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+@node Variable Argument Facilities, Memory Allocation, Common Definitions, Top
+@chapter Variable Argument Facilities
+@cindex variadic argument functions
+@cindex variadic functions
+@cindex variable number of arguments
+@cindex optional arguments
+
+ANSI C defines a syntax as part of the kernel language for specifying
+functions that take a variable number or type of arguments.  (Such
+functions are also referred to as @dfn{variadic functions}.)  However,
+the kernel language provides no mechanism for actually accessing
+non-required arguments; instead, you use the variable arguments macros
+defined in @file{stdarg.h}.
+@pindex stdarg.h
+
+@menu
+* Why Variable Arguments are Used::     Using variable arguments can
+                                         save you time and effort.
+* How Variable Arguments are Used::     An overview of the facilities for
+				         receiving variable arguments.
+* Variable Arguments Interface::        Detailed specification of the
+					 library facilities.
+* Example of Variable Arguments::       A complete example.
+@end menu
+
+@node Why Variable Arguments are Used, How Variable Arguments are Used,  , Variable Argument Facilities
+@section Why Variable Arguments are Used
+
+Most C functions take a fixed number of arguments.  When you define a
+function, you also supply a specific data type for each argument.
+Every call to the function should supply the same number and type of
+arguments as specified in the function definition.
+
+On the other hand, sometimes a function performs an operation that can
+meaningfully accept an unlimited number of arguments.  
+
+For example, consider a function that joins its arguments into a linked
+list.  It makes sense to connect any number of arguments together into a
+list of arbitrary length.  Without facilities for variable arguments,
+you would have to define a separate function for each possible number of
+arguments you might want to link together.  This is an example of a
+situation where some kind of mapping or iteration is performed over an
+arbitrary number of arguments of the same type.
+
+Another kind of application where variable arguments can be useful is
+for functions where values for some arguments can simply be omitted in
+some calls, either because they are not used at all or because the
+function can determine appropriate defaults for them if they're missing.
+
+The library function @code{printf} (@pxref{Formatted Output}) is an
+example of still another class of function where variable arguments are
+useful.  This function prints its arguments (which can vary in type as
+well as number) under the control of a format template string.
+
+@node How Variable Arguments are Used, Variable Arguments Interface, Why Variable Arguments are Used, Variable Argument Facilities
+@section How Variable Arguments are Used
+
+This section describes how you can define and call functions that take
+variable arguments, and how to access the values of the non-required
+arguments.
+
+@menu
+* Syntax for Variable Arguments::       How to make a prototype for a
+                                         function with variable arguments.
+* Receiving the Argument Values::       Steps you must follow to access the
+					 optional argument values.
+* How Many Arguments::                  How to decide whether there are more
+					 arguments.
+* Calling Variadic Functions::          Things you need to know about calling
+					 variable arguments functions.
+@end menu
+
+@node Syntax for Variable Arguments, Receiving the Argument Values,  , How Variable Arguments are Used
+@subsection Syntax for Variable Arguments
+
+A function that accepts a variable number of arguments must have at
+least one required argument with a specified type.  In the function
+definition or prototype declaration, you indicate the fact that a
+function can accept additional arguments of unspecified type by putting
+@samp{@dots{}} at the end of the arguments.  For example,
+
+@example
+int 
+func (const char *a, int b, @dots{})
+@{
+  @dots{}
+@}	
+@end example
+
+@noindent
+outlines a definition of a function @code{func} which returns an
+@code{int} and takes at least two arguments, the first two being a
+@code{const char *} and an @code{int}.@refill
+
+An obscure restriction placed by the ANSI C standard is that the last
+required argument must not be declared @code{register} in the function
+definition.  Furthermore, this argument must not be of a function or
+array type, and may not be, for example, a @code{char} or @code{short
+int} (whether signed or not) or a @code{float}.
+
+@strong{Compatibility Note:} Many older C dialects provide a similar,
+but incompatible, mechanism for defining functions with variable numbers
+of arguments.  In particular, the @samp{@dots{}} syntax is a new feature
+of ANSI C.
+
+
+@node Receiving the Argument Values, How Many Arguments, Syntax for Variable Arguments, How Variable Arguments are Used
+@subsection Receiving the Argument Values
+
+Inside the definition of a variadic function, to access the optional
+arguments with the following three step process:
+
+@enumerate
+@item
+You initialize an argument pointer variable of type @code{va_list} using
+@code{va_start}.
+
+@item
+You access the optional arguments by successive calls to @code{va_arg}.
+
+@item
+You call @code{va_end} to indicate that you are finished accessing the
+arguments.
+@end enumerate
+
+Steps 1 and 3 must be performed in the function that is defined to
+accept variable arguments.  However, you can pass the @code{va_list}
+variable as an argument to another function and perform all or part of
+step 2 there.  After doing this, the value of the @code{va_list}
+variable in the calling function becomes undefined for further calls to
+@code{va_arg}; you should just pass it to @code{va_end}.
+
+You can perform the entire sequence of the three steps multiple times
+within a single function invocation.  And, if the function doesn't want
+to look at its optional arguments at all, it doesn't have to do any of
+these steps.  It is also perfectly all right for a function to access
+fewer arguments than were supplied in the call, but you will get garbage
+values if you try to access too many arguments.
+
+
+@node How Many Arguments, Calling Variadic Functions, Receiving the Argument Values, How Variable Arguments are Used
+@subsection How Many Arguments Were Supplied
+
+There is no general way for a function to determine the number and type
+of the actual values that were passed as optional arguments.  Typically,
+the value of one of the required arguments is used to tell the function
+this information.  It is up to you to define an appropriate calling
+convention for each function, and write all calls accordingly.
