comma expressions with gcc 2.95.1

[Roland Winkler <roland dot winkler at physik dot uni-erlangen dot de>]

Hi

I wanted to have a macro `QU(x)' which gives me the square of its
argument. And I wanted to do this in a way that the argument is
evaluated only once. Thus my macro was based on a comma expression:

static double dqarg;
#define QU(x) (dqarg=(x), dqarg*dqarg)

For an expression like `z = QU(x) + QU(y);' the expanded code is

  z =  (dqarg = x, dqarg * dqarg)
     + (dqarg = y, dqarg * dqarg);

However, using gcc-2.95.1 this piece of expanded code is evaluated
like `z=2*x*x' and not `z=x*x+y*y', see the example program attached
below. My system is:

gcc --version  2.95.1

uname -a:
Linux tfkp03 2.2.10 #4 Wed Oct 13 12:35:36 CEST 1999 i686 unknown

libc 2.1.1

Distribution: SuSE Linux 6.2

=================================================================

/* Example programm */

#include <stdio.h>
#include <stdlib.h>

main (int argc, char *argv[])
{
  double x, y, z, dqarg;
  if (argc < 3) {
    printf ("%s: two arguments required\n", argv [0]); 
    exit (1);
  };
  sscanf (argv [1], "%lf", &x);
  sscanf (argv [2], "%lf", &y);
  z =  (dqarg = x, dqarg * dqarg)
     + (dqarg = y, dqarg * dqarg);
  printf ("%f %f %f\n", x, y, z);
}

-- 
-----------------------------------------------------------------------
 Dr. Roland Winkler
 Lst. fuer Theoretische Festkoerperphysik     Phone  ++49-9131-85 28834
 Institut fuer Technische Physik III          Fax    ++49-9131-85 28833
 Universitaet Erlangen-Nuernberg
 Staudtstr. 7
 D-91058 Erlangen         e-mail: roland.winkler@physik.uni-erlangen.de
 Germany                  http://www.tfkp.physik.uni-erlangen.de/
-----------------------------------------------------------------------