diff options
Diffstat (limited to 'gcc/gcc.info-8')
| -rw-r--r-- | gcc/gcc.info-8 | 1202 |
1 files changed, 0 insertions, 1202 deletions
diff --git a/gcc/gcc.info-8 b/gcc/gcc.info-8 deleted file mode 100644 index 3f49097..0000000 --- a/gcc/gcc.info-8 +++ /dev/null @@ -1,1202 +0,0 @@ -This is Info file gcc.info, produced by Makeinfo version 1.68 from the -input file gcc.texi. - - This file documents the use and the internals of the GNU compiler. - - Published by the Free Software Foundation 59 Temple Place - Suite 330 -Boston, MA 02111-1307 USA - - Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997 Free -Software Foundation, Inc. - - Permission is granted to make and distribute verbatim copies of this -manual provided the copyright notice and this permission notice are -preserved on all copies. - - Permission is granted to copy and distribute modified versions of -this manual under the conditions for verbatim copying, provided also -that the sections entitled "GNU General Public License," "Funding for -Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are -included exactly as in the original, and provided that the entire -resulting derived work is distributed under the terms of a permission -notice identical to this one. - - Permission is granted to copy and distribute translations of this -manual into another language, under the above conditions for modified -versions, except that the sections entitled "GNU General Public -License," "Funding for Free Software," and "Protect Your Freedom--Fight -`Look And Feel'", and this permission notice, may be included in -translations approved by the Free Software Foundation instead of in the -original English. - - -File: gcc.info, Node: Cross Runtime, Next: Build Cross, Prev: Cross Headers, Up: Cross-Compiler - -`libgcc.a' and Cross-Compilers ------------------------------- - - Code compiled by GNU CC uses certain runtime support functions -implicitly. Some of these functions can be compiled successfully with -GNU CC itself, but a few cannot be. These problem functions are in the -source file `libgcc1.c'; the library made from them is called -`libgcc1.a'. - - When you build a native compiler, these functions are compiled with -some other compiler-the one that you use for bootstrapping GNU CC. -Presumably it knows how to open code these operations, or else knows how -to call the run-time emulation facilities that the machine comes with. -But this approach doesn't work for building a cross-compiler. The -compiler that you use for building knows about the host system, not the -target system. - - So, when you build a cross-compiler you have to supply a suitable -library `libgcc1.a' that does the job it is expected to do. - - To compile `libgcc1.c' with the cross-compiler itself does not work. -The functions in this file are supposed to implement arithmetic -operations that GNU CC does not know how to open code for your target -machine. If these functions are compiled with GNU CC itself, they will -compile into infinite recursion. - - On any given target, most of these functions are not needed. If GNU -CC can open code an arithmetic operation, it will not call these -functions to perform the operation. It is possible that on your target -machine, none of these functions is needed. If so, you can supply an -empty library as `libgcc1.a'. - - Many targets need library support only for multiplication and -division. If you are linking with a library that contains functions for -multiplication and division, you can tell GNU CC to call them directly -by defining the macros `MULSI3_LIBCALL', and the like. These macros -need to be defined in the target description macro file. For some -targets, they are defined already. This may be sufficient to avoid the -need for libgcc1.a; if so, you can supply an empty library. - - Some targets do not have floating point instructions; they need other -functions in `libgcc1.a', which do floating arithmetic. Recent -versions of GNU CC have a file which emulates floating point. With a -certain amount of work, you should be able to construct a floating -point emulator that can be used as `libgcc1.a'. Perhaps future -versions will contain code to do this automatically and conveniently. -That depends on whether someone wants to implement it. - - Some embedded targets come with all the necessary `libgcc1.a' -routines written in C or assembler. These targets build `libgcc1.a' -automatically and you do not need to do anything special for them. -Other embedded targets do not need any `libgcc1.a' routines since