1SETUID(2)                  Linux Programmer's Manual                 SETUID(2)
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NAME

6       setuid - set user identity
7

SYNOPSIS

9       #include <sys/types.h>
10       #include <unistd.h>
11
12       int setuid(uid_t uid);
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DESCRIPTION

15       setuid()  sets  the  effective  user ID of the calling process.  If the
16       calling process is privileged (more precisely: if the process  has  the
17       CAP_SETUID  capability  in  its user namespace), the real UID and saved
18       set-user-ID are also set.
19
20       Under Linux, setuid() is implemented like the POSIX  version  with  the
21       _POSIX_SAVED_IDS  feature.  This allows a set-user-ID (other than root)
22       program to drop all of its user privileges, do some un-privileged work,
23       and then reengage the original effective user ID in a secure manner.
24
25       If  the  user  is root or the program is set-user-ID-root, special care
26       must be taken: setuid() checks the effective user ID of the caller  and
27       if  it  is the superuser, all process-related user ID's are set to uid.
28       After this has occurred, it is impossible for  the  program  to  regain
29       root privileges.
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31       Thus, a set-user-ID-root program wishing to temporarily drop root priv‐
32       ileges, assume the identity of an unprivileged user,  and  then  regain
33       root privileges afterward cannot use setuid().  You can accomplish this
34       with seteuid(2).
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RETURN VALUE

37       On success, zero is returned.  On error, -1 is returned, and  errno  is
38       set appropriately.
39
40       Note:  there  are cases where setuid() can fail even when the caller is
41       UID 0; it is a grave security error to omit checking for a failure  re‐
42       turn from setuid().
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ERRORS

45       EAGAIN The  call would change the caller's real UID (i.e., uid does not
46              match the caller's real UID), but there was a temporary  failure
47              allocating the necessary kernel data structures.
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49       EAGAIN uid  does not match the real user ID of the caller and this call
50              would bring the number of processes belonging to the  real  user
51              ID  uid  over  the  caller's RLIMIT_NPROC resource limit.  Since
52              Linux 3.1, this error case no longer occurs (but robust applica‐
53              tions  should  check for this error); see the description of EA‐
54              GAIN in execve(2).
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56       EINVAL The user ID specified in uid is not valid  in  this  user  name‐
57              space.
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59       EPERM  The  user is not privileged (Linux: does not have the CAP_SETUID
60              capability in its user namespace) and uid  does  not  match  the
61              real UID or saved set-user-ID of the calling process.
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CONFORMING TO

64       POSIX.1-2001, POSIX.1-2008, SVr4.  Not quite compatible with the 4.4BSD
65       call, which sets all of the real, saved, and effective user IDs.
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NOTES

68       Linux has the concept of the filesystem user ID, normally equal to  the
69       effective  user ID.  The setuid() call also sets the filesystem user ID
70       of the calling process.  See setfsuid(2).
71
72       If uid is different from the old effective UID,  the  process  will  be
73       forbidden from leaving core dumps.
74
75       The original Linux setuid() system call supported only 16-bit user IDs.
76       Subsequently, Linux 2.4 added setuid32() supporting  32-bit  IDs.   The
77       glibc  setuid() wrapper function transparently deals with the variation
78       across kernel versions.
79
80   C library/kernel differences
81       At the kernel level, user IDs and group IDs are a per-thread attribute.
82       However,  POSIX  requires  that all threads in a process share the same
83       credentials.  The NPTL threading implementation handles the  POSIX  re‐
84       quirements  by providing wrapper functions for the various system calls
85       that change process UIDs and GIDs.  These wrapper functions  (including
86       the  one  for  setuid()) employ a signal-based technique to ensure that
87       when one thread changes credentials, all of the other  threads  in  the
88       process also change their credentials.  For details, see nptl(7).
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SEE ALSO

91       getuid(2),  seteuid(2), setfsuid(2), setreuid(2), capabilities(7), cre‐
92       dentials(7), user_namespaces(7)
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COLOPHON

95       This page is part of release 5.10 of the Linux  man-pages  project.   A
96       description  of  the project, information about reporting bugs, and the
97       latest    version    of    this    page,    can     be     found     at
98       https://www.kernel.org/doc/man-pages/.
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102Linux                             2019-03-06                         SETUID(2)
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