getgrouplist(3) — Linux manual page


GETGROUPLIST(3)         Linux Programmer's Manual        GETGROUPLIST(3)

NAME         top

       getgrouplist - get list of groups to which a user belongs

SYNOPSIS         top

       #include <grp.h>

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

   Feature Test Macro Requirements for glibc (see

           Since glibc 2.19:
           Glibc 2.19 and earlier:

DESCRIPTION         top

       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.

RETURN VALUE         top

       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().

VERSIONS         top

       This function is present since glibc 2.2.4.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface                      Attribute     Value          │
       │getgrouplist()                 │ Thread safety │ MT-Safe locale │

CONFORMING TO         top

       This function is nonstandard; it appears on most BSDs.

BUGS         top

       In glibc versions before 2.3.3, the implementation of this
       function contains a buffer-overrun bug: it returns the complete
       list of groups for user in the array groups, even when the number
       of groups exceeds *ngroups.

EXAMPLES         top

       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>

       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]);

           ngroups = atoi(argv[2]);

           gid_t *groups = malloc(sizeof(*groups) * ngroups);
           if (groups == NULL) {

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

           pw = getpwnam(argv[1]);
           if (pw == NULL) {

           /* Retrieve group list. */

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

           /* 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);


SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.13 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at

GNU                            2021-03-22                GETGROUPLIST(3)

Pages that refer to this page: capsh(1)getent(1)getgroups(2)getgrent(3)group_member(3)nsswitch.conf(5)