all -the necessary operations are supported by the hardware. - - If your target system has another C compiler, you can configure GNU -CC as a native compiler on that machine, build just `libgcc1.a' with -`make libgcc1.a' on that machine, and use the resulting file with the -cross-compiler. To do this, execute the following on the target -machine: - - cd TARGET-BUILD-DIR - ./configure --host=sparc --target=sun3 - make libgcc1.a - -And then this on the host machine: - - ftp TARGET-MACHINE - binary - cd TARGET-BUILD-DIR - get libgcc1.a - quit - - Another way to provide the functions you need in `libgcc1.a' is to -define the appropriate `perform_...' macros for those functions. If -these definitions do not use the C arithmetic operators that they are -meant to implement, you should be able to compile them with the -cross-compiler you are building. (If these definitions already exist -for your target file, then you are all set.) - - To build `libgcc1.a' using the perform macros, use -`LIBGCC1=libgcc1.a OLDCC=./xgcc' when building the compiler. -Otherwise, you should place your replacement library under the name -`libgcc1.a' in the directory in which you will build the -cross-compiler, before you run `make'. - - -File: gcc.info, Node: Cross Headers, Next: Cross Runtime, Prev: Tools and Libraries, Up: Cross-Compiler - -Cross-Compilers and Header Files --------------------------------- - - If you are cross-compiling a standalone program or a program for an -embedded system, then you may not need any header files except the few -that are part of GNU CC (and those of your program). However, if you -intend to link your program with a standard C library such as `libc.a', -then you probably need to compile with the header files that go with -the library you use. - - The GNU C compiler does not come with these files, because (1) they -are system-specific, and (2) they belong in a C library, not in a -compiler. - - If the GNU C library supports your target machine, then you can get -the header files from there (assuming you actually use the GNU library -when you link your program). - - If your target machine comes with a C compiler, it probably comes -with suitable header files also. If you make these files accessible -from the host machine, the cross-compiler can use them also. - - Otherwise, you're on your own in finding header files to use when -cross-compiling. - - When you have found suitable header files, put them in the directory -`/usr/local/TARGET/include', before building the cross compiler. Then -installation will run fixincludes properly and install the corrected -versions of the header files where the compiler will use them. - - Provide the header files before you build the cross-compiler, because -the build stage actually runs the cross-compiler to produce parts of -`libgcc.a'. (These are the parts that *can* be compiled with GNU CC.) -Some of them need suitable header files. - - Here's an example showing how to copy the header files from a target -machine. On the target machine, do this: - - (cd /usr/include; tar cf - .) > tarfile - - Then, on the host machine, do this: - - ftp TARGET-MACHINE - lcd /usr/local/TARGET/include - get tarfile - quit - tar xf tarfile - - -File: gcc.info, Node: Build Cross, Prev: Cross Runtime, Up: Cross-Compiler - -Actually Building the Cross-Compiler ------------------------------------- - - Now you can proceed just as for compiling a single-machine compiler -through the step of building stage 1. If you have not provided some -sort of `libgcc1.a', then compilation will give up at the point where -it needs that file, printing a suitable error message. If you do -provide `libgcc1.a', then building the compiler will automatically -compile and link a test program called `libgcc1-test'; if you get -errors in the linking, it means that not all of the necessary routines -in `libgcc1.a' are available. - - You must provide the header file `float.h'. One way to do this is -to compile `enquire' and run it on your target machine. The job of -`enquire' is to run on the target machine and figure out by experiment -the nature of its floating point representation. `enquire' records its -findings in the header file `float.h'. If you can't produce this file -by running `enquire' on the target machine, then you will need to come -up with a suitable `float.h' in some other way (or else, avoid using it -in your programs). - - Do not try to build stage 2 for a cross-compiler. It doesn't work to -rebuild GNU CC as a cross-compiler using the cross-compiler, because -that would produce a program that runs on the target machine, not on the -host. For example, if you compile a 386-to-68030 cross-compiler with -itself, the result will not be right either for the 386 (because it was -compiled into 68030 code) or for the 68030 (because it was configured -for a 386 as the host). If you want to compile GNU CC into 68030 code, -whether you compile it on a 68030 or with a cross-compiler on a 386, you -must specify a 68030 as the host when you configure it. - - To install the cross-compiler, use `make install', as usual. - - -File: gcc.info, Node: Sun Install, Next: VMS Install, Prev: Cross-Compiler, Up: Installation - -Installing GNU CC on the Sun -============================ - - On Solaris (version 2.1), do not use the linker or other tools in -`/usr/ucb' to build GNU CC. Use `/usr/ccs/bin'. - - Make sure the environment variable `FLOAT_OPTION' is not set when -you compile `libgcc.a'. If this option were set to `f68881' when -`libgcc.a' is compiled, the resulting code would demand to be linked -with a special startup file and would not link properly without special -pains. - - There is a bug in `alloca' in certain versions of the Sun library. -To avoid this bug, install the binaries of GNU CC that were compiled by -GNU CC. They use `alloca' as a built-in function and never the one in -the library. - - Some versions of the Sun compiler crash when compiling GNU CC. The -problem is a segmentation fault in cpp. This problem seems to be due to -the bulk of data in the environment variables. You may be able to avoid -it by using the following command to compile GNU CC with Sun CC: - - make CC="TERMCAP=x OBJS=x LIBFUNCS=x STAGESTUFF=x cc" - - SunOS 4.1.3 and 4.1.3_U1 have bugs that can cause intermittent core -dumps when compiling GNU CC. A common symptom is an internal compiler -error which does not recur if you run it again. To fix the problem, -install Sun recommended patch 100726 (for SunOS 4.1.3) or 101508 (for -SunOS 4.1.3_U1), or upgrade to a later SunOS release. - - -File: gcc.info, Node: VMS Install, Next: Collect2, Prev: Sun Install, Up: Installation - -Installing GNU CC on VMS -======================== - - The VMS version of GNU CC is distributed in a backup saveset -containing both source code and precompiled binaries. - - To install the `gcc' command so you can use the compiler easily, in -the same manner as you use the VMS C compiler, you must install the VMS -CLD file for GNU CC as follows: - - 1. Define the VMS logical names `GNU_CC' and `GNU_CC_INCLUDE' to - point to the directories where the GNU CC executables - (`gcc-cpp.exe', `gcc-cc1.exe', etc.) and the C include files are - kept respectively. This should be done with the commands: - - $ assign /system /translation=concealed - - disk:[gcc.] gnu_cc - $ assign /system /translation=concealed - - disk:[gcc.include.] gnu_cc_include - - with the appropriate disk and directory names. These commands can - be placed in your system startup file so they will be executed - whenever the machine is rebooted. You may, if you choose, do this - via the `GCC_INSTALL.COM' script in the `[GCC]' directory. - - 2. Install the `GCC' command with the command line: - - $ set command /table=sys$common:[syslib]dcltables - - /output=sys$common:[syslib]dcltables gnu_cc:[000000]gcc - $ install replace sys$common:[syslib]dcltables - - 3. To install the help file, do the following: - - $ library/help sys$library:helplib.hlb gcc.hlp - - Now you can invoke the compiler with a command like `gcc /verbose - file.c', which is equivalent to the command `gcc -v -c file.c' in - Unix. - - If you wish to use GNU C++ you must first install GNU CC, and then -perform the following steps: - - 1. Define the VMS logical name `GNU_GXX_INCLUDE' to point to the - directory where the preprocessor will search for the C++ header - files. This can be done with the command: - - $ assign /system /translation=concealed - - disk:[gcc.gxx_include.] gnu_gxx_include - - with the appropriate disk and directory name. If you are going to - be using libg++, this is where the libg++ install procedure will - install the libg++ header files. - - 2. Obtain the file `gcc-cc1plus.exe', and place this in the same - directory that `gcc-cc1.exe' is kept. - - The GNU C++ compiler can be invoked with a command like `gcc /plus - /verbose file.cc', which is equivalent to the command `g++ -v -c - file.cc' in Unix. - - We try to put corresponding binaries and sources on the VMS -distribution tape. But sometimes the binaries will be from an older -version