setfsgid — set group identity used for filesystem checks


#include <sys/fsuid.h>
int setfsgid( uid_t fsgid);


The system call setfsgid() changes the value of the caller's filesystem group ID—the group ID that the Linux kernel uses to check for all accesses to the filesystem. Normally, the value of the filesystem group ID will shadow the value of the effective group ID. In fact, whenever the effective group ID is changed, the filesystem group ID will also be changed to the new value of the effective group ID.

Explicit calls to setfsuid(2) and setfsgid() are usually used only by programs such as the Linux NFS server that need to change what user and group ID is used for file access without a corresponding change in the real and effective user and group IDs. A change in the normal user IDs for a program such as the NFS server is a security hole that can expose it to unwanted signals. (But see below.)

setfsgid() will succeed only if the caller is the superuser or if fsgid matches either the caller's real group ID, effective group ID, saved set-group-ID, or current the filesystem user ID.


On both success and failure, this call returns the previous filesystem group ID of the caller.


This system call is present in Linux since version 1.2.


setfsgid() is Linux-specific and should not be used in programs intended to be portable.


When glibc determines that the argument is not a valid group ID, it will return −1 and set errno to EINVAL without attempting the system call.

Note that at the time this system call was introduced, a process could send a signal to a process with the same effective user ID. Today signal permission handling is slightly different. See setfsuid(2) for a discussion of why the use of both setfsuid(2) and setfsgid() is nowadays unneeded.

The original Linux setfsgid() system call supported only 16-bit group IDs. Subsequently, Linux 2.4 added setfsgid32() supporting 32-bit IDs. The glibc setfsgid() wrapper function transparently deals with the variation across kernel versions.


No error indications of any kind are returned to the caller, and the fact that both successful and unsuccessful calls return the same value makes it impossible to directly determine whether the call succeeded or failed. Instead, the caller must resort to looking at the return value from a further call such as setfsgid(−1) (which will always fail), in order to determine if a preceding call to setfsgid() changed the filesystem group ID. At the very least, EPERM should be returned when the call fails (because the caller lacks the CAP_SETGID capability).


kill(2), setfsuid(2), capabilities(7), credentials(7)


This page is part of release 3.54 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at−pages/.

  Copyright (C) 1995, Thomas K. Dyas <>

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.

Created   1995-08-06 Thomas K. Dyas <>
Modified  2000-07-01 aeb
Modified  2002-07-23 aeb
Modified, 27 May 2004, Michael Kerrisk <>
    Added notes on capability requirements