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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
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 sig-
nal permission handling is slightly different. See setfsuid(2) for a
discussion of why the use of both setfsuid(2) and setfsgid() is nowa-
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.
Linux 2013-08-08 SETFSGID(2)