than the sources, because we don't always have time to update -them. (Use the `/version' option to determine the version number of -the binaries and compare it with the source file `version.c' to tell -whether this is so.) In this case, you should use the binaries you get -to recompile the sources. If you must recompile, here is how: - - 1. Execute the command procedure `vmsconfig.com' to set up the files - `tm.h', `config.h', `aux-output.c', and `md.', and to create files - `tconfig.h' and `hconfig.h'. This procedure also creates several - linker option files used by `make-cc1.com' and a data file used by - `make-l2.com'. - - $ @vmsconfig.com - - 2. Setup the logical names and command tables as defined above. In - addition, define the VMS logical name `GNU_BISON' to point at the - to the directories where the Bison executable is kept. This - should be done with the command: - - $ assign /system /translation=concealed - - disk:[bison.] gnu_bison - - You may, if you choose, use the `INSTALL_BISON.COM' script in the - `[BISON]' directory. - - 3. Install the `BISON' command with the command line: - - $ set command /table=sys$common:[syslib]dcltables - - /output=sys$common:[syslib]dcltables - - gnu_bison:[000000]bison - $ install replace sys$common:[syslib]dcltables - - 4. Type `@make-gcc' to recompile everything (alternatively, submit - the file `make-gcc.com' to a batch queue). If you wish to build - the GNU C++ compiler as well as the GNU CC compiler, you must - first edit `make-gcc.com' and follow the instructions that appear - in the comments. - - 5. In order to use GCC, you need a library of functions which GCC - compiled code will call to perform certain tasks, and these - functions are defined in the file `libgcc2.c'. To compile this - you should use the command procedure `make-l2.com', which will - generate the library `libgcc2.olb'. `libgcc2.olb' should be built - using the compiler built from the same distribution that - `libgcc2.c' came from, and `make-gcc.com' will automatically do - all of this for you. - - To install the library, use the following commands: - - $ library gnu_cc:[000000]gcclib/delete=(new,eprintf) - $ library gnu_cc:[000000]gcclib/delete=L_* - $ library libgcc2/extract=*/output=libgcc2.obj - $ library gnu_cc:[000000]gcclib libgcc2.obj - - The first command simply removes old modules that will be replaced - with modules from `libgcc2' under different module names. The - modules `new' and `eprintf' may not actually be present in your - `gcclib.olb'--if the VMS librarian complains about those modules - not being present, simply ignore the message and continue on with - the next command. The second command removes the modules that - came from the previous version of the library `libgcc2.c'. - - Whenever you update the compiler on your system, you should also - update the library with the above procedure. - - 6. You may wish to build GCC in such a way that no files are written - to the directory where the source files reside. An example would - be the when the source files are on a read-only disk. In these - cases, execute the following DCL commands (substituting your - actual path names): - - $ assign dua0:[gcc.build_dir.]/translation=concealed, - - dua1:[gcc.source_dir.]/translation=concealed gcc_build - $ set default gcc_build:[000000] - - where the directory `dua1:[gcc.source_dir]' contains the source - code, and the directory `dua0:[gcc.build_dir]' is meant to contain - all of the generated object files and executables. Once you have - done this, you can proceed building GCC as described above. (Keep - in mind that `gcc_build' is a rooted logical name, and thus the - device names in each element of the search list must be an actual - physical device name rather than another rooted logical name). - - 7. *If you are building GNU CC with a previous version of GNU CC, you - also should check to see that you have the newest version of the - assembler*. In particular, GNU CC version 2 treats global constant - variables slightly differently from GNU CC version 1, and GAS - version 1.38.1 does not have the patches required to work with GCC - version 2. If you use GAS 1.38.1, then `extern const' variables - will not have the read-only bit set, and the linker will generate - warning messages about mismatched psect attributes for these - variables. These warning messages are merely a nuisance, and can - safely be ignored. - - If you are compiling with a version of GNU CC older than 1.33, - specify `/DEFINE=("inline=")' as an option in all the - compilations. This