stdarg(3STDC)

NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | STANDARDS | COMPATIBILITY | RESTRICTIONS | ATTRIBUTES

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);

DESCRIPTION

A function may be called with a varying number of arguments of a number of types. The include file 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 type va_list which is used by the macros va_start, va_arg, and va_end.

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

The parameter last is the name of the last parameter before the variable argument list, in other words, the last parameter of which the calling function knows the type.

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

The va_start macro does not return a value.

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 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 to allow the type of pointer to an object of the specified type can be obtained simply by adding a * to type.

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

The first use of the va_arg macro after that of the va_start6 macro returns the argument after last. Successive invocations return the values of the remaining arguments.

The va_end macro handles a normal return from the function whose variable argument list was initialized using va_start.

The va_end macro does not return a value.

EXAMPLES

---

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 */
                        c = va_arg(ap, char);
                        printf("char %c\n", c);
                        break;
                }
        va_end(ap);
}

---

STANDARDS

The va_start, va_arg, and va_end macros conform to ANSI-C.

COMPATIBILITY

These macros are not compatible with the macros they replace. A backward compatible version can be found in the include file varargs.h.

RESTRICTIONS

Unlike the varargs macros, the stdarg macros do not permit programmers to code a function with no fixed arguments.

ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Interface Stability	Evolving

NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | STANDARDS | COMPATIBILITY | RESTRICTIONS | ATTRIBUTES