setuid(2)
NAME
setuid - set user identity
SYNOPSIS
#include <sys/types.h> #include <unistd.h> int setuid(uid_t uid);
DESCRIPTION
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 afterwards cannot use setuid(). You can accomplish
this with seteuid(2).
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
ERRORS
- 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 setuser-ID of the calling process.
CONFORMING TO
SVr4, POSIX.1-2001. Not quite compatible with the 4.4BSD call, which
sets all of the real, saved, and effective user IDs.
NOTES
- Linux Notes
- Linux has the concept of 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.
SEE ALSO
getuid(2), seteuid(2), setfsuid(2), setreuid(2), capabilities(7), credentials(7)
COLOPHON
- This page is part of release 3.25 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/.