GNU C Library (libc) Programming Guide

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29.7 Setting the Group IDs

This section describes the functions for altering the group IDs (real and effective) of a process. To use these facilities, you must include the header files sys/types.h and unistd.h.

— Function: int setegid (gid_t newgid)

This function sets the effective group ID of the process to newgid, provided that the process is allowed to change its group ID. Just as with seteuid, if the process is privileged it may change its effective group ID to any value; if it isn't, but it has a file group ID, then it may change to its real group ID or file group ID; otherwise it may not change its effective group ID.
Note that a process is only privileged if its effective user ID is zero. The effective group ID only affects access permissions.
The return values and error conditions for setegid are the same as those for seteuid.
This function is only present if _POSIX_SAVED_IDS is defined.

— Function: int setgid (gid_t newgid)

This function sets both the real and effective group ID of the process to newgid, provided that the process is privileged. It also deletes the file group ID, if any.
If the process is not privileged, then setgid behaves like setegid.
The return values and error conditions for setgid are the same as those for seteuid.

— Function: int setregid (gid_t rgid, gid_t egid)

This function sets the real group ID of the process to rgid and the effective group ID to egid. If rgid is -1, it means not to change the real group ID; likewise if egid is -1, it means not to change the effective group ID.
The setregid function is provided for compatibility with 4.3 BSD Unix, which does not support file IDs. You can use this function to swap the effective and real group IDs of the process. (Privileged processes are not limited to this usage.) If file IDs are supported, you should use that feature instead of using this function. See Enable/Disable Setuid.
The return values and error conditions for setregid are the same as those for setreuid.

setuid and setgid behave differently depending on whether the effective user ID at the time is zero. If it is not zero, they behave like seteuid and setegid. If it is, they change both effective and real IDs and delete the file ID. To avoid confusion, we recommend you always use seteuid and setegid except when you know the effective user ID is zero and your intent is to change the persona permanently. This case is rare—most of the programs that need it, such as login and su, have already been written.

Note that if your program is setuid to some user other than root, there is no way to drop privileges permanently.

The system also lets privileged processes change their supplementary group IDs. To use setgroups or initgroups, your programs should include the header file grp.h.

— Function: int setgroups (size_t count, gid_t *groups)

This function sets the process's supplementary group IDs. It can only be called from privileged processes. The count argument specifies the number of group IDs in the array groups.
This function returns 0 if successful and -1 on error. The following errno error conditions are defined for this function:

EPERM
The calling process is not privileged.

— Function: int initgroups (const char *user, gid_t group)

The initgroups function sets the process's supplementary group IDs to be the normal default for the user name user. The group group is automatically included.
This function works by scanning the group database for all the groups user belongs to. It then calls setgroups with the list it has constructed.
The return values and error conditions are the same as for setgroups.

If you are interested in the groups a particular user belongs to, but do not want to change the process's supplementary group IDs, you can use getgrouplist. To use getgrouplist, your programs should include the header file grp.h.

— Function: int getgrouplist (const char *user, gid_t group, gid_t *groups, int *ngroups)

The getgrouplist function scans the group database for all the groups user belongs to. Up to *ngroups group IDs corresponding to these groups are stored in the array groups; the return value from the function is the number of group IDs actually stored. If *ngroups is smaller than the total number of groups found, then getgrouplist returns a value of -1 and stores the actual number of groups in *ngroups. The group group is automatically included in the list of groups returned by getgrouplist.
Here's how to use getgrouplist to read all supplementary groups for user:
          gid_t *
          supplementary_groups (char *user)
          {
            int ngroups = 16;
            gid_t *groups
              = (gid_t *) xmalloc (ngroups * sizeof (gid_t));
            struct passwd *pw = getpwnam (user);
          
            if (pw == NULL)
              return NULL;
          
            if (getgrouplist (pw->pw_name, pw->pw_gid, groups, &ngroups) < 0)
              {
                groups = xrealloc (ngroups * sizeof (gid_t));
                getgrouplist (pw->pw_name, pw->pw_gid, groups, &ngroups);
              }
            return groups;
          }
     