requires editing all the `gcc' commands in - `make-cc1.com'. (The older versions had problems supporting - `inline'.) Once you have a working 1.33 or newer GNU CC, you can - change this file back. - - 8. If you want to build GNU CC with the VAX C compiler, you will need - to make minor changes in `make-cccp.com' and `make-cc1.com' to - choose alternate definitions of `CC', `CFLAGS', and `LIBS'. See - comments in those files. However, you must also have a working - version of the GNU assembler (GNU as, aka GAS) as it is used as - the back-end for GNU CC to produce binary object modules and is - not included in the GNU CC sources. GAS is also needed to compile - `libgcc2' in order to build `gcclib' (see above); `make-l2.com' - expects to be able to find it operational in - `gnu_cc:[000000]gnu-as.exe'. - - To use GNU CC on VMS, you need the VMS driver programs `gcc.exe', - `gcc.com', and `gcc.cld'. They are distributed with the VMS - binaries (`gcc-vms') rather than the GNU CC sources. GAS is also - included in `gcc-vms', as is Bison. - - Once you have successfully built GNU CC with VAX C, you should use - the resulting compiler to rebuild itself. Before doing this, be - sure to restore the `CC', `CFLAGS', and `LIBS' definitions in - `make-cccp.com' and `make-cc1.com'. The second generation - compiler will be able to take advantage of many optimizations that - must be suppressed when building with other compilers. - - Under previous versions of GNU CC, the generated code would -occasionally give strange results when linked with the sharable -`VAXCRTL' library. Now this should work. - - Even with this version, however, GNU CC itself should not be linked -with the sharable `VAXCRTL'. The version of `qsort' in `VAXCRTL' has a -bug (known to be present in VMS versions V4.6 through V5.5) which -causes the compiler to fail. - - The executables are generated by `make-cc1.com' and `make-cccp.com' -use the object library version of `VAXCRTL' in order to make use of the -`qsort' routine in `gcclib.olb'. If you wish to link the compiler -executables with the shareable image version of `VAXCRTL', you should -edit the file `tm.h' (created by `vmsconfig.com') to define the macro -`QSORT_WORKAROUND'. - - `QSORT_WORKAROUND' is always defined when GNU CC is compiled with -VAX C, to avoid a problem in case `gcclib.olb' is not yet available. - - -File: gcc.info, Node: Collect2, Next: Header Dirs, Prev: VMS Install, Up: Installation - -`collect2' -========== - - Many target systems do not have support in the assembler and linker -for "constructors"--initialization functions to be called before the -official "start" of `main'. On such systems, GNU CC uses a utility -called `collect2' to arrange to call these functions at start time. - - The program `collect2' works by linking the program once and looking -through the linker output file for symbols with particular names -indicating they are constructor functions. If it finds any, it creates -a new temporary `.c' file containing a table of them, compiles it, and -links the program a second time including that file. - - The actual calls to the constructors are carried out by a subroutine -called `__main', which is called (automatically) at the beginning of -the body of `main' (provided `main' was compiled with GNU CC). Calling -`__main' is necessary, even when compiling C code, to allow linking C -and C++ object code together. (If you use `-nostdlib', you get an -unresolved reference to `__main', since it's defined in the standard -GCC library. Include `-lgcc' at the end of your compiler command line -to resolve this reference.) - - The program `collect2' is installed as `ld' in the directory where -the passes of the compiler are installed. When `collect2' needs to -find the *real* `ld', it tries the following file names: - - * `real-ld' in the directories listed in the compiler's search - directories. - - * `real-ld' in the directories listed in the environment variable - `PATH'. - - * The file specified in the `REAL_LD_FILE_NAME' configuration macro, - if specified. - - * `ld' in the compiler's search directories, except that `collect2' - will not execute itself recursively. - - * `ld' in `PATH'. - - "The compiler's search directories" means all the directories where -`gcc' searches for passes of the compiler. This includes directories -that you specify with `-B'. - - Cross-compilers search a little differently: - - * `real-ld' in the compiler's search directories. - - * `TARGET-real-ld' in `PATH'. - - * The file specified in the `REAL_LD_FILE_NAME' configuration macro, - if specified. - - * `ld' in the compiler's search directories. - - * `TARGET-ld' in `PATH'. - - `collect2' explicitly avoids running `ld' using the file name under -which `collect2' itself was invoked. In fact, it remembers up a list -of such names--in case one copy of `collect2' finds another copy (or -version) of `collect2' installed as `ld' in a second place in the -search path. - - `collect2' searches for the utilities `nm' and `strip' using the -same algorithm as above for `ld'. - - -File: gcc.info, Node: Header Dirs, Prev: Collect2, Up: Installation - -Standard Header File Directories -================================ - - `GCC_INCLUDE_DIR' means the same thing for native and cross. It is -where GNU CC stores its private include files, and also where GNU CC -stores the fixed include files. A cross compiled GNU CC runs -`fixincludes' on the header files in `$(tooldir)/include'. (If the -cross compilation header files need to be fixed, they must be installed -before GNU CC is built. If the cross compilation header files are -already suitable for ANSI C and GNU CC, nothing special need be done). - - `GPLUS_INCLUDE_DIR' means the same thing for native and cross. It -is where `g++' looks first for header files. `libg++' installs only -target independent header files in that directory. - - `LOCAL_INCLUDE_DIR' is used only for a native compiler. It is -normally `/usr/local/include'. GNU CC searches this directory so that -users can install header files in `/usr/local/include'. - - `CROSS_INCLUDE_DIR' is used only for a cross compiler. GNU CC -doesn't install anything there. - - `TOOL_INCLUDE_DIR' is used for both native and cross compilers. It -is the place for other packages to install header files that GNU CC will -use. For a cross-compiler, this is the equivalent of `/usr/include'. -When you build a cross-compiler, `fixincludes' processes any header -files in this directory. - - -File: gcc.info, Node: C Extensions, Next: C++ Extensions, Prev: Installation, Up: Top - -Extensions to the C Language Family -*********************************** - - GNU C provides several language features not found in ANSI standard -C. (The `-pedantic' option directs GNU CC 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 GNU CC. - - These extensions are available in C and Objective C. Most of them -are also available in C++. *Note Extensions to the C++ Language: C++ -Extensions, for extensions that apply *only* to C++. - -* Menu: - -* Statement Exprs:: Putting statements and declarations inside expressions. -* Local Labels:: Labels local to a statement-expression. -* Labels as Values:: Getting pointers to labels, and computed gotos. -* Nested Functions:: As in Algol and Pascal, lexical scoping of functions. -* Constructing Calls:: Dispatching a call to another function. -* Naming Types:: Giving a name to the type of some expression. -* Typeof:: `typeof': referring to the type of an expression. -* Lvalues:: Using `?:', `,' and casts in lvalues. -* Conditionals:: Omitting the middle operand of a `?:' expression. -* Long Long:: Double-word integers--`long long int'. -* Complex:: Data types for complex numbers. -* Zero Length:: Zero-length arrays. -* Variable Length:: Arrays whose length is computed at run time. -* Macro Varargs:: Macros with variable number of arguments. -* Subscripting:: Any array can be subscripted, even if not an lvalue. -* Pointer Arith:: Arithmetic on `void'-pointers and function pointers. -* Initializers:: Non-constant initializers. -* Constructors:: Constructor expressions give structures, unions - or arrays as values. -* Labeled Elements:: Labeling elements of initializers. -* Cast to Union:: Casting to union type from any member of the union. -* Case Ranges:: `case 1 ... 9' and such. -* Function Attributes:: Declaring that functions have no side effects, - or that they can never return. -* Function Prototypes:: Prototype declarations and old-style definitions. -* C++ Comments:: C++ comments are recognized. -* Dollar Signs:: Dollar sign is allowed in identifiers. -* Character Escapes:: `\e' stands for the character <ESC>. -* Variable Attributes:: Specifying attributes of variables. -* Type Attributes:: Specifying attributes of types. -* Alignment:: Inquiring about the alignment of a type or variable. -* Inline:: Defining inline functions (as fast as macros). -* Extended Asm:: Assembler instructions with C expressions as operands. - (With them you can define "built-in" functions.) -* Asm Labels:: Specifying the assembler name to use for a C symbol. -* Explicit Reg Vars:: Defining variables residing in specified registers. -* Alternate Keywords:: `__const__', `__asm__', etc., for header files. -* Incomplete Enums:: `enum foo;', with details to follow. -* Function Names:: Printable strings which are the name of the current - function. -* Return Address:: Getting the return or frame address of a function. - - -File: gcc.info, Node: Statement Exprs, Next: Local Labels, Up: C Extensions - -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' (*note Typeof::.) or type naming (*note -Naming Types::.). - - -File: gcc.info, Node: Local Labels, Next: Labels as Values, Prev: Statement Exprs, Up: C Extensions - -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; \ - }) - - -File: gcc.info, Node: Labels as Values, Next: Nested Functions, Prev: Local Labels, Up: C Extensions - -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(1), `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 can 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. - - ---------- Footnotes ---------- - - (1) 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. - - -File: gcc.info, Node: Nested Functions, Next: Constructing Calls, Prev: Labels as Values, Up: C Extensions - -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. - - GNU CC implements taking the address of a nested function using a -technique called "trampolines". A paper describing them is available -from `maya.idiap.ch' in directory `pub/tmb', file `usenix88-lexic.ps.Z'. - - A nested function can jump to a label inherited from a containing -function, provided the label was explicitly declared in the containing -function (*note 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]; - } - ... - } - - -File: gcc.info, Node: Constructing Calls, Next: Naming Types, Prev: Nested Functions, Up: C Extensions - -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). - -`__builtin_apply_args ()' - This built-in function returns a pointer of type `void *' 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. - -`__builtin_apply (FUNCTION, ARGUMENTS, SIZE)' - This built-in function invokes FUNCTION (type `void (*)()') with a - copy of the parameters described by ARGUMENTS (type `void *') and - SIZE (type `int'). - - 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 of type `void *' 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. - -`__builtin_return (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'. - - -File: gcc.info, Node: Naming Types, Next: Typeof, Prev: Constructing Calls, Up: C Extensions - -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. - - -File: gcc.info, Node: Typeof, Next: Lvalues, Prev: Naming Types, Up: C Extensions - -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 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 -ANSI C programs, write `__typeof__' instead of `typeof'. *Note -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'. - - -File: gcc.info, Node: Lvalues, Next: Conditionals, Prev: Typeof, Up: C Extensions - -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'. - - -File: gcc.info, Node: Conditionals, Next: Long Long, Prev: Lvalues, Up: C Extensions - -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. - - -File: gcc.info, Node: Long Long, Next: Complex, Prev: Conditionals, Up: C Extensions - -Double-Word Integers -==================== - - GNU C supports data types for integers that are twice as long as -`int'. 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 GNU CC. - - 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. - - -File: gcc.info, Node: Complex, Next: Zero Length, Prev: Long Long, Up: C Extensions - -Complex Numbers -=============== - - GNU C supports complex data types. You can declare both complex -integer types and complex floating types, using the keyword -`__complex__'. - - 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. - - To extract the real part of a complex-valued expression EXP, write -`__real__ EXP'. Likewise, use `__imag__' to extract the imaginary part. - - The operator `~' performs complex conjugation when used on a value -with a complex type. - - GNU CC 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 GNU CC 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. - - -File: gcc.info, Node: Zero Length, Next: Variable Length, Prev: Complex, Up: C Extensions - -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 standard C, you would have to give `contents' a length of 1, which -means either you waste space or complicate the argument to `malloc'. - |