+
+One calling convention is to make one of the required arguments be an
+explicit argument count.  This convention is usable if all of the
+optional arguments are of the same type.
+
+A required argument can be used as a pattern to specify both the number
+and types of the optional arguments.  The format template string
+argument to @code{printf} is one example of this.
+
+A similar technique that is sometimes used is to have one of the
+required arguments be a bit mask, with a bit for each possible optional
+argument that might be supplied.  The bits are tested in a predefined
+sequence; if the bit is set, the value of the next argument is
+retrieved, and otherwise a default value is used.
+
+Another technique that is sometimes used is to pass an ``end marker''
+value as the last optional argument.  For example, for a function that
+manipulates an arbitrary number of pointer arguments, a null pointer
+might indicate the end of the argument list, provided that a null
+pointer isn't otherwise meaningful to the function.
+
+
+@node Calling Variadic Functions,  , How Many Arguments, How Variable Arguments are Used
+@subsection Calling Variadic Functions
+
+Functions that are @emph{defined} to be variadic must also be
+@emph{declared} to be variadic using a function prototype in the scope
+of all calls to it.  This is because C compilers might use a different
+internal function call protocol for variadic functions than for
+functions that take a fixed number and type of arguments.  If the
+compiler can't determine in advance that the function being called is
+variadic, it may end up trying to call it incorrectly and your program
+won't work.
+@cindex function prototypes
+@cindex prototypes for variadic functions
+@cindex variadic functions need prototypes
+
+Since the prototype doesn't specify types for optional arguments, in a
+call to a variadic function the @dfn{default argument promotions} are
+performed on the optional argument values.  This means the objects of
+type @code{char} or @code{short int} (whether signed or not) are
+promoted to either @code{int} or @code{unsigned int}, as appropriate;
+and that objects of type @code{float} are promoted to type
+@code{double}.  So, if the caller passes a @code{char} as an optional
+argument, it is promoted to a @code{int}, and the function should get it
+with @code{va_arg (@var{ap}, int)}.
+
+Promotions of the required arguments are determined by the function
+prototype in the usual way (as if by assignment to the types of the
+corresponding formal parameters).
+@cindex default argument promotions
+@cindex argument promotion
+
+@node Variable Arguments Interface, Example of Variable Arguments, How Variable Arguments are Used, Variable Argument Facilities
+@section Variable Arguments Interface
+
+Here are descriptions of the macros used to retrieve variable arguments.
+These macros are defined in the header file @file{stdarg.h}.
+@pindex stdarg.h
+
+@comment stdarg.h
+@comment ANSI
+@deftp {Data Type} va_list
+The type @code{va_list} is used for argument pointer variables.
+@end deftp
+
+@comment stdarg.h
+@comment ANSI
+@deftypefn {Macro} void va_start (va_list @var{ap}, @var{last_required})
+This macro initialized the argument pointer variable @var{ap} to point
+to the first of the optional arguments of the current function;
+@var{last_required} must be the last required argument to the function.
+@end deftypefn
+
+@comment stdarg.h
+@comment ANSI
+@deftypefn {Macro} @var{type} va_arg (va_list @var{ap}, @var{type})
+The @code{va_arg} macro returns the value of the next optional argument,
+and changes the internal state of @var{ap} to move past this argument.
+Thus, successive uses of @code{va_arg} return successive optional 
+arguments.
+The type of the value returned by @code{va_arg} is the @var{type}
+specified in the call.  
+
+The @var{type} must match the type of the actual argument, and must not
+be @code{char} or @code{short int} or @code{float}.  (Remember that the
+default argument promotions apply to optional arguments.)
+@end deftypefn
+
+@comment stdarg.h
+@comment ANSI
+@deftypefn {Macro} void va_end (va_list @var{ap})
+This ends the use of @var{ap}.  After a @code{va_end} call, further
+@code{va_arg} calls with the same @var{ap} may not work.  You should invoke
+@code{va_end} before returning from the function in which @code{va_start}
+was invoked with the same @var{ap} argument.
+
+In the GNU C library, @code{va_end} does nothing, and you need not ever
+use it except for reasons of portability.
+@refill
+@end deftypefn
+
+
+@node Example of Variable Arguments,  , Variable Arguments Interface, Variable Argument Facilities
+@section Example of Variable Arguments
+
+Here is a complete sample function that accepts variable numbers of
+arguments.  The first argument to the function is the count of remaining
+arguments, which are added up and the result returned.  (This is
+obviously a rather pointless function, but it serves to illustrate the
+way the variable arguments facility is commonly used.)
+
+@comment Yes, this example has been tested.
+
+@example
+#include <stdarg.h>
+
+int 
+add_em_up (int count, @dots{})
+@{
+  va_list ap;
+  int i, sum;
+
+  va_start (ap, count);           /* @r{Initialize the argument list.} */
+
+  sum = 0;
+  for (i = 0; i < count; i++)
+    sum = sum + va_arg (ap, int); /* @r{Get the next argument value.} */
+
+  va_end (ap);                    /* @r{Clean up.} */
+  return sum;
+@}
+
+void main (void)
+@{
+  /* @r{This call prints 16.} */
+  printf ("%d\n", add_em_up (3, 5, 5, 6));
+
+  /* @r{This call prints 55.} */
+  printf ("%d\n", add_em_up (10, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10));
+@}
+@end example