STDARG(3) BSD Library Functions Manual 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
<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_copy(), and va_end().
The va_start() macro must be called first, and it initializes ap, which can be passed to va_arg() for
each argument to be processed. Calling va_end() signals that there are no further arguments, and
causes ap to be invalidated. Note that each call to va_start() must be matched by a call to va_end(),
from within the same function.
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 function or 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), 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 return the values of the remaining arguments.
The va_copy() macro copies the state of the variable argument list, src, previously initialized by
va_start(), to the variable argument list, dest, which must not have been previously initialized by
va_start(), without an intervening call to va_end(). The state preserved in dest is equivalent to
calling va_start() and va_arg() on dest in the same way as was used on src. The copied variable argu-
ment list can subsequently be passed to va_arg(), and must finally be passed to va_end() when through
with it.
After a variable argument list is invalidated by va_end(), it can be reinitialized with va_start() or
made a copy of another variable argument list with va_copy().
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, ap2;
int d;
char c, *s;
va_start(ap, fmt);
va_copy(ap2, ap);
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 */
/* Note: char is promoted to int. */
c = va_arg(ap, int);
printf("char %c\n", c);
break;
}
va_end(ap);
...
/* use ap2 to iterate over the arguments again */
...
va_end(ap2);
}
COMPATIBILITY
These macros are not compatible with the historic macros they replace. A backward compatible version
can be found in the include file <varargs.h>.
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 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).
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