NAME

       getgrouplist - get list of groups to which a user belongs

SYNOPSIS

       #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)):

       getgrouplist():
           Since glibc 2.19:
               _DEFAULT_SOURCE
           Glibc 2.19 and earlier:
               _BSD_SOURCE

DESCRIPTION

       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

       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

       This function is present since glibc 2.2.4.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).
       ┌────────────────┬───────────────┬────────────────┐
       │ Interface      │ Attribute     │ Value          │
       ├────────────────┼───────────────┼────────────────┤
       │ getgrouplist() │ Thread safety │ MT-Safe locale │
       └────────────────┴───────────────┴────────────────┘

CONFORMING TO

       This function is nonstandard; it appears on most BSDs.

BUGS

       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

       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);
       }

SEE ALSO

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

COLOPHON

       This  page  is  part  of  release  5.10  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
       https://www.kernel.org/doc/man-pages/.

GNU                                                2020-11-01                                    GETGROUPLIST(3)