setfsuid - set user identity used for file system checks
#include <unistd.h> /* glibc uses <sys/fsuid.h> */
int setfsuid(uid_t fsuid);
setfsuid sets the user ID that the Linux kernel uses to
check for all accesses to the file system. Normally, the
value of fsuid will shadow the value of the effective user
ID. In fact, whenever the effective user ID is changed,
fsuid will also be changed to new value of effective user
An explict call to setfsuid is usually only used by pro
grams such as the Linux NFS server that need to change
what user ID is used for file access without a correspond
ing change in the real and effective user 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
from other user IDs.
setfsuid will only succeed if the caller is the superuser
or if fsuid matches either the real user ID, effective
user ID, saved set-user-ID, or the current value of fsuid.
On success, the previous value of fsuid is returned. On
error, the current value of fsuid is returned.
setfsuid 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
uid, it will return -1 and set errno to EINVAL without
attempting the system call.