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.

SEE ALSO

       getegid(),  geteuid(),  getgid(),  getuid(),  setegid(),  seteuid(), setgid(), setregid(),
       setuid()

       The Base Definitions volume of POSIX.1‐2008, <unistd.h>

COPYRIGHT

       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1,  2013  Edition,  Standard  for Information Technology -- Portable Operating System
       Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013  by  the
       Institute  of  Electrical  and  Electronics  Engineers,  Inc and The Open Group.  (This is
       POSIX.1-2008 with the  2013  Technical  Corrigendum  1  applied.)  In  the  event  of  any
       discrepancy  between  this  version and the original IEEE and The Open Group Standard, the
       original IEEE and The Open Group Standard is the referee document. The  original  Standard
       can be obtained online at http://www.unix.org/online.html .

       Any  typographical  or  formatting errors that appear in this page are most likely to have
       been introduced during the conversion of the source files to man page  format.  To  report
       such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .