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C in a Nutshell by Tony Crawford, Peter Prinz

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Name

va_arg, va_copy, va_end, va_start

Synopsis

Manage variable-argument lists

#include <stdarg.h>
voidva_start( va_list argptr, last_fixed_arg );
type va_arg( va_list argptr, type );
void va_copy( va_list dest, va_list src );        (C99)
void va_end( va_list argptr );

The macros va_arg(), va_start(), and va_end() allow you to define C functions with variable numbers of arguments. Such functions use these macros to access the optional arguments, which are managed as anonymous objects in a list referenced by a pointer object of type va_list.

The prototype syntax for a function that takes a variable number of arguments is as follows:

fn_type fn_name( [arg_type_1 fixed_arg_1, [arg_type_2 fixed_arg_2, [etc.]]]
                  last_arg_type last_fixed_arg, ... );

The ellipsis (...) after last_fixed_arg in this syntax is a literal ellipsis, which must appear in the function prototype to represent the optional arguments. A function with optional arguments must also take at least one mandatory argument. The ellipsis representing the optional arguments must be the last item in the parameter list, after the last mandatory parameter. The following example shows the prototype of the function vop(), which takes two mandatory arguments—one with the type pointer to const char and one with the type int—and a variable number of optional arguments:

double vop( const char * op, int argcount, ... );

In a function definition, the macros va_start(), va_arg(), va_copy(), and va_end() allow you to access the optional arguments.

va_start ...

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