setuid - set user identity
int setuid(uid_t uid);
() sets the effective user ID of the calling process. If the
calling process is privileged (more precisely: if the process has the
capability in its user namespace), the real UID and saved
set-user-ID are also set.
Under Linux, setuid
() is implemented like the POSIX version with the
feature. This allows a set-user-ID (other than root)
program to drop all of its user privileges, do some un-privileged work, and
then reengage the original effective user ID in a secure manner.
If the user is root or the program is set-user-ID-root, special care must be
taken. The setuid
() function checks the effective user ID of the caller
and if it is the superuser, all process-related user ID's are set to
. After this has occurred, it is impossible for the program to
regain root privileges.
Thus, a set-user-ID-root program wishing to temporarily drop root privileges,
assume the identity of an unprivileged user, and then regain root privileges
afterward cannot use setuid
(). You can accomplish this with
On success, zero is returned. On error, -1 is returned, and errno
: there are cases where setuid
() can fail even when the caller
is UID 0; it is a grave security error to omit checking for a failure return
- The call would change the caller's real UID (i.e.,
uid does not match the caller's real UID), but there was a
temporary failure allocating the necessary kernel data structures.
- uid does not match the real user ID of the caller
and this call would bring the number of processes belonging to the real
user ID uid over the caller's RLIMIT_NPROC resource limit.
Since Linux 3.1, this error case no longer occurs (but robust applications
should check for this error); see the description of EAGAIN in
- The user ID specified in uid is not valid in this
- The user is not privileged (Linux: does not have the
CAP_SETUID capability) and uid does not match the real UID
or saved set-user-ID of the calling process.
POSIX.1-2001, POSIX.1-2008, SVr4. Not quite compatible with the 4.4BSD call,
which sets all of the real, saved, and effective user IDs.
Linux has the concept of the filesystem user ID, normally equal to the effective
user ID. The setuid
() call also sets the filesystem user ID of the
calling process. See setfsuid
is different from the old effective UID, the process will be
forbidden from leaving core dumps.
The original Linux setuid
() system call supported only 16-bit user IDs.
Subsequently, Linux 2.4 added setuid32
() supporting 32-bit IDs. The
() wrapper function transparently deals with the variation
across kernel versions.
At the kernel level, user IDs and group IDs are a per-thread attribute. However,
POSIX requires that all threads in a process share the same credentials. The
NPTL threading implementation handles the POSIX requirements by providing
wrapper functions for the various system calls that change process UIDs and
GIDs. These wrapper functions (including the one for setuid
()) employ a
signal-based technique to ensure that when one thread changes credentials, all
of the other threads in the process also change their credentials. For
details, see nptl
This page is part of release 4.13 of the Linux man-pages
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