VARARGS(3) GENIX Programmer's Manual VARARGS(3)
NAME
varargs - variable argument list
SYNOPSIS
#include <varargs.h>
function(va_alist)
va_dcl
va_list pvar;
va_start(pvar);
f = va_arg(pvar, type);
va_end(pvar);
DESCRIPTION
This set of macros allows portable procedures that accept variable
argument lists to be written. Routines which have variable argument
lists (such as printf(3)) that do not use varargs are inherently
nonportable, since different machines use different argument passing
conventions.
va_alist is used in a function header to declare a variable argument
list.
va_dcl is a declaration for va_alist. Note that there is no semi-
colon after va_dcl.
va_list is a type which can be used for the variable pvar, which is
used to traverse the list. One such variable must always be
declared.
va_start (pvar) is called to initialize pvar to the beginning of the
list.
va_arg (pvar,type) will return the next argument in the list pointed
to by pvar. Type is the type the argument is expected to be. Dif-
ferent types can be mixed, but it is up to the routine to know what
type of argument is expected, since it cannot be determined at run-
time.
va_end (pvar) is used to finish up.
Multiple traversals, each bracketted by va_start .. va_end, are
possible.
EXAMPLE
#include <varargs.h>
execl(va_alist)
va_dcl
{
va_list ap;
char *file;
char *args[100];
int argno = 0;
Printed 11/16/83 1
VARARGS(3) GENIX Programmer's Manual VARARGS(3)
va_start(ap);
file = va_arg(ap, char *);
while (args[argno++] = va_arg(ap, char *))
;
va_end(ap);
return execv(file, args);
}
BUGS
It is up to the calling routine to determine how many arguments
there are, since it is not possible to determine this from the stack
frame. For example, execl passes a 0 to signal the end of the list.
Printf can tell how many arguments are there by the format.
Printed 11/16/83 2