getgrouplist - get list of groups to which a user belongs

#include <grp.h>

int getgrouplist(const char *user, gid_t group,
gid_t *groups, int *ngroups);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

The getgrouplist() function scans the group database (see group(5)) to obtain the list of groups that user belongs to. Up to *ngroups of these groups are returned in the array groups.

If it was not among the groups defined for user in the group database, then group is included in the list of groups returned by getgrouplist(); typically this argument is specified as the group ID from the password record for user.

The ngroups argument is a value-result argument: on return it always contains the number of groups found for user, including group; this value may be greater than the number of groups stored in groups.

If the number of groups of which user is a member is less than or equal to *ngroups, then the value *ngroups is returned.

If the user is a member of more than *ngroups groups, then getgrouplist() returns -1. In this case, the value returned in *ngroups can be used to resize the buffer passed to a further call getgrouplist().

The program below displays the group list for the user named in its first command-line argument. The second command-line argument specifies the ngroups value to be supplied to getgrouplist(). The following shell session shows examples of the use of this program:

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

Program source

#include <stdio.h>
#include <stdlib.h>
#include <grp.h>
#include <pwd.h>

int
main(int argc, char *argv[])
{
    int ngroups;
    struct passwd *pw;
    struct group *gr;

    if (argc != 3) {
        fprintf(stderr, "Usage: %s <user> <ngroups>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    ngroups = atoi(argv[2]);

    gid_t *groups = malloc(sizeof(*groups) * ngroups);
    if (groups == NULL) {
        perror("malloc");
        exit(EXIT_FAILURE);
    }

    /* Fetch passwd structure (contains first group ID for user) */

    pw = getpwnam(argv[1]);
    if (pw == NULL) {
        perror("getpwnam");
        exit(EXIT_SUCCESS);
    }

    /* Retrieve group list */

    if (getgrouplist(argv[1], pw->pw_gid, groups, &ngroups) == -1) {
        fprintf(stderr, "getgrouplist() returned -1; ngroups = %d\n",
                ngroups);
        exit(EXIT_FAILURE);
    }

    /* Display list of retrieved groups, along with group names */

    fprintf(stderr, "ngroups = %d\n", ngroups);
    for (int j = 0; j < ngroups; j++) {
        printf("%d", groups[j]);
        gr = getgrgid(groups[j]);
        if (gr != NULL)
            printf(" (%s)", gr->gr_name);
        printf("\n");
    }

    exit(EXIT_SUCCESS);
}

getgroups(2), setgroups(2), getgrent(3), group_member(3), group(5), passwd(5)