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.\"	@(#)stdarg.3	6.8 (Berkeley) 6/29/91
.\"
.\" Converted for Linux, Mon Nov 29 15:11:11 1993, faith@cs.unc.edu
.\"
.TH STDARG 3  "29 November 1993" "BSD MANPAGE" "Linux Programmer's Manual"
.SH NAME
stdarg \- variable argument lists
.SH SYNOPSIS
.B #include <stdarg.h>
.sp
.BI "void va_start( va_list " ap ", " last );
.br
.BI "" type " va_arg( va_list " ap ", " type );
.br
.BI "void va_end( va_list " ap );
.SH DESCRIPTION
A function may be called with a varying number of arguments of varying
types.  The include file
.I stdarg.h
declares a type
.B va_list
and defines three macros for stepping through a list of arguments whose
number and types are not known to the called function.
.PP
The called function must declare an object of type
.B va_list
which is used by the macros
.BR va_start ,
.BR va_arg ,
and
.BR va_end .
.PP
The
.B va_start
macro initializes
.I ap
for subsequent use by
.B va_arg
and
.BR va_end ,
and must be called first.
.PP
The parameter
.I 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.
.PP
Because the address of this parameter is used in the
.B va_start
macro, it should not be declared as a register variable, or as a function
or an array type.
.PP
The
.B va_start
macro returns no value.
.PP
The
.B va_arg
macro expands to an expression that has the type and value of the next
argument in the call.  The parameter
.I ap
is the 
.BI va_list " " ap
initialized by
.BR va_start .
Each call to
.B va_arg
modifies
.I ap
so that the next call returns the next argument.  The parameter
.I 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
.IR type .
.PP
If there is no next argument, or if
.I type
is not compatible with the type of the actual next argument (as promoted
according to the default argument promotions), random errors will occur.
.PP
The first use of the
.B va_arg
macro after that of the 
.B va_start
macro returns the argument after 
.IR last .
Successive invocations return the values of the remaining arguments.
.PP
The
.B va_end
macro handles a normal return from the function whose variable argument
list was initialized by
.BR va_start .
.PP
The
.B va_end
macro returns no value.
.SH EXAMPLES
The function
.I foo
takes a string of format characters and prints out the argument associated
with each format character based on the type.
.RS
.nf
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\en", s);
			break;
		case 'd':			/* int */
			d = va_arg(ap, int);
			printf("int %d\en", d);
			break;
		case 'c':			/* char */
			/* need a cast here since va_arg only
			   takes fully promoted types */
			c = (char) va_arg(ap, int);
			printf("char %c\en", c);
			break;
		}
	va_end(ap);
}
.fi
.RE
.SH "CONFORMING TO"
The
.BR va_start ,
.BR va_arg ,
and
.B va_end
macros conform to ANSI X3.159-1989 (``ANSI C'').
.SH COMPATIBILITY
These macros are
.I not
compatible with the historic macros they replace.  A backward compatible
version can be found in the include file
.IR varargs.h .
.SH BUGS
Unlike the
.B varargs
macros, the
.B stdarg
macros do not permit programmers to code a function with no fixed
arguments.  This problem generates work mainly when converting
.B varargs
code to
.B 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
.B va_list
argument, such as
.BR vfprintf (3).