.\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
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.\" Created   1995-08-06 Thomas K. Dyas <tdyas@eden.rutgers.edu>
.\" Modified  2000-07-01 aeb
.\" Modified  2002-07-23 aeb
.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"     Added notes on capability requirements
.\"
.TH SETFSUID 2 2010-11-22 "Linux" "Linux Programmer's Manual"
.SH NAME
setfsuid \- set user identity used for file system checks
.SH SYNOPSIS
.B #include <unistd.h>
/* glibc uses <sys/fsuid.h> */
.sp
.BI "int setfsuid(uid_t " fsuid );
.SH DESCRIPTION
The system call
.BR setfsuid ()
sets the user ID that the Linux kernel uses to check for all accesses
to the file system.
Normally, the value of
.I fsuid
will shadow the value of the effective user ID.
In fact, whenever the
effective user ID is changed,
.I fsuid
will also be changed to the new value of the effective user ID.

Explicit calls to
.BR setfsuid ()
and
.BR setfsgid (2)
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.)

.BR setfsuid ()
will succeed only if the caller is the superuser or if
.I fsuid
matches either the real user ID, effective user ID, saved set-user-ID, or
the current value of
.IR fsuid .
.SH RETURN VALUE
On success, the previous value of
.I fsuid
is returned.
On error, the current value of
.I fsuid
is returned.
.SH VERSIONS
This system call is present in Linux since version 1.2.
.\" This system call is present since Linux 1.1.44
.\" and in libc since libc 4.7.6.
.SH CONFORMING TO
.BR setfsuid ()
is Linux-specific and should not be used in programs intended
to be portable.
.SH NOTES
When glibc determines that the argument is not a valid user ID,
it will return \-1 and set \fIerrno\fP to
.B EINVAL
without attempting
the system call.
.LP
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.

The original Linux
.BR setfsuid ()
system call supported only 16-bit user IDs.
Subsequently, Linux 2.4 added
.BR setfsuid32 ()
supporting 32-bit IDs.
The glibc
.BR setfsuid ()
wrapper function transparently deals with the variation across kernel versions.
.SH BUGS
No error messages of any kind are returned to the caller.
At the very
least,
.B EPERM
should be returned when the call fails (because the caller lacks the
.B CAP_SETUID
capability).
.SH SEE ALSO
.BR kill (2),
.BR setfsgid (2),
.BR capabilities (7),
.BR credentials (7)
.SH COLOPHON
This page is part of release 3.53 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
\%http://www.kernel.org/doc/man\-pages/.
