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<stdarg.h>        -        variable arguments

jump to: demonstration of <stdarg.h>

The header provides funtions that allow the programmer to write functions containing variable-lenght paramater lists. The variable argument list macros provide a portable method of accessing arguments to a function taking a variable number of arguments.

type va_arg(va_list argptr, type); returns the current and succeeding arguments
void va_end(va_list argptr); sets the argument pointer to NULL
void va_start(va_list argptr); sets the argument poointer (argptr), declared as type va_list, to the first argument in the list passed to the function
   

The <stdarg.h> header contains a set of macros which allows portable functions that accept variable argument lists to be written. Functions that have variable argument lists (such as printf()) but do not use these macros are inherently non-portable, as different systems use different argument-passing conventions.
The type va_list is defined for variables used to traverse the list.

The va_start() macro is invoked to initialise ap to the beginning of the list before any calls to va_arg().

The object argptr may be passed as an argument to another function; if that function invokes the va_arg() macro with parameter argptr, the value of argptr in the calling function is indeterminate and must be passed to the va_end() macro prior to any further reference to argptr. The parameter argN is the identifier of the rightmost parameter in the variable parameter list in the function definition (the one just before the , ...). If the parameter argN is declared with the register storage class, with a function type or array type, or with a type that is not compatible with the type that results after application of the default argument promotions, the behaviour is undefined.

The va_arg() macro will return the next argument in the list pointed to by argptr. Each invocation of va_arg() modifies argptr so that the values of successive arguments are returned in turn. The type parameter is the type the argument is expected to be. This is the type name specified such that the type of a pointer to an object that has the specified type can be obtained simply by suffixing a * to type. Different types can be mixed, but it is up to the routine to know what type of argument is expected.

The va_end() macro is used to clean up; it invalidates argptr for use (unless va_start() is invoked again).

Multiple traversals, each bracketed by va_start() va_end(), are possible.

Example:

#include <stdarg.h>

#define MAXARGS 31

int execl (const char *file, const char *args, ...)
{
    va_list ap;
    char *array[MAXARGS];
    int argno = 0;
    va_start(ap, args);

    while (args != 0) {
        array[argno++] = args;
        args = va_arg(ap, const char *);
    }

    va_end(ap);

    return execv(file, array);
}

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