Re: Wrapping variable length agument lists

[Sisyphus]

[EMAIL PROTECTED] wrote:
An alternative in c:
You can also make a new va_list out of the old one.  It's basically an array of 
void pointers.  So you va_start, va_arg and repopulate the second array with 
appropriate void casted pointers.  You can then vprintf on this hacked va_list.

I wasn't familiar with va_list and friends, so I've dug around a bit and 
come up with the following C program which wraps vprintf. It's a totally 
useless script for any practical purpose, but demonstrates some sort of 
usage regarding the va_list type.

--- start ---
#include <stdio.h>
#include <stdarg.h>
void wrap_printf(char *, ...);
int main() {
  int w = 37, x = 22, y = 11, z = 10;
  char * fmt = "%#o %#o %#o %#o\n";
  wrap_printf(fmt, w, x, y, z);
  return 0;
}
void wrap_printf(char * a, ...) {
  va_list ap;
  va_start(ap, a);
  vprintf(a, ap);
  va_end(ap);
}
--- end ---
Is that usage sane (ignoring the pointlessness of it) ? Note that 
there's no mention of 'va_arg'.

Because that compiled without issuing any warnings and ran as desired, I 
came up with the following Inline C script:

--- start ---
use warnings;
use Inline C => <<'EOC';
#include <stdio.h>
#include <stdarg.h>
void wrap_printf(SV * a, ...) {
     va_list ap;
     va_start(ap, a);
     vprintf(SvPV_nolen(a), ap);
     va_end(ap);
}
EOC
wrap_printf("%#o %#o %#o %#o\n", 37, 22, 11, 10);
--- end ---
That also compiles without warning and runs fine, except that it doesn't 
give the correct answer. The output I get is '044170444 06 0 0' - 
presumably because the values that are being printf'd are not the 
integer values being passed to wrap_printf() - but some other values 
associated with 'ap', instead.

So ... can I modify the line 'vprintf(SvPV_nolen(a), ap);' in such a way 
that it's the integers 37, 22, 11, and 10 that get printf'd ?

Or am I faced with the task of converting the argument list received by 
the Inline C wrap_printf() function to an array of char* and ints, and 
then somehow passing that array to vprintf ? (This seems to be what your 
original reply is suggesting. Is that what I need to concentrate on 
achieving ?)

Thanks R.
Cheers,
Rob