getgrouplist - get list of groups to which a user belongs

       For an explanation of the terms used in this section, see attributes(7).
       ┌──────────────────────────────────────────────────────────────────────┬───────────────┬────────────────┐
       │ Interface                                                            │ Attribute     │ Value          │
       ├──────────────────────────────────────────────────────────────────────┼───────────────┼────────────────┤
       │ getgrouplist()                                                       │ Thread safety │ MT-Safe locale │
       └──────────────────────────────────────────────────────────────────────┴───────────────┴────────────────┘

       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.

       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.

       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.outcecilia0
           getgrouplist() returned -1; ngroups = 3
           $ ./a.outcecilia3
           ngroups = 3
           16 (dialout)
           33 (video)
           100 (users)

   Programsource

       #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);
       }

       glibc 2.2.4.

       Standard C library (libc, -lc)

       getgrouplist - get list of groups to which a user belongs

       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().

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

Linux man-pages 6.9.1                              2024-06-15                                    getgrouplist(3)

       None.

#include<grp.h>intgetgrouplist(constchar*user,gid_tgroup,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