Linux / Unix Command: va_arg

NAME

stdarg - variable argument lists  

SYNOPSIS

#include <stdarg.h>

void va_start(va_list ap, last);
type va_arg(va_list ap, type);
void va_end(va_list ap);
void va_copy(va_list dest, va_list src);  

DESCRIPTION

A function may be called with a varying number of arguments of varying types. The include file stdarg.h declares a type va_list and defines three macros for stepping through a list of arguments whose number and types are not known to the called function.

The called function must declare an object of typeva_list which is used by the macrosva_start,va_arg, andva_end. 

va_start

The va_start macro initializes ap for subsequent use by va_arg and va_end, and must be called first.

The parameterlast is the name of the last parameter before the variable argument list, i.e., the last parameter of which the calling function knows the type.

Because the address of this parameter may be used in theva_start macro, it should not be declared as a register variable, or as a function or an array type. 

va_arg

The va_arg macro expands to an expression that has the type and value of the next argument in the call. The parameter ap is the va_list ap initialized by va_start. Each call to va_arg modifies ap so that the next call returns the next argument. The parameter type is a type name specified so that the type of a pointer to an object that has the specified type can be obtained simply by adding a * to type.

The first use of theva_arg macro after that of the va_start macro returns the argument after last. Successive invocations return the values of the remaining arguments.

If there is no next argument, or iftype is not compatible with the type of the actual next argument (as promoted according to the default argument promotions), random errors will occur.

Ifap is passed to a function that usesva_arg(ap,type) then the value ofap is undefined after the return of that function. 

va_end

Each invocation of va_start must be matched by a corresponding invocation of va_end in the same function. After the call va_end( ap ) the variable ap is undefined. Multiple transversals of the list, each bracketed by va_start and va_end are possible. va_end may be a macro or a function.  

va_copy

An obvious implementation would have a va_list a pointer to the stack frame of the variadic function. In such a setup (by far the most common) there seems nothing against an assignment

        va_list aq = ap;

Unfortunately, there are also systems that make it an array of pointers (of length 1), and there one needs

        va_list aq;
        *aq = *ap;

Finally, on systems where parameters are passed in registers, it may be necessary for va_start to allocate memory, store the parameters there, and also an indication of which parameter is next, so that va_arg can step through the list. Now va_end can free the allocated memory again. To accommodate this situation, C99 adds a macro va_copy, so that the above assignment can be replaced by

        va_list aq;
        va_copy(aq, ap);
        ...
        va_end(aq);

Each invocation of va_copy must be matched by a corresponding invocation of va_end in the same function. Some systems that do not supply va_copy have __va_copy instead, since that was the name used in the draft proposal.  

EXAMPLES

The function foo takes a string of format characters and prints out the argument associated with each format character based on the type.

#include <stdio.h>
#include <stdarg.h>

void foo(char *fmt, ...) {
        va_list ap;
        int d;
        char c, *p, *s;

        va_start(ap, fmt);
        while (*fmt)
                switch(*fmt++) {
                case 's':                       /* string */
                        s = va_arg(ap, char *);
                        printf("string %s\n", s);
                        break;
                case 'd':                       /* int */
                        d = va_arg(ap, int);
                        printf("int %d\n", d);
                        break;
                case 'c':                       /* char */
                        /* need a cast here since va_arg only
                           takes fully promoted types */
                        c = (char) va_arg(ap, int);
                        printf("char %c\n", c);
                        break;
                }
        va_end(ap);
}

 

CONFORMING TO

The va_start, va_arg, and va_end macros conform to ANSI X3.159-1989 (``C89''). C99 defines the va_copy macro.  

Important: Use the man command (% man) to see how a command is used on your particular computer.