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.\" @(#)stdarg.3 8.1 (Berkeley) 6/5/93
.\" $FreeBSD: src/share/man/man3/stdarg.3,v 1.15 2005/01/21 08:36:36 ru Exp $
.\"
.Dd October 25, 2002
.Dt STDARG 3
.Os
.Sh NAME
.Nm stdarg
.Nd variable argument lists
.Sh SYNOPSIS
.In stdarg.h
.Ft void
.Fn va_start "va_list ap" last
.Ft type
.Fn va_arg "va_list ap" type
.Ft void
.Fn va_copy "va_list dest" "va_list src"
.Ft void
.Fn va_end "va_list ap"
.Sh DESCRIPTION
A function may be called with a varying number of arguments of varying
types.
The include file
.In stdarg.h
declares a type
.Pq Em 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
.Em va_list
which is used by the macros
.Fn va_start ,
.Fn va_arg ,
.Fn va_copy ,
and
.Fn va_end .
.Pp
The
.Fn va_start
macro must be called first, and it initializes
.Fa ap ,
which can be passed to
.Fn va_arg
for each argument to be processed.
Calling
.Fn va_end
signals that there are no further arguments, and causes
.Fa ap
to be invalidated.
Note that each call to
.Fn va_start
must be matched by a call to
.Fn va_end ,
from within the same function.
.Pp
The parameter
.Fa 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
.Fn va_start
macro, it should not be declared as a register variable, or as a
function or an array type.
.Pp
The
.Fn va_arg
macro expands to an expression that has the type and value of the next
argument in the call.
The parameter
.Fa ap
is the
.Em va_list Fa ap
initialized by
.Fn va_start .
Each call to
.Fn va_arg
modifies
.Fa ap
so that the next call returns the next argument.
The parameter
.Fa 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
.Fa type .
.Pp
If there is no next argument, or if
.Fa 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
.Fn va_arg
macro after that of the
.Fn va_start
macro returns the argument after
.Fa last .
Successive invocations return the values of the remaining
arguments.
.Pp
The
.Fn va_copy
macro copies the state of the variable argument list,
.Fa src ,
previously initialized by
.Fn va_start ,
to the variable argument list,
.Fa dest ,
which must not have been previously initialized by
.Fn va_start ,
without an intervening call to
.Fn va_end .
The state preserved in
.Fa dest
is equivalent to calling
.Fn va_start
and
.Fn va_arg
on
.Fa dest
in the same way as was used on
.Fa src .
The copied variable argument list can subsequently be passed to
.Fn va_arg ,
and must finally be passed to
.Fn va_end
when through with it.
.Pp
After a variable argument list is invalidated by
.Fn va_end ,
it can be reinitialized with
.Fn va_start
or made a copy of another variable argument list with
.Fn va_copy .
.Sh EXAMPLES
The function
.Em foo
takes a string of format characters and prints out the argument
associated with each format character based on the type.
.Bd -literal -offset indent
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\en", s);
break;
case 'd': /* int */
d = va_arg(ap, int);
printf("int %d\en", d);
break;
case 'c': /* char */
/* Note: char is promoted to int. */
c = va_arg(ap, int);
printf("char %c\en", c);
break;
}
va_end(ap);
...
/* use ap2 to iterate over the arguments again */
...
va_end(ap2);
}
.Ed
.Sh COMPATIBILITY
These macros are
.Em not
compatible with the historic macros they replace.
A backward compatible version can be found in the include
file
.In varargs.h .
.Sh STANDARDS
The
.Fn va_start ,
.Fn va_arg ,
.Fn va_copy ,
and
.Fn va_end
macros conform to
.St -isoC-99 .
.Sh BUGS
Unlike the
.Em varargs
macros, the
.Nm
macros do not permit programmers to
code a function with no fixed arguments.
This problem generates work mainly when converting
.Em varargs
code to
.Nm
code,
but it also creates difficulties for variadic functions that
wish to pass all of their arguments on to a function
that takes a
.Em va_list
argument, such as
.Xr vfprintf 3 .