VA_START(3) | Library Functions Manual | VA_START(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_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 `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_end`() macro handles a normal return from the function whose variable argument list was initialized by `va_start`() or `va_copy`().

# RETURN VALUES

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_start`(), `va_copy`() and `va_end`() macros return 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 ISO/IEC 9899:1999 (“ISO C99”).

# HISTORY

The `va_start`(), `va_arg`() and `va_end`() macros were introduced in ANSI X3.159-1989 (“ANSI C89”). The `va_copy`() macro was introduced in ISO/IEC 9899:1999 (“ISO C99”).

# 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 an argument of type va_list, such as vfprintf(3).

August 30, 2019 | Debian