Section: Linux Programmer's Manual (2)
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setfsgid - set group identity used for filesystem checks
int setfsgid(uid_t fsgid);
The system call
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
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.)
will succeed only if the caller is the superuser or if
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.
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
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
system call supported only 16-bit group IDs.
Subsequently, Linux 2.4 added
supporting 32-bit IDs.
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
(which will always fail), in order to determine if a preceding call to
changed the filesystem group ID.
At the very
should be returned when the call fails (because the caller lacks the
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- RETURN VALUE
- CONFORMING TO
- SEE ALSO