GETGROUPLIST(3)           (2020-11-01)            GETGROUPLIST(3)

     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 spec-
          ified 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).  allbox; lb lb lb l l l.

     Page 1                         GNU              (printed 5/20/22)

     GETGROUPLIST(3)           (2020-11-01)            GETGROUPLIST(3)

          Interface Attribute Value T{ getgrouplist() T}   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 com-
          plete 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);

     Page 2                         GNU              (printed 5/20/22)

     GETGROUPLIST(3)           (2020-11-01)            GETGROUPLIST(3)

              }

              /* 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/.

     Page 3                         GNU              (printed 5/20/22)