# setreuid

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## NAME



           setreuid — set real and effective user IDs
    


## SYNOPSIS



           #include <unistd.h>
    
           int setreuid(uid_t _ruid_ , uid_t _euid_);
    


## DESCRIPTION



           The _setreuid_() function shall set the real and effective user IDs
           of the current process to the values specified by the _ruid_ and
           _euid_ arguments. If _ruid_ or _euid_ is -1, the corresponding effective
           or real user ID of the current process shall be left unchanged.
    
           A process with appropriate privileges can set either ID to any
           value.  An unprivileged process can only set the effective user ID
           if the _euid_ argument is equal to either the real, effective, or
           saved user ID of the process.
    
           If the real user ID is being set (_ruid_ is not -1), or the
           effective user ID is being set to a value not equal to the real
           user ID, then the saved set-user-ID of the current process shall
           be set equal to the new effective user ID.
    
           It is unspecified whether a process without appropriate privileges
           is permitted to change the real user ID to match the current
           effective user ID or saved set-user-ID of the process.
    


## RETURN VALUE



           Upon successful completion, 0 shall be returned. Otherwise, -1
           shall be returned and _errno_ set to indicate the error.
    


## ERRORS



           The _setreuid_() function shall fail if:
    
           **EINVAL** The value of the _ruid_ or _euid_ argument is invalid or out-
                  of-range.
    
           **EPERM** The current process does not have appropriate privileges,
                  and either an attempt was made to change the effective user
                  ID to a value other than the real user ID or the saved set-
                  user-ID or an attempt was made to change the real user ID
                  to a value not permitted by the implementation.
    
           _The following sections are informative._
    


## EXAMPLES



       **Setting the Effective User ID to the Real User ID**
           The following example sets the effective user ID of the calling
           process to the real user ID, so that files created later will be
           owned by the current user. It also sets the saved set-user-ID to
           the real user ID, so any future attempt to set the effective user
           ID back to its previous value will fail.
    
               #include <unistd.h>
               #include <sys/types.h>
               ...
               setreuid(getuid(), getuid());
               ...
    


## APPLICATION USAGE



           None.
    


## RATIONALE



           Earlier versions of this standard did not specify whether the
           saved set-user-ID was affected by _setreuid_() calls. This version
           specifies common existing practice that constitutes an important
           security feature. The ability to set both the effective user ID
           and saved set-user-ID to be the same as the real user ID means
           that any security weakness in code that is executed after that
           point cannot result in malicious code being executed with the
           previous effective user ID. Privileged applications could already
           do this using just _setuid_(), but for non-privileged applications
           the only standard method available is to use this feature of
           _setreuid_().
    


## FUTURE DIRECTIONS



           None.
    


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