __restrict__ in a function definition, rather than in a function prototype as well.

Vague Linkage
=============

There are several constructs in C++ which require space in the object file but are not clearly tied to a single translation unit.  We say that these constructs have ``vague linkage''.  Typically such constructs are emitted wherever they are needed, though sometimes we can be more clever.

Inline Functions
     Inline functions are typically defined in a header file which can be included in many different compilations.  Hopefully they can usually be inlined, but sometimes an out-of-line copy is necessary, if the address of the function is taken or if inlining fails.  In general, we emit an out-of-line copy in all translation units where one is needed.  As an exception, we only emit inline virtual functions with the vtable, since it will always require a copy.

     Local static variables and string constants used in an inline function are also considered to have vague linkage, since they must be shared between all inlined and out-of-line instances of the function.

VTables
     C++ virtual functions are implemented in most compilers using a lookup table, known as a vtable.  The vtable contains pointers to the virtual functions provided by a class, and each object of the class contains a pointer to its vtable (or vtables, in some multiple-inheritance situations).  If the class declares any non-inline, non-pure virtual functions, the first one is chosen as the ``key method'' for the class, and the vtable is only emitted in the translation unit where the key method is defined.

     Note: If the chosen key method is later defined as inline, the vtable will still be emitted in every translation unit which defines it. Make sure that any inline virtuals are declared inline in the class body, even if they are not defined there.

type_info objects
     C++ requires information about types to be written out in order to implement dynamic_cast, typeid and exception handling. For polymorphic classes (classes with virtual functions), the type_info object is written out along with the vtable so that dynamic_cast can determine the dynamic type of a class object at runtime.  For all other types, we write out the type_info object when it is used: when applying typeid to an expression, throwing an object, or referring to a type in a catch clause or exception specification.

Template Instantiations
     Most everything in this section also applies to template instantiations, but there are other options as well. See Where's the Template?.

When used with GNU ld version 2.8 or later on an ELF system such as Linux/GNU or Solaris 2, or on Microsoft Windows, duplicate copies of these constructs will be discarded at link time.  This is known as COMDAT support.

On targets that don't support COMDAT, but do support weak symbols, GCC will use them.  This way one copy will override all the others, but the unused copies will still take up space in the executable.

For targets which do not support either COMDAT or weak symbols, most entities with vague linkage will be emitted as local symbols to avoid duplicate definition errors from the linker.  This will not happen for local statics in inlines, however, as having multiple copies will almost certainly break things.

See Declarations and Definitions in One Header, for another way to control placement of these constructs.

Declarations and Definitions in One Header
==========================================

C++ object definitions can be quite complex.  In principle, your source code will need two kinds of things for each object that you use across more than one source file.  First, you need an interface specification, describing its structure with type declarations and function prototypes.  Second, you need the implementation itself. It can be tedious to maintain a separate interface description in a header file, in parallel to the actual implementation.  It is also dangerous, since separate interface and implementation definitions may not remain parallel.

With GNU C++, you can use a single header file for both purposes.

     Warning: The mechanism to specify this is in transition.  For the nonce, you must use one of two #pragma commands; in a future release of GNU C++, an alternative mechanism will make these #pragma commands unnecessary.

The header file contains the full definitions, but is marked with #pragma interface in the source code.  This allows the compiler to use the header file only as an interface specification when ordinary source files incorporate it with #include.  In the single source file where the full implementation belongs, you can use either a naming convention or #pragma implementation to indicate this alternate use of the header file.

#pragma interface
#pragma interface "SUBDIR/OBJECTS.h"
     Use this directive in header files that define object classes, to save space in most of the object files that use those classes.  Normally, local copies of certain information (backup copies of inline member functions, debugging information, and the internal tables that implement virtual functions) must be kept in each object file that includes class definitions.  You can use this pragma to avoid such duplication.  When a header file containing #pragma interface is included in a compilation, this auxiliary information will not be generated (unless the main input source file itself uses #pragma implementation). Instead, the object files will contain references to be resolved at link time.

     The second form of this directive is useful for the case where you have multiple headers with the same name in different directories.  If you use this form, you must specify the same string to #pragma implementation.

