getgrouplist - get list of groups to which a user belongs


   #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)):

       Since glibc 2.19:
       Glibc 2.19 and earlier:


   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.


   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


   This function is present since glibc 2.2.4.


   For  an  explanation  of  the  terms  used   in   this   section,   see

   Interface       Attribute      Value          
   getgrouplist()  Thread safety  MT-Safe locale 


   This function is nonstandard; it appears on most BSDs.


   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


   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>

   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]);

       ngroups = atoi(argv[2]);

       groups = malloc(ngroups * sizeof (gid_t));
       if (groups == NULL) {

       /* Fetch passwd structure (contains first group ID for user) */

       pw = getpwnam(argv[1]);
       if (pw == NULL) {

       /* Retrieve group list */

       if (getgrouplist(argv[1], pw->pw_gid, groups, &ngroups) == -1) {
           fprintf(stderr, "getgrouplist() returned -1; ngroups = %d\n",

       /* 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);



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


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