This is an integer data type used to represent user IDs. In the GNU library, this is an alias for unsigned int.

This is an integer data type used to represent group IDs. In the GNU library, this is an alias for unsigned int.

The getuid function returns the real user ID of the process.

The getgid function returns the real group ID of the process.

The geteuid function returns the effective user ID of the process.

The getegid function returns the effective group ID of the process.

The getgroups function is used to inquire about the supplementary group IDs of the process. Up to count of these group IDs are stored in the array groups; the return value from the function is the number of group IDs actually stored. If count is smaller than the total number of supplementary group IDs, then getgroups returns a value of -1 and errno is set to EINVAL.

If count is zero, then getgroups just returns the total number of supplementary group IDs. On systems that do not support supplementary groups, this will always be zero.

Here's how to use getgroups to read all the supplementary group IDs:

          gid_t *
          read_all_groups (void)
          {
            int ngroups = getgroups (0, NULL);
            gid_t *groups
              = (gid_t *) xmalloc (ngroups * sizeof (gid_t));
            int val = getgroups (ngroups, groups);
            if (val < 0)
              {
                free (groups);
                return NULL;
              }
            return groups;
          }