#pragma implementation
#pragma implementation "OBJECTS.h"
     Use this pragma in a main input file, when you want full output from included header files to be generated (and made globally visible).  The included header file, in turn, should use #pragma interface. Backup copies of inline member functions, debugging information, and the internal tables used to implement virtual functions are all generated in implementation files.

     If you use #pragma implementation with no argument, it applies to an include file with the same basename (A file's basename was the name stripped of all leading path information and of trailing suffixes, such as .h or .C or .cc.) as your source file.  For example, in allclass.cc, giving just #pragma implementation by itself is equivalent to #pragma implementation "allclass.h".

     In versions of GNU C++ prior to 2.6.0 allclass.h was treated as an implementation file whenever you would include it from allclass.cc even if you never specified #pragma implementation.  This was deemed to be more trouble than it was worth, however, and disabled.

     If you use an explicit #pragma implementation, it must appear in your source file before you include the affected header files.

     Use the string argument if you want a single implementation file to include code from multiple header files.  (You must also use #include to include the header file; #pragma implementation only specifies how to use the file---it doesn't actually include it.)

     There is no way to split up the contents of a single header file into multiple implementation files.

#pragma implementation and #pragma interface also have an effect on function inlining.

If you define a class in a header file marked with #pragma interface, the effect on a function defined in that class is similar to an explicit extern declaration---the compiler emits no code at all to define an independent version of the function.  Its definition is used only for inlining with its callers.

Conversely, when you include the same header file in a main source file that declares it as #pragma implementation, the compiler emits code for the function itself; this defines a version of the function that can be found via pointers (or by callers compiled without inlining).  If all calls to the function can be inlined, you can avoid emitting the function by compiling with -fno-implement-inlines. If any calls were not inlined, you will get linker errors.

Where's the Template?
=====================

C++ templates are the first language feature to require more intelligence from the environment than one usually finds on a UNIX system.  Somehow the compiler and linker have to make sure that each template instance occurs exactly once in the executable if it is needed, and not at all otherwise.  There are two basic approaches to this problem, which I will refer to as the Borland model and the Cfront model.

Borland model
     Borland C++ solved the template instantiation problem by adding the code equivalent of common blocks to their linker; the compiler emits template instances in each translation unit that uses them, and the linker collapses them together.  The advantage of this model is that the linker only has to consider the object files themselves; there is no external complexity to worry about.  This disadvantage is that compilation time is increased because the template code is being compiled repeatedly. Code written for this model tends to include definitions of all templates in the header file, since they must be seen to be instantiated.

Cfront model
     The AT&T C++ translator, Cfront, solved the template instantiation problem by creating the notion of a template repository, an automatically maintained place where template instances are stored.  A more modern version of the repository works as follows: As individual object files are built, the compiler places any template definitions and instantiations encountered in the repository.  At link time, the link wrapper adds in the objects in the repository and compiles any needed instances that were not previously emitted.  The advantages of this model are more optimal compilation speed and the ability to use the system linker; to implement the Borland model a compiler vendor also needs to replace the linker.  The disadvantages are vastly increased complexity, and thus potential for error; for some code this can be just as transparent, but in practice it can been very difficult to build multiple programs in one directory and one program in multiple directories.  Code written for this model tends to separate definitions of non-inline member templates into a separate file, which should be compiled separately.

When used with GNU ld version 2.8 or later on an ELF system such as Linux/GNU or Solaris 2, or on Microsoft Windows, g++ supports the Borland model.  On other systems, g++ implements neither automatic model.

A future version of g++ will support a hybrid model whereby the compiler will emit any instantiations for which the template definition is included in the compile, and store template definitions and instantiation context information into the object file for the rest. The link wrapper will extract that information as necessary and invoke the compiler to produce the remaining instantiations.  The linker will then combine duplicate instantiations.

In the mean time, you have the following options for dealing with template instantiations:

 1. Compile your template-using code with -frepo.  The compiler will generate files with the extension .rpo listing all of the template instantiations used in the corresponding object files which could be instantiated there; the link wrapper, collect2, will then update the .rpo files to tell the compiler where to place those instantiations and rebuild any affected object files.  The link-time overhead is negligible after the first pass, as the compiler will continue to place the instantiations in the same files.

     This is your best option for application code written for the Borland model, as it will just work.  Code written for the Cfront model will need to be modified so that the template definitions are available at one or more points of instantiation; usually this is as simple as adding #include <tmethods.cc> to the end of each template header.

