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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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SETUID(3P) POSIX Programmer's Manual SETUID(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
setuid — set user ID
#include <unistd.h>
int setuid(uid_t uid);
If the process has appropriate privileges, setuid() shall set the
real user ID, effective user ID, and the saved set-user-ID of the
calling process to uid.
If the process does not have appropriate privileges, but uid is
equal to the real user ID or the saved set-user-ID, setuid() shall
set the effective user ID to uid; the real user ID and saved set-
user-ID shall remain unchanged.
The setuid() function shall not affect the supplementary group
list in any way.
Upon successful completion, 0 shall be returned. Otherwise, -1
shall be returned and errno set to indicate the error.
The setuid() function shall fail, return -1, and set errno to the
corresponding value if one or more of the following are true:
EINVAL The value of the uid argument is invalid and not supported
by the implementation.
EPERM The process does not have appropriate privileges and uid
does not match the real user ID or the saved set-user-ID.
The following sections are informative.
None.
None.
The various behaviors of the setuid() and setgid() functions when
called by non-privileged processes reflect the behavior of
different historical implementations. For portability, it is
recommended that new non-privileged applications use the seteuid()
and setegid() functions instead.
The saved set-user-ID capability allows a program to regain the
effective user ID established at the last exec call. Similarly,
the saved set-group-ID capability allows a program to regain the
effective group ID established at the last exec call. These
capabilities are derived from System V. Without them, a program
might have to run as superuser in order to perform the same
functions, because superuser can write on the user's files. This
is a problem because such a program can write on any user's files,
and so must be carefully written to emulate the permissions of the
calling process properly. In System V, these capabilities have
traditionally been implemented only via the setuid() and setgid()
functions for non-privileged processes. The fact that the behavior
of those functions was different for privileged processes made
them difficult to use. The POSIX.1‐1990 standard defined the
setuid() function to behave differently for privileged and
unprivileged users. When the caller had appropriate privileges,
the function set the real user ID, effective user ID, and saved
set-user ID of the calling process on implementations that
supported it. When the caller did not have appropriate privileges,
the function set only the effective user ID, subject to permission
checks. The former use is generally needed for utilities like
login and su, which are not conforming applications and thus
outside the scope of POSIX.1‐2008. These utilities wish to change
the user ID irrevocably to a new value, generally that of an
unprivileged user. The latter use is needed for conforming
applications that are installed with the set-user-ID bit and need
to perform operations using the real user ID.
POSIX.1‐2008 augments the latter functionality with a mandatory
feature named _POSIX_SAVED_IDS. This feature permits a set-user-ID
application to switch its effective user ID back and forth between
the values of its exec-time real user ID and effective user ID.
Unfortunately, the POSIX.1‐1990 standard did not permit a
conforming application using this feature to work properly when it
happened to be executed with (implementation-defined) appropriate
privileges. Furthermore, the application did not even have a means
to tell whether it had this privilege. Since the saved set-user-ID
feature is quite desirable for applications, as evidenced by the
fact that NIST required it in FIPS 151‐2, it has been mandated by
POSIX.1‐2008. However, there are implementors who have been
reluctant to support it given the limitation described above.
The 4.3BSD system handles the problem by supporting separate
functions: setuid() (which always sets both the real and effective
user IDs, like setuid() in POSIX.1‐2008 for privileged users), and
seteuid() (which always sets just the effective user ID, like
setuid() in POSIX.1‐2008 for non-privileged users). This
separation of functionality into distinct functions seems
desirable. 4.3BSD does not support the saved set-user-ID feature.
It supports similar functionality of switching the effective user
ID back and forth via setreuid(), which permits reversing the real
and effective user IDs. This model seems less desirable than the
saved set-user-ID because the real user ID changes as a side-
effect. The current 4.4BSD includes saved effective IDs and uses
them for seteuid() and setegid() as described above. The
setreuid() and setregid() functions will be deprecated or removed.
The solution here is:
* Require that all implementations support the functionality of
the saved set-user-ID, which is set by the exec functions and
by privileged calls to setuid().
* Add the seteuid() and setegid() functions as portable
alternatives to setuid() and setgid() for non-privileged and
privileged processes.
Historical systems have provided two mechanisms for a set-user-ID
process to change its effective user ID to be the same as its real
user ID in such a way that it could return to the original
effective user ID: the use of the setuid() function in the
presence of a saved set-user-ID, or the use of the BSD setreuid()
function, which was able to swap the real and effective user IDs.
The changes included in POSIX.1‐2008 provide a new mechanism using
seteuid() in conjunction with a saved set-user-ID. Thus, all
implementations with the new seteuid() mechanism will have a saved
set-user-ID for each process, and most of the behavior controlled
by _POSIX_SAVED_IDS has been changed to agree with the case where
the option was defined. The kill() function is an exception.
Implementors of the new seteuid() mechanism will generally be
required to maintain compatibility with the older mechanisms
previously supported by their systems. However, compatibility with
this use of setreuid() and with the _POSIX_SAVED_IDS behavior of
kill() is unfortunately complicated. If an implementation with a
saved set-user-ID allows a process to use setreuid() to swap its
real and effective user IDs, but were to leave the saved set-user-
ID unmodified, the process would then have an effective user ID
equal to the original real user ID, and both real and saved set-
user-ID would be equal to the original effective user ID. In that
state, the real user would be unable to kill the process, even
though the effective user ID of the process matches that of the
real user, if the kill() behavior of _POSIX_SAVED_IDS was used.
This is obviously not acceptable. The alternative choice, which is
used in at least one implementation, is to change the saved set-
user-ID to the effective user ID during most calls to setreuid().
The standard developers considered that alternative to be less
correct than the retention of the old behavior of kill() in such
systems. Current conforming applications shall accommodate either
behavior from kill(), and there appears to be no strong reason for
kill() to check the saved set-user-ID rather than the effective
user ID.
None.
exec(1p), getegid(3p), geteuid(3p), getgid(3p), getuid(3p),
setegid(3p), seteuid(3p), setgid(3p), setregid(3p), setreuid(3p)
The Base Definitions volume of POSIX.1‐2017, sys_types.h(0p),
unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 SETUID(3P)
Pages that refer to this page: unistd.h(0p), getegid(3p), geteuid(3p), getgid(3p), getuid(3p), posix_spawn(3p), setegid(3p), seteuid(3p), setgid(3p), setregid(3p), setreuid(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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