List all groups the current user is member of.

groups.c

#include <stdio.h>
#include <unistd.h>
#include <grp.h>
#include <stdlib.h>
#include <malloc.h>

int main()
{
    struct group *gr;
    int i, n;
    gid_t *groups;

    n = getgroups(0, NULL);

    if (n < 0) {
        fprintf(stderr, "Unable to retrieve groups: %m\n");
        return EXIT_FAILURE;
    }

    groups = malloc(n * sizeof(gid_t));
    if (!groups)
        return EXIT_FAILURE;

    if (getgroups(n, groups) < 0) {
        fprintf(stderr, "Unable to retrieve groups: %m\n");
        free(groups);
        return EXIT_FAILURE;
    }

    printf("Found %d groups:\n", n);
    for (i = 0; i < n; i++) {
        gr = getgrgid(groups[i]);
        printf(" * %s\n", gr->gr_name);
    }

    free(groups);

    return 0;
}