     For library code, if you want the library to provide all of the template instantiations it needs, just try to link all of its object files together; the link will fail, but cause the instantiations to be generated as a side effect.  Be warned, however, that this may cause conflicts if multiple libraries try to provide the same instantiations. For greater control, use explicit instantiation as described in the next option.

 2. Compile your code with -fno-implicit-templates to disable the implicit generation of template instances, and explicitly instantiate all the ones you use.  This approach requires more knowledge of exactly which instances you need than do the others, but it's less mysterious and allows greater control.  You can scatter the explicit instantiations throughout your program, perhaps putting them in the translation units where the instances are used or the translation units that define the templates themselves; you can put all of the explicit instantiations you need into one big file; or you can create small files like

          #include "Foo.h"
          #include "Foo.cc"

          template class Foo<int>;
          template ostream& operator <<
                          (ostream&, const Foo<int>&);

     for each of the instances you need, and create a template instantiation library from those.

     If you are using Cfront-model code, you can probably get away with not using -fno-implicit-templates when compiling files that don't #include the member template definitions.

     If you use one big file to do the instantiations, you may want to compile it without -fno-implicit-templates so you get all of the instances required by your explicit instantiations (but not by any other files) without having to specify them as well.

     g++ has extended the template instantiation syntax outlined in the Working Paper to allow forward declaration of explicit instantiations (with extern), instantiation of the compiler support data for a template class (i.e. the vtable) without instantiating any of its members (with inline), and instantiation of only the static data members of a template class, without the support data or member functions (with (static):

          extern template int max (int, int);
          inline template class Foo<int>;
          static template class Foo<int>;

 3. Do nothing.  Pretend g++ does implement automatic instantiation management.  Code written for the Borland model will work fine, but each translation unit will contain instances of each of the templates it uses.  In a large program, this can lead to an unacceptable amount of code duplication.

 4. Add #pragma interface to all files containing template definitions.  For each of these files, add #pragma implementation "FILENAME" to the top of some .C file which #includes it.  Then compile everything with -fexternal-templates.  The templates will then only be expanded in the translation unit which implements them (i.e. has a #pragma implementation line for the file where they live); all other files will use external references.  If you're lucky, everything should work properly.  If you get undefined symbol errors, you need to make sure that each template instance which is used in the program is used in the file which implements that template.  If you don't have any use for a particular instance in that file, you can just instantiate it explicitly, using the syntax from the latest C++ working paper:

          template class A<int>;
          template ostream& operator << (ostream&, const A<int>&);

     This strategy will work with code written for either model.  If you are using code written for the Cfront model, the file containing a class template and the file containing its member templates should be implemented in the same translation unit.

 5. A slight variation on this approach is to use the flag -falt-external-templates instead.  This flag causes template instances to be emitted in the translation unit that implements the header where they are first instantiated, rather than the one which implements the file where the templates are defined.  This header must be the same in all translation units, or things are likely to break.

     See Declarations and Definitions in One Header, for more discussion of these pragmas.

Extracting the function pointer from a bound pointer to member function
=======================================================================

In C++, pointer to member functions (PMFs) are implemented using a wide pointer of sorts to handle all the possible call mechanisms; the PMF needs to store information about how to adjust the this pointer, and if the function pointed to is virtual, where to find the vtable, and where in the vtable to look for the member function.  If you are using PMFs in an inner loop, you should really reconsider that decision.  If that is not an option, you can extract the pointer to the function that would be called for a given object/PMF pair and call it directly inside the inner loop, to save a bit of time.

Note that you will still be paying the penalty for the call through a function pointer; on most modern architectures, such a call defeats the branch prediction features of the CPU.  This is also true of normal virtual function calls.

The syntax for this extension is

     extern A a;
     extern int (A::*fp)();
     typedef int (*fptr)(A *);

     fptr p = (fptr)(a.*fp);

For PMF constants (i.e. expressions of the form &Klasse::Member), no object is needed to obtain the address of the function.  They can be converted to function pointers directly:

     fptr p1 = (fptr)(&A::foo);

You must specify -Wno-pmf-conversions to use this extension.

