The system call
sets the user ID that the Linux kernel uses to check for all accesses
to the file system.
Normally, the value of
will shadow the value of the effective user ID.
In fact, whenever the
effective user ID is changed,
will also be changed to the new value of the effective user ID.
Explicit calls to
are usually only used 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 only succeed if the caller is the superuser or if
matches either the real user ID, effective user ID, saved set-user-ID, or
the current value of
On success, the previous value of
On error, the current value of
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 user ID,
it will return -1 and set errno to
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.
No error messages of any kind are returned to the caller.
At the very
should be returned when the call fails (because the caller lacks the