VARARGS(5) DOMAIN/IX Reference Manual (SYS5) VARARGS(5)
NAME
varargs - handle variable argument list
USAGE
#include <varargs.h> va_alist va_dcl void va_start(pvar)
va_list pvar; type va_arg(pvar, type)
va_list pvar; void va_end(pvar)
va_list pvar;
DESCRIPTION
This set of macros allows portable procedures that accept
variable argument lists to be written. Routines that have
variable argument lists (such as printf(3S)) but do not use
varargs are inherently nonportable, as different machines
use different argument-passing conventions.
va_alist is used as the parameter list in a function header.
va_dcl is a declaration for va_alist. No semicolon should
follow va_dcl.
va_list is a type defined for the variable used to traverse
the list.
va_start is called to initialize pvar to the beginning of
the list.
va_arg will return the next argument in the list pointed to
by pvar. Type is the type the argument is expected to be.
Different types can be mixed, but it is up to the routine to
know what type of argument is expected, as it cannot be
determined at runtime.
va_end is used to clean up.
Multiple traversals, each bracketed by va_start ... va_end,
are possible.
EXAMPLE
This example is a possible implementation of execl(2).
#include <varargs.h>
#define MAXARGS 100
/* execl is called by
execl(file, arg1, arg2, ..., (char *)0);
*/
execl(va_alist)
va_dcl
{
va_list ap;
char *file;
char *args[MAXARGS];
Printed 5/10/85 VARARGS-1
VARARGS(5) DOMAIN/IX Reference Manual (SYS5) VARARGS(5)
int argno = 0;
va_start(ap);
file = va_arg(ap, char *);
while ((args[argno++] = va_arg(ap, char *)) != (char *)0)
;
va_end(ap);
return execv(file, args);
}
NOTES
It is up to the calling routine to specify how many argu-
ments there are, since it is not always possible to deter-
mine this from the stack frame. For example, execl is
passed a zero pointer to signal the end of the list. Printf
can tell how many arguments are there by the format.
It is non-portable to specify a second argument of char,
short, or float to va_arg, since arguments seen by the
called function are not char, short, or float. C converts
char and short arguments to int and converts float arguments
to double before passing them to a function.
RELATED INFORMATION
exec(2), printf(3S).
VARARGS-2 Printed 5/10/85