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 pro-
gram to regain root privileges.
Thus, a set-user-ID-root program wishing to temporarily drop root priv-
ileges, assume the identity of an unprivileged user, and then regain
root privileges afterward cannot use setuid(). You can accomplish this
On success, zero is returned. On error, -1 is returned, and errno is
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
Linux 2010-11-22 SETUID(2)
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