The following program:

tcsh> cat bug.c
#include <stdio.h>
#include <stdlib.h>

union u {
  unsigned i;
  struct {
    unsigned low : 16;
    unsigned high : 16;
  } s;
};

static void cvt (unsigned *v) {
  union u u;
  u.i = *v;
  u.s.low = 3;
  memcpy (v, &u.i, sizeof (unsigned));
}

int main () {
  unsigned i = 0x00010002;
  cvt (&i);
  printf ("res = %08x\n", i);
  return EXIT_SUCCESS;
}

Compiled on:

tcsh> uname -a
Linux hostname 2.2.12 #3 SMP Thu Sep 2 03:34:03 MDT 1999
i586 unknown

Using cc:

tcsh> cc --version
egcs-2.91.66

With the command:

tcsh> cc -O1 -o bug bug.c

Produces the following result:

tcsh> ./bug
res = 00000003

I would prefer it to produce a "00010003" result.