🚀 go-pugleaf

mgk@sql.sybase.com (Michael Keirnan aka Joe Cool) writes:
>Is using the macro va_start more than once in a function legal in
>ANSI C? I was thinking of something along the lines of:
>#include <stdarg.h>
>void two_pass_arg_eater(int first_arg, ...)
>  {
>    va_list ap;
>    va_start(ap, first_arg);
>    /* while ... va_arg(ap, type) ... process args pass 1 */
>    va_end(ap);
>    va_start(ap, first_arg);
>    /* while ... va_arg(ap, type) ... process args pass 2 */
>    va_end(ap);
>  }
>My trusty Harbison and Steele (3rd ed.) does not forbid it as far as I
>can tell, but I'm looking for a more authoritative answer.

Looking in the standard, I don't see an explicit statement that va_start()
may be used more than once, but there certainly is none saying it may not,
and va_end() is described (partially) like this:  "The va_end macro may
modify ap so that it is no longer usable (without an intervening invocation
of va_start)."  Additionally, it is mentioned that if you pass ap to another
function, "the value of ap in the calling function is indeterminate and
shall be passed to the va_end macro prior to any further reference to ap."
Of course, there may be no further reference to ap after calling va_end
unless you call va_start afterwards.  Sounds like clear documentation of
the Committee's intent, and strong circumstantial evidence that it may be
invoked more than once.