STDARG(3)

NAME
va_start, va_arg, va_copy, va_end - variable argument lists

SYNOPSIS
#include <stdarg.h>
void
va_start(va_list ap, last);
type
va_arg(va_list ap, type);
void
va_copy(va_list dst, va_list src);
void
va_end(va_list ap);

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 type va_list which is used by the macros va_start(), va_arg(), va_end(), and, optionally, va_copy().

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

The parameter last 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 is used in the va_start() macro, it should not be declared as a register variable, nor as a function, nor an array type.

The va_start() macro returns no 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 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.

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

If the type in question is one that would normally be promoted, the promoted type should be used as the argument to va_arg(). The following describes which types should be promoted (and to what):
-   short is promoted to int
-   float is promoted to double
-   char is promoted to int

The same rules apply to unsigned versions of the above types, as well as their bit-type equivalents (e.g. int8_t and int16_t).

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

The va_copy() macro makes dst a copy of src as if the va_start() macro had been applied to it followed by the same sequence of uses of the va_arg() macro as had previously been used to reach the present state of src.

The va_copy() macro returns no value.

The va_end() macro handles a normal return from the function whose variable argument list was initialized by va_start() or va_copy().

The va_end() macro returns no value.

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

    void
    foo(char *fmt, ...)
    {
    	va_list ap;
    	int d, c;
    	char *s;
    	double f;
    	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, int);	/* promoted */
    			printf("char %c\n", c);
    			break;
    		case 'f':			/* float */
    			f = va_arg(ap, double); /* promoted */
    			printf("float %f\n", f);
    		}
    	va_end(ap);
    }

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

The va_start(), va_arg() and va_end() macros conform to ansiC-99.

HISTORY
The va_start(), va_arg() and va_end() macros were introduced in ansiC-89. The va_copy() macro was introduced in ansiC-99.

BUGS
Unlike the varargs macros, the stdarg macros do not permit programmers to code a function with no fixed arguments. This problem generates work mainly when converting varargs code to stdarg code, but it also creates difficulties for variadic functions that wish to pass all of their arguments on to a function that takes a va_list argument, such as vfprintf(3).