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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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setuid(2) System Calls Manual setuid(2)
setuid - set user identity
Standard C library (libc, -lc)
#include <unistd.h>
int setuid(uid_t uid);
setuid() sets the effective user ID of the calling process. If
the calling process is privileged (more precisely: if the process
has the CAP_SETUID 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 _POSIX_SAVED_IDS 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: setuid() checks the effective user ID of the
caller and if it is the superuser, all process-related user ID's
are set to uid. 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 seteuid(2).
On success, zero is returned. On error, -1 is returned, and errno
is set to indicate the error.
Note: 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 from setuid().
EAGAIN 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.
EAGAIN 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 execve(2).
EINVAL The user ID specified in uid is not valid in this user
namespace.
EPERM The user is not privileged (Linux: does not have the
CAP_SETUID capability in its user namespace) and uid does
not match the real UID or saved set-user-ID of the calling
process.
C library/kernel differences
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(7).
POSIX.1-2008.
POSIX.1-2001, SVr4.
Not quite compatible with the 4.4BSD call, which sets all of the
real, saved, and effective user IDs.
The original Linux setuid() system call supported only 16-bit user
IDs. Subsequently, Linux 2.4 added setuid32() supporting 32-bit
IDs. The glibc setuid() wrapper function transparently deals with
the variation across kernel versions.
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(2).
If uid is different from the old effective UID, the process will
be forbidden from leaving core dumps.
getuid(2), seteuid(2), setfsuid(2), setreuid(2), capabilities(7),
credentials(7), user_namespaces(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 setuid(2)
Pages that refer to this page: capsh(1), access(2), execve(2), getresuid(2), getuid(2), seccomp(2), seteuid(2), setresuid(2), setreuid(2), syscalls(2), vfork(2), cap_get_proc(3), euidaccess(3), posix_spawn(3), systemd.exec(5), capabilities(7), credentials(7), nptl(7), signal-safety(7), user_namespaces(7)
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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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