SETFSUID(2) Linux Programmer's Manual SETFSUID(2)
setfsuid - set user identity used for file system checks
#include <unistd.h> /* glibc uses <sys/fsuid.h> */
int setfsuid(uid_t fsuid);
The system call 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 the new value of the effective user ID.
Explicit calls to setfsuid() and 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 with-
out 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.)
setfsuid() will succeed only 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
This system call is present in Linux since version 1.2.
setfsuid() 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 EINVAL without attempting 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
The original Linux setfsuid() system call supported only 16-bit user IDs. Subsequently,
Linux 2.4 added setfsuid32() supporting 32-bit IDs. The glibc setfsuid() wrapper function
transparently deals with the variation across kernel versions.
No error messages of any kind are returned to the caller. At the very least, EPERM should
be returned when the call fails (because the caller lacks the CAP_SETUID capability).
kill(2), setfsgid(2), capabilities(7), credentials(7)
This page is part of release 3.53 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
Linux 2010-11-22 SETFSUID(2)