NAME
stdarg - variable argument lists
SYNOPSIS
#include <stdarg.h>
void va_start( va_list ap, last));
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, 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 function 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 speci-
fied 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 invoca-
tions return the values of the remaining arguments.
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 charac-
ter based on the type.
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
C3.159-1989 (``ANSI C'').
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.
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).