source: branches/samba-3.3.x/testsuite/nsswitch/initgroups.c

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

int main(int argc, char **argv)
{
        int result, ngroups, i;
        gid_t *groups;
        struct passwd *pw;

        if (!(pw = getpwnam(argv[1]))) {
                printf("FAIL: no passwd entry for %s\n", argv[1]);
                return 1;
        }

        result = initgroups(argv[1], pw->pw_gid);

        if (result == -1) {
                printf("FAIL");
                return 1;
        }

        ngroups = getgroups(0, NULL);

        groups = (gid_t *)malloc(sizeof(gid_t) * ngroups);
        ngroups = getgroups(ngroups, groups);

        printf("%s is a member of groups:\n", argv[1]);

        for (i = 0; i < ngroups; i++) {
                struct group *grp;

                grp = getgrgid(groups[i]);

                printf("%d (%s)\n", groups[i], grp ? grp->gr_name : "?");
        }

        printf("PASS\n");
        return 0;
}