setfsgid - set group identity used for file system checks
#include <unistd.h> /* glibc uses <sys/fsuid.h> */
int setfsgid(uid_t fsgid);
setfsgid sets the group ID that the Linux kernel uses to
check for all accesses to the file system. Normally, the
value of fsgid will shadow the value of the effective
group ID. In fact, whenever the effective group ID is
changed, fsgid will also be changed to new value of effec
tive group ID.
An explicit call to setfsgid is usually only used by pro
grams such as the Linux NFS server that need to change
what group ID is used for file access without a corre
sponding change in the real and effective group IDs. A
change in the normal group IDs for a program such as the
NFS server is a security hole that can expose it to
unwanted signals from other group IDs.
setfsgid will only succeed if the caller is the superuser
or if fsgid matches either the real group ID, effective
group ID, saved set-group-ID, or the current value of
On success, the previous value of fsgid is returned. On
error, the current value of fsgid is returned.
setfsgid is Linux specific and should not be used in pro
grams intended to be portable.
No error messages of any kind are returned to the caller.
At the very least, EPERM should be returned when the call
When glibc determines that the argument is not a valid
gid, it will return -1 and set errno to EINVAL without
attempting the system call.