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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():
           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  getgrou-
       plist();  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 getgrou-
       plist() 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).
       +---------------+---------------+----------------+
       |Interface      | Attribute     | Value          |
       +---------------+---------------+----------------+
       |getgrouplist() | 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 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  speci-
       fies 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_member(3),  group(5),
       passwd(5)
COLOPHON
       This  page  is  part of release 4.15 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/.
GNU                               2017-09-15                   GETGROUPLIST(3)