getgrouplist(3)

#include <grp.h>

int getgrouplist(const char *\nuser\n, gid_t \ngroup\n,\n
\n                 gid_t *\ngroups\n, int *\nngroups\n);

\n
    Since glibc 2.19:
\n
        _DEFAULT_SOURCE
\n
    glibc 2.19 and earlier:
\n
        _BSD_SOURCE

$\n ./a.out cecilia 0\n
getgrouplist() returned -1; ngroups = 3
$\n ./a.out cecilia 3\n
ngroups = 3
16 (dialout)
33 (video)
100 (users)

#include <errno.h>
#include <grp.h>
#include <pwd.h>
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
\n
    int            ngroups;
\n
    gid_t          *groups;
\n
    struct group   *gr;
\n
    struct passwd  *pw;
\n
    if (argc != 3) {
\n
        fprintf(stderr, "Usage: %s <user> <ngroups>\n", argv[0]);
\n
        exit(EXIT_FAILURE);
\n
    }
\n
    ngroups = atoi(argv[2]);
\n
    groups = malloc(sizeof(*groups) * ngroups);
\n
    if (groups == NULL) {
\n
        perror("malloc");
\n
        exit(EXIT_FAILURE);
\n
    }
\n
    /* Fetch passwd structure (contains first group ID for user). */
\n
    errno = 0;
\n
    pw = getpwnam(argv[1]);
\n
    if (pw == NULL) {
\n
        if (errno)
\n
            perror("getpwnam");
\n
        else
\n
            fprintf(stderr, "no such user\n");
\n
        exit(EXIT_FAILURE);
\n
    }
\n
    /* Retrieve group list. */
\n
    if (getgrouplist(argv[1], pw->pw_gid, groups, &ngroups) == -1) {
\n
        fprintf(stderr, "getgrouplist() returned -1; ngroups = %d\n",
\n
                ngroups);
\n
        exit(EXIT_FAILURE);
\n
    }
\n
    /* Display list of retrieved groups, along with group names. */
\n
    fprintf(stderr, "ngroups = %d\n", ngroups);
\n
    for (int j = 0; j < ngroups; j++) {
\n
        printf("%d", groups[j]);
\n
        gr = getgrgid(groups[j]);
\n
        if (gr != NULL)
\n
            printf(" (%s)", gr->gr_name);
\n
        printf("\n");
\n
    }
\n
    exit(EXIT_SUCCESS);
}