varargs(5)
_________________________________________________________________
varargs Miscellany
handle variable argument list
_________________________________________________________________
SYNTAX
#include <varargs.h>
vaalist
vadcl
void vastart(pvar)
valist pvar;
type vaarg(pvar, type)
valist pvar;
void vaend(pvar)
valist 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.
vaalist is used as the parameter list in a function header.
vadcl is a declaration for va_alist. No semicolon should follow
va_dcl.
valist is a type defined for the variable used to traverse the
list.
vastart is called to initialize pvar to the beginning of the
list.
vaarg 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.
vaend is used to clean up.
DG/UX 4.00 Page 1
Licensed material--property of copyright holder(s)
varargs(5)
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];
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);
}
SEE ALSO
exec(2), printf(3S), vprintf(3S).
NOTES
It is up to the calling routine to specify how many arguments
there are, since it is not always possible to determine 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.
DG/UX 4.00 Page 2
Licensed material--property of copyright holder(s)