# setuid

## NAME



           setuid - set user identity
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <unistd.h>**
    
           **int setuid(uid_t** _uid_**);**
    


## DESCRIPTION



           **setuid**() sets the effective user ID of the calling process.  If
           the calling process is privileged (more precisely: if the process
           has the **CAP_SETUID** capability in its user namespace), 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: **setuid**() 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).
    


## RETURN VALUE



           On success, zero is returned.  On error, -1 is returned, and _errno_
           is set to indicate the error.
    
           _Note_ : there are cases where **setuid**() can fail even when the caller
           is UID 0; it is a grave security error to omit checking for a
           failure return from **setuid**().
    


## ERRORS



           **EAGAIN** The call would change the caller's real UID (i.e., _uid_ does
                  not match the caller's real UID), but there was a temporary
                  failure allocating the necessary kernel data structures.
    
           **EAGAIN** _uid_ does not match the real user ID of the caller and this
                  call would bring the number of processes belonging to the
                  real user ID _uid_ over the caller's **RLIMIT_NPROC** resource
                  limit.  Since Linux 3.1, this error case no longer occurs
                  (but robust applications should check for this error); see
                  the description of **EAGAIN** in execve(2).
    
           **EINVAL** The user ID specified in _uid_ is not valid in this user
                  namespace.
    
           **EPERM** The user is not privileged (Linux: does not have the
                  **CAP_SETUID** capability in its user namespace) and _uid_ does
                  not match the real UID or saved set-user-ID of the calling
                  process.
    


## VERSIONS



       **C library/kernel differences**
           At the kernel level, user IDs and group IDs are a per-thread
           attribute.  However, POSIX requires that all threads in a process
           share the same credentials.  The NPTL threading implementation
           handles the POSIX requirements by providing wrapper functions for
           the various system calls that change process UIDs and GIDs.  These
           wrapper functions (including the one for **setuid**()) employ a
           signal-based technique to ensure that when one thread changes
           credentials, all of the other threads in the process also change
           their credentials.  For details, see nptl(7).
    


## STANDARDS



           POSIX.1-2008.
    


## NOTES



           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.
    


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