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NAMEsetfsgid - set group identity used for filesystem checks
int setfsgid(uid_t fsgid);
DESCRIPTIONThe 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.
RETURN VALUEOn both success and failure, this call returns the previous filesystem group ID of the caller.
VERSIONSThis system call is present in Linux since version 1.2.
CONFORMING TOsetfsgid() is Linux-specific and should not be used in programs intended to be portable.
NOTESWhen 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.
for a discussion of why the use of both
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.