setfsuid - set user identity used for file system checks
#include /* glibc uses
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
An explict call to setfsuid is usually only used by
programs such as the Linux NFS server that need to change
what user ID is used for file access without a corresponding
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
setfsuid is Linux specific and should not be used in
programs 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
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.