source: branches/samba-3.3.x/source/tests/getgroups.c

/* this tests whether getgroups actually returns lists of integers
   rather than gid_t. The test only works if the user running
   the test is in at least 1 group 

   The test is designed to check for those broken OSes that define
   getgroups() as returning an array of gid_t but actually return a
   array of ints! Ultrix is one culprit
  */

#if defined(HAVE_UNISTD_H)
#include <unistd.h>
#endif

#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>
#include <grp.h>

main()
{
        int i;
        int *igroups;
        char *cgroups;
        int grp = 0;
        int  ngroups = getgroups(0,&grp);

        if (sizeof(gid_t) == sizeof(int)) {
                fprintf(stderr,"gid_t and int are the same size\n");
                exit(1);
        }

        if (ngroups <= 0)
                ngroups = 32;

        igroups = (int *)malloc(sizeof(int)*ngroups);

        for (i=0;i<ngroups;i++)
                igroups[i] = 0x42424242;

        ngroups = getgroups(ngroups,(gid_t *)igroups);

        if (igroups[0] == 0x42424242)
                ngroups = 0;

        if (ngroups == 0) {
                printf("WARNING: can't determine getgroups return type\n");
                exit(1);
        }
        
        cgroups = (char *)igroups;

        if (ngroups == 1 && 
            cgroups[2] == 0x42 && cgroups[3] == 0x42) {
                fprintf(stderr,"getgroups returns gid_t\n");
                exit(1);
        }
          
        for (i=0;i<ngroups;i++) {
                if (igroups[i] == 0x42424242) {
                        fprintf(stderr,"getgroups returns gid_t\n");
                        exit(1);
                }
        }

        exit(0);
}