setuid - set user identity


   #include <sys/types.h>
   #include <unistd.h>

   int setuid(uid_t uid);


   setuid()  sets  the  effective  user ID of the calling process.  If the
   calling process is privileged (more precisely: if the process  has  the
   CAP_SETUID  capability  in  its user namespace), the real UID and saved
   set-user-ID are also set.

   Under Linux, setuid() is implemented like the POSIX  version  with  the
   _POSIX_SAVED_IDS  feature.  This allows a set-user-ID (other than root)
   program to drop all of its user privileges, do some un-privileged work,
   and then reengage the original effective user ID in a secure manner.

   If  the  user  is root or the program is set-user-ID-root, special care
   must be taken.  The setuid() function checks the effective user  ID  of
   the  caller  and  if it is the superuser, all process-related user ID's
   are set to uid.  After this has occurred,  it  is  impossible  for  the
   program to regain root privileges.

   Thus,  a  set-user-ID-root  program  wishing  to  temporarily drop root
   privileges, assume the identity  of  an  unprivileged  user,  and  then
   regain   root  privileges  afterward  cannot  use  setuid().   You  can
   accomplish this with seteuid(2).


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

   Note:  there  are cases where setuid() can fail even when the caller is
   UID 0; it is a grave security error to  omit  checking  for  a  failure
   return from setuid().


   EAGAIN The  call would change the caller's real UID (i.e., uid does not
          match the caller's real UID), but there was a temporary  failure
          allocating the necessary kernel data structures.

   EAGAIN uid  does not match the real user ID of the caller and this call
          would bring the number of processes belonging to the  real  user
          ID  uid  over  the  caller's RLIMIT_NPROC resource limit.  Since
          Linux  3.1,  this  error  case  no  longer  occurs  (but  robust
          applications  should  check for this error); see the description
          of EAGAIN in execve(2).

   EINVAL The user  ID  specified  in  uid  is  not  valid  in  this  user

   EPERM  The  user is not privileged (Linux: does not have the CAP_SETUID
          capability) and uid does not match the real UID  or  saved  set-
          user-ID of the calling process.


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


   Linux has the concept of the filesystem user ID, normally equal to  the
   effective  user ID.  The setuid() call also sets the filesystem user ID
   of the calling process.  See setfsuid(2).

   If uid is different from the old effective UID,  the  process  will  be
   forbidden from leaving core dumps.

   The original Linux setuid() system call supported only 16-bit user IDs.
   Subsequently, Linux 2.4 added setuid32() supporting  32-bit  IDs.   The
   glibc  setuid() wrapper function transparently deals with the variation
   across kernel versions.

   C library/kernel differences
   At the kernel level, user IDs and group IDs are a per-thread attribute.
   However,  POSIX  requires  that all threads in a process share the same
   credentials.  The  NPTL  threading  implementation  handles  the  POSIX
   requirements  by  providing  wrapper  functions  for the various system
   calls that change process  UIDs  and  GIDs.   These  wrapper  functions
   (including  the  one  for  setuid()) employ a signal-based technique to
   ensure that when one thread  changes  credentials,  all  of  the  other
   threads in the process also change their credentials.  For details, see


   getuid(2),  seteuid(2),  setfsuid(2),   setreuid(2),   capabilities(7),
   credentials(7), user_namespaces(7)


   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at

More Linux Commands

newaliases(1) - Postfix to Sendmail compatibility interface
The Postfix sendmail(1) command implements the Postfix to Sendmail compatibility interface. For the sake of compatibility with existing applications, some Sendm

dup(2) - duplicate a file descriptor - Linux manual page....
These system calls create a copy of the file descriptor oldfd. dup() uses the lowest-numbered unused descriptor for the new descriptor. dup2() makes newfd be th

start_color_sp(3ncurses) - curses screen-pointer extension
This implementation can be configured to provide a set of functions which improve the ability to manage multiple screens. This feature can be added to any of th

change_hat(2) - change to or from a "hat" within a AppArmor
An AppArmor profile applies to an executable program; if a portion of the program needs different access permissions than other portions, the program can change

getsubopt(3) - parse suboption arguments from a string......
getsubopt() parses the list of comma-separated suboptions provided in optionp. (Such a suboption list is typically produced when getopt(3) is used to parse a co


XtRegisterDrawable(3) - register a drawable with the Intrins
XtRegisterDrawable associates the specified drawable with the specified widget so that future calls to XtWindowToWidget with the drawable will return the widget

clnt_destroy(3) - library routines for remote procedure call
These routines allow C programs to make procedure calls on other machines across the network. First, the client calls a procedure to send a data packet to the s

zic(8) - timezone compiler (Administration - Linux man page)
Zic reads text from the file(s) named on the command line and creates the time conversion information files specified in this input. If a filename is -, the sta

snmpd.conf(5) - configuration file for the Net-SNMP SNMP age
The Net-SNMP agent uses one or more configuration files to control its operation and the management information provided. These files (snmpd.conf and snmpd.loca

TIFFPrintDirectory(3tiff) - print a description of a TIFF di
TIFFPrintDirectory prints a description of the current directory in the specified TIFF file to the standard I/O output stream fd. The flags parameter is used to

gnutls_openpgp_privkey_get_subkey_expiration_time.3 - Get subkey expiration time. A value of 0 means that the key doesnt expire at all. RETURNS the time when th

We can't live, work or learn in freedom unless the software we use is free.