getgrouplist(3) — Linux manual page

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | BUGS | EXAMPLES | SEE ALSO | COLOPHON

getgrouplist(3)         Library Functions Manual         getgrouplist(3)

NAME         top

       getgrouplist - get list of groups to which a user belongs

LIBRARY         top

       Standard C library (libc, -lc)

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
   feature_test_macros(7)):

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

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 to getgrouplist().

ATTRIBUTES         top

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

STANDARDS         top

       None.

HISTORY         top

       glibc 2.2.4.

BUGS         top

       Before glibc 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 <errno.h>
       #include <grp.h>
       #include <pwd.h>
       #include <stdio.h>
       #include <stdlib.h>

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

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

           ngroups = atoi(argv[2]);

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

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

           errno = 0;
           pw = getpwnam(argv[1]);
           if (pw == NULL) {
               if (errno)
                   perror("getpwnam");
               else
                   fprintf(stderr, "no such user\n");
               exit(EXIT_FAILURE);
           }

           /* 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         top

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

COLOPHON         top

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Linux man-pages 6.9.1          2024-06-15                getgrouplist(3)

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