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GETGROUPLIST(3)            Linux Programmer's Manual           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(): _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.

CONFORMING TO
       This function is non-standard; 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.

EXAMPLE
       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 ses-
       sion 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 j, ngroups;
           gid_t *groups;
           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]);

           groups = malloc(ngroups * sizeof (gid_t));
           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 (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(5), passwd(5)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



GNU                               2008-07-03                   GETGROUPLIST(3)