1SETREUID(3P)               POSIX Programmer's Manual              SETREUID(3P)
2
3
4

PROLOG

6       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
7       implementation of this interface may differ (consult the  corresponding
8       Linux  manual page for details of Linux behavior), or the interface may
9       not be implemented on Linux.
10

NAME

12       setreuid — set real and effective user IDs
13

SYNOPSIS

15       #include <unistd.h>
16
17       int setreuid(uid_t ruid, uid_t euid);
18

DESCRIPTION

20       The setreuid() function shall set the real and effective  user  IDs  of
21       the  current process to the values specified by the ruid and euid argu‐
22       ments. If ruid or euid is -1, the corresponding effective or real  user
23       ID of the current process shall be left unchanged.
24
25       A  process  with appropriate privileges can set either ID to any value.
26       An unprivileged process can only set the effective user ID if the  euid
27       argument  is  equal  to either the real, effective, or saved user ID of
28       the process.
29
30       If the real user ID is being set (ruid is not  -1),  or  the  effective
31       user ID is being set to a value not equal to the real user ID, then the
32       saved set-user-ID of the current process shall be set equal to the  new
33       effective user ID.
34
35       It  is  unspecified whether a process without appropriate privileges is
36       permitted to change the real user ID to  match  the  current  effective
37       user ID or saved set-user-ID of the process.
38

RETURN VALUE

40       Upon successful completion, 0 shall be returned. Otherwise, -1 shall be
41       returned and errno set to indicate the error.
42

ERRORS

44       The setreuid() function shall fail if:
45
46       EINVAL The value of the ruid or euid argument  is  invalid  or  out-of-
47              range.
48
49       EPERM  The  current  process  does not have appropriate privileges, and
50              either an attempt was made to change the effective user ID to  a
51              value other than the real user ID or the saved set-user-ID or an
52              attempt was made to change the real user ID to a value not  per‐
53              mitted by the implementation.
54
55       The following sections are informative.
56

EXAMPLES

58   Setting the Effective User ID to the Real User ID
59       The following example sets the effective user ID of the calling process
60       to the real user ID, so that files created later will be owned  by  the
61       current  user.  It also sets the saved set-user-ID to the real user ID,
62       so any future attempt to set the effective user ID back to its previous
63       value will fail.
64
65
66           #include <unistd.h>
67           #include <sys/types.h>
68           ...
69           setreuid(getuid(), getuid());
70           ...
71

APPLICATION USAGE

73       None.
74

RATIONALE

76       Earlier  versions  of  this  standard did not specify whether the saved
77       set-user-ID was affected by setreuid() calls.  This  version  specifies
78       common  existing  practice  that constitutes an important security fea‐
79       ture. The ability to set both the effective user ID and saved set-user-
80       ID  to be the same as the real user ID means that any security weakness
81       in code that is executed after that point cannot  result  in  malicious
82       code  being  executed  with  the previous effective user ID. Privileged
83       applications could already do this using just setuid(),  but  for  non-
84       privileged  applications  the  only standard method available is to use
85       this feature of setreuid().
86

FUTURE DIRECTIONS

88       None.
89

SEE ALSO

91       getegid(), geteuid(), getgid(), getuid(),  setegid(),  seteuid(),  set‐
92       gid(), setregid(), setuid()
93
94       The Base Definitions volume of POSIX.1‐2017, <unistd.h>
95
97       Portions  of  this text are reprinted and reproduced in electronic form
98       from IEEE Std 1003.1-2017, Standard for Information Technology --  Por‐
99       table  Operating System Interface (POSIX), The Open Group Base Specifi‐
100       cations Issue 7, 2018 Edition, Copyright (C) 2018 by the  Institute  of
101       Electrical  and  Electronics Engineers, Inc and The Open Group.  In the
102       event of any discrepancy between this version and the original IEEE and
103       The  Open Group Standard, the original IEEE and The Open Group Standard
104       is the referee document. The original Standard can be  obtained  online
105       at http://www.opengroup.org/unix/online.html .
106
107       Any  typographical  or  formatting  errors that appear in this page are
108       most likely to have been introduced during the conversion of the source
109       files  to  man page format. To report such errors, see https://www.ker
110       nel.org/doc/man-pages/reporting_bugs.html .
111
112
113
114IEEE/The Open Group                  2017                         SETREUID(3P)
Impressum