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.



The uid does not match the current uid and uid brings process over its RLIMIT_NPROC resource limit.


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 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.

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 3.54 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at−pages/.

  Copyright (C), 1994, Graeme W. Wilford (Wilf).

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Fri Jul 29th 12:56:44 BST 1994  Wilf. <>
Changes inspired by patch from Richard Kettlewell
  <>, aeb 970616.
Modified, 27 May 2004, Michael Kerrisk <>
    Added notes on capability requirements