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SETUID(2) Linux Programmer's Manual SETUID(2)
setuid - set user identity
#include <sys/types.h>
#include <unistd.h>
int setuid(uid_t uid);
setuid() sets the effective user ID of the calling process. If the effective
UID of the caller is root, 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. 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 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
appropriately.
EAGAIN The uid does not match the current uid and uid brings process over its
RLIMIT_NPROC resource limit.
EPERM 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.
SVr4, POSIX.1-2001. 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 file system user ID, normally equal to the
effective user ID. The setuid() call also sets the file system 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.
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.
getuid(2), seteuid(2), setfsuid(2), setreuid(2), capabilities(7),
credentials(7)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2010-11-22 SETUID(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface