SETFSGID(2) Linux Programmer's Manual SETFSGID(2)
setfsgid - set group identity used for filesystem checks
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. Nor-
mally, 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 with-
out 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
On both success and failure, this call returns the previous filesystem group ID of the
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 setfs-
gid() 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 suc-
cessful 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 look-
ing 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 3.55 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
Linux 2013-08-08 SETFSGID(2)