stdarg(3)

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_copy(va_list dest, 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 #include <stdarg.h> declares a type (va_list) and defines three macros for step

ping 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_copy(), 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, 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, or as a func

tion or 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), random errors will occur.

The first use of the va_arg() macro after that of the

va_start() macro
returns the argument after last. Successive invocations re

turn the values of the remaining arguments.

The va_copy() macro copies a variable argument list, previ

ously initialized by va_start(), from src to dest. The state is pre

served such that
it is equivalent to calling va_start() with the same second

argument used
with src, and calling va_arg() the same number of times as

called with
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().

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;
char c, *s;

va_start(ap, fmt);
while (*fmt)

switch(*fmt++) {
case 's': /*

string */

s = va_arg(ap, char *);
printf("string %s0, s);
break;

case 'd': /* int

*/

d = va_arg(ap, int);
printf("int %d0, d);
break;

case 'c': /*

char */

/* Note: char is promoted to

int. */
c = va_arg(ap, int);
printf("char %c0, c);
break;

}

va_end(ap);

}

COMPATIBILITY

These macros are not compatible with the historic macros

they replace. A
backward compatible version can be found in the include file

STANDARDS

The va_start(), va_arg(), va_copy(), and va_end() macros

conform to
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 gen

erates 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).

BSD October 25, 2002

BSD