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

6       setuid - set user identity
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SYNOPSIS

9       #include <sys/types.h>
10       #include <unistd.h>
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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       effective UID of the caller is root, the real UID and saved set-user-ID
17       are also set.
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19       Under  Linux,  setuid()  is implemented like the POSIX version with the
20       _POSIX_SAVED_IDS feature.  This allows a set-user-ID (other than  root)
21       program to drop all of its user privileges, do some un-privileged work,
22       and then reengage the original effective user ID in a secure manner.
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24       If the user is root or the program is  set-user-ID-root,  special  care
25       must  be  taken.  The setuid() function checks the effective user ID of
26       the caller and if it is the superuser, all  process-related  user  ID's
27       are set to uid.  After this has occurred, it is impossible for the pro‐
28       gram to regain root privileges.
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30       Thus, a set-user-ID-root program wishing to temporarily drop root priv‐
31       ileges,  assume  the  identity of an unprivileged user, and then regain
32       root privileges afterward cannot use setuid().  You can accomplish this
33       with seteuid(2).
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RETURN VALUE

36       On  success,  zero is returned.  On error, -1 is returned, and errno is
37       set appropriately.
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ERRORS

40       EAGAIN The uid does not match the current uid and  uid  brings  process
41              over its RLIMIT_NPROC resource limit.
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43       EPERM  The  user is not privileged (Linux: does not have the CAP_SETUID
44              capability) and uid does not match the real UID  or  saved  set-
45              user-ID of the calling process.
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CONFORMING TO

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

52       Linux has the concept of the file system user ID, normally equal to the
53       effective user ID.  The setuid() call also sets the file system user ID
54       of the calling process.  See setfsuid(2).
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56       If uid is different from the old effective UID,  the  process  will  be
57       forbidden from leaving core dumps.
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59       The original Linux setuid() system call supported only 16-bit user IDs.
60       Subsequently, Linux 2.4 added setuid32() supporting  32-bit  IDs.   The
61       glibc  setuid() wrapper function transparently deals with the variation
62       across kernel versions.
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SEE ALSO

65       getuid(2), seteuid(2), setfsuid(2), setreuid(2), capabilities(7),  cre‐
66       dentials(7)
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COLOPHON

69       This  page  is  part of release 3.53 of the Linux man-pages project.  A
70       description of the project, and information about reporting  bugs,  can
71       be found at http://www.kernel.org/doc/man-pages/.
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75Linux                             2010-11-22                         SETUID(2)
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