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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 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.

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 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 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

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       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.