NAME

       getgrouplist - get list of groups to which a user belongs

LIBRARY

       Standard C library (libc, -lc)

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 to getgrouplist().

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌────────────────────────────────────────────────────────┬───────────────┬────────────────┐
       │Interface                                               │ Attribute     │ Value          │
       ├────────────────────────────────────────────────────────┼───────────────┼────────────────┤
       │getgrouplist()                                          │ Thread safety │ MT-Safe locale │
       └────────────────────────────────────────────────────────┴───────────────┴────────────────┘

STANDARDS

       None.

HISTORY

       glibc 2.2.4.

BUGS

       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

       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

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