C++-Specific Variable, Function, and Type Attributes
====================================================

Some attributes only make sense for C++ programs.

init_priority (PRIORITY)

     In Standard C++, objects defined at namespace scope are guaranteed to be initialized in an order in strict accordance with that of their definitions in a given translation unit.  No guarantee is made for initializations across translation units.  However, GNU C++ allows users to control the order of initialization of objects defined at namespace scope with the init_priority attribute by specifying a relative PRIORITY, a constant integral expression currently bounded between 101 and 65535 inclusive.  Lower numbers indicate a higher priority.

     In the following example, A would normally be created before B, but the init_priority attribute has reversed that order:

          Some_Class  A  __attribute__ ((init_priority (2000)));
          Some_Class  B  __attribute__ ((init_priority (543)));

     Note that the particular values of PRIORITY do not matter; only their relative ordering.

java_interface

     This type attribute informs C++ that the class is a Java interface.  It may only be applied to classes declared within an extern "Java" block. Calls to methods declared in this interface will be dispatched using GCJ's interface table mechanism, instead of regular virtual table dispatch.

Java Exceptions
===============

The Java language uses a slightly different exception handling model from C++.  Normally, GNU C++ will automatically detect when you are writing C++ code that uses Java exceptions, and handle them appropriately.  However, if C++ code only needs to execute destructors when Java exceptions are thrown through it, GCC will guess incorrectly. Sample problematic code is:

       struct S { ~S(); };
       extern void bar();    // is written in Java, and may throw exceptions
       void foo()
       {
         S s;
         bar();
       }

The usual effect of an incorrect guess is a link failure, complaining of a missing routine called __gxx_personality_v0.

You can inform the compiler that Java exceptions are to be used in a translation unit, irrespective of what it might think, by writing #pragma GCC java_exceptions at the head of the file.  This #pragma must appear before any functions that throw or catch exceptions, or run destructors when exceptions are thrown through them.

You cannot mix Java and C++ exceptions in the same translation unit.  It is believed to be safe to throw a C++ exception from one file through another file compiled for the Java exception model, or vice versa, but there may be bugs in this area.

Deprecated Features
===================

In the past, the GNU C++ compiler was extended to experiment with new features, at a time when the C++ language was still evolving.  Now that the C++ standard is complete, some of those features are superseded by superior alternatives.  Using the old features might cause a warning in some cases that the feature will be dropped in the future.  In other cases, the feature might be gone already.

While the list below is not exhaustive, it documents some of the options that are now deprecated:

-fexternal-templates
-falt-external-templates
     These are two of the many ways for g++ to implement template instantiation.  See Template Instantiation.  The C++ standard clearly defines how template definitions have to be organized across implementation units.  g++ has an implicit instantiation mechanism that should work just fine for standard-conforming code.

-fstrict-prototype
-fno-strict-prototype
     Previously it was possible to use an empty prototype parameter list to indicate an unspecified number of parameters (like C), rather than no parameters, as C++ demands.  This feature has been removed, except where it is required for backwards compatibility see Backwards Compatibility.

The named return value extension has been deprecated, and is now removed from g++.

The use of initializer lists with new expressions has been deprecated, and is now removed from g++.

Floating and complex non-type template parameters have been deprecated, and are now removed from g++.

The implicit typename extension has been deprecated and will be removed from g++ at some point.  In some cases g++ determines that a dependant type such as TPL<T>::X is a type without needing a typename keyword, contrary to the standard.

Backwards Compatibility
=======================

Now that there is a definitive ISO standard C++, G++ has a specification to adhere to.  The C++ language evolved over time, and features that used to be acceptable in previous drafts of the standard, such as the ARM [Annotated C++ Reference Manual], are no longer accepted.  In order to allow compilation of C++ written to such drafts, G++ contains some backwards compatibilities.  All such backwards compatibility features are liable to disappear in future versions of G++. They should be considered deprecated see Deprecated Features.

For scope
     If a variable is declared at for scope, it used to remain in scope until the end of the scope which contained the for statement (rather than just within the for scope).  G++ retains this, but issues a warning, if such a variable is accessed outside the for scope.

Implicit C language
     Old C system header files did not contain an extern "C" {...} scope to set the language.  On such systems, all header files are implicitly scoped inside a C language scope.  Also, an empty prototype () will be treated as an unspecified number of arguments, rather than no arguments, as C++ demands.