setuid() - Unix, Linux System Call
setuid - set user identity
int setuid(uid_t uid);
setuid() sets the effective user ID of the current process.
If the effective UID of the caller is root,
the real UID and saved set-user-ID are also set.
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 re-engage 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
setuid() function checks the effective user ID of the caller and if it is
the superuser, all process related user IDs are set to
uid. After this has occurred, it is impossible for the program to regain root
Thus, a set-user-ID-root program wishing to temporarily drop root
privileges, assume the identity of a non-root user, and then regain
root privileges afterwards cannot use
setuid(). You can accomplish this with the (non-POSIX, BSD) call
On success, zero is returned. On error, -1 is returned, and
errno is set appropriately.
uid does not match the current uid and
uid brings process over its NPROC rlimit.
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.
Not quite compatible with the 4.4BSD call, which
sets all of the real, saved, and effective user IDs.
Linux has the concept of filesystem user ID, normally equal to the
effective user ID. The
setuid() call also sets the filesystem user ID of the current process.
uid is different from the old effective uid, the process will
be forbidden from leaving core dumps.