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EXIT(3P)                   POSIX Programmer's Manual                  EXIT(3P)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may dif-
       fer (consult the corresponding Linux manual page for details of Linux behavior), or the interface  may  not  be
       implemented on Linux.

NAME
       exit, _Exit, _exit - terminate a process

SYNOPSIS
       #include <stdlib.h>

       void exit(int status);
       void _Exit(int status);


       #include <unistd.h>
       void _exit(int status);


DESCRIPTION
       For  exit()  and  _Exit():   The functionality described on this reference page is aligned with the ISO C stan-
       dard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume
       of IEEE Std 1003.1-2001 defers to the ISO C standard.

       The  value  of status may be 0, EXIT_SUCCESS, EXIT_FAILURE,  or any other value, though only the least signifi-
       cant 8 bits (that is, status & 0377) shall be available to a waiting parent process.

       The exit() function shall first call all functions registered by atexit(), in the reverse order of their regis-
       tration,  except  that  a  function  is  called after any previously registered functions that had already been
       called at the time it was registered. Each function is called as many times as it was  registered.  If,  during
       the  call  to  any such function, a call to the longjmp() function is made that would terminate the call to the
       registered function, the behavior is undefined.

       If a function registered by a call to atexit() fails to return, the remaining registered functions shall not be
       called  and  the  rest of the exit() processing shall not be completed. If exit() is called more than once, the
       behavior is undefined.

       The exit() function shall then flush all open streams with unwritten buffered data, close all open streams, and
       remove  all  files  created  by tmpfile(). Finally, control shall be terminated with the consequences described
       below.

       The _Exit() and _exit() functions shall be functionally equivalent.

       The _Exit()  and _exit()  functions shall not call functions registered with atexit() nor any registered signal
       handlers.   Whether  open  streams  are  flushed  or  closed, or temporary files are removed is implementation-
       defined. Finally, the calling process is terminated with the consequences described below.

       These functions shall terminate the calling process  with the following consequences:

       Note:  These consequences are all extensions to the ISO C standard and are not further CX shaded. However,  XSI
              extensions are shaded.


        * All  of  the  file  descriptors, directory streams,  conversion descriptors, and message catalog descriptors
          open in the calling process shall be closed.


        * If the parent process of the calling process is executing a wait() or waitpid(),  and has  neither  set  its
          SA_NOCLDWAIT  flag nor set SIGCHLD to SIG_IGN,  it shall be notified of the calling process' termination and
          the low-order eight bits (that is, bits 0377) of status shall be made available to it. If the parent is  not
          waiting,  the  child's  status shall be made available to it when the parent subsequently executes wait() or
          waitpid().

       The semantics of the waitid() function shall be equivalent to wait().


        * If the parent process of the calling process is not executing a wait() or waitpid(),  and  has  neither  set
          its  SA_NOCLDWAIT  flag  nor set SIGCHLD to SIG_IGN,  the calling process shall be transformed into a zombie
          process. A zombie process is an inactive process and it shall be deleted at some later time when its  parent
          process executes wait() or waitpid().

       The semantics of the waitid() function shall be equivalent to wait().


        * Termination  of  a  process  does  not  directly  terminate its children.  The sending of a SIGHUP signal as
          described below indirectly terminates children in some circumstances.


        * Either:

       If the implementation supports the SIGCHLD signal, a SIGCHLD shall be sent to the parent process.

       Or:

       If the parent process has set its SA_NOCLDWAIT flag, or set SIGCHLD to SIG_IGN, the status shall be  discarded,
       and  the  lifetime  of the calling process shall end immediately. If SA_NOCLDWAIT is set, it is implementation-
       defined whether a SIGCHLD signal is sent to the parent process.


        * The parent process ID of all of the calling process' existing child processes and zombie processes shall  be
          set  to the process ID of an implementation-defined system process. That is, these processes shall be inher-
          ited by a special system process.


        * Each attached shared-memory segment is detached and the value of  shm_nattch  (see  shmget())  in  the  data
          structure associated with its shared memory ID shall be decremented by 1.


        * For  each  semaphore for which the calling process has set a semadj value (see semop()), that value shall be
          added to the semval of the specified semaphore.


        * If the process is a controlling process, the SIGHUP signal shall be sent to each process in  the  foreground
          process group of the controlling terminal belonging to the calling process.


        * If  the process is a controlling process, the controlling terminal associated with the session shall be dis-
          associated from the session, allowing it to be acquired by a new controlling process.


        * If the exit of the process causes a process group to become orphaned,  and  if  any  member  of  the  newly-
          orphaned  process  group is stopped, then a SIGHUP signal followed by a SIGCONT signal shall be sent to each
          process in the newly-orphaned process group.


        * All open named semaphores in the calling process shall be closed as if by appropriate calls to  sem_close().


        * Any  memory  locks established by the process via calls to mlockall() or mlock() shall be removed. If locked
          pages in the address space of the calling process are also mapped into the address spaces of other processes
          and  are  locked by those processes, the locks established by the other processes shall be unaffected by the
          call by this process to _Exit() or _exit().


        * Memory mappings that were created in the process shall be unmapped before the process is destroyed.


        * Any blocks of typed memory that were mapped in the calling process shall be unmapped,  as  if  munmap()  was
          implicitly called to unmap them.


        * All  open  message  queue  descriptors  in the calling process shall be closed as if by appropriate calls to
          mq_close().


        * Any outstanding cancelable asynchronous I/O operations may be canceled.  Those asynchronous  I/O  operations
          that  are  not  canceled shall complete as if the _Exit() or _exit() operation had not yet occurred, but any
          associated signal notifications shall be suppressed. The _Exit() or _exit()  operation  may  block  awaiting
          such  I/O completion. Whether any I/O is canceled, and which I/O may be canceled upon _Exit() or _exit(), is
          implementation-defined.


        * Threads terminated by a call to _Exit() or _exit() shall not invoke their cancellation cleanup  handlers  or
          per-thread data destructors.


        * If  the  calling  process  is a trace controller process, any trace streams that were created by the calling
          process shall be shut down as described by the posix_trace_shutdown() function, and any process' mapping  of
          trace event names to trace event type identifiers built for these trace streams may be deallocated.


RETURN VALUE
       These functions do not return.

ERRORS
       No errors are defined.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       Normally applications should use exit() rather than _Exit() or _exit().

RATIONALE
   Process Termination
       Early  proposals  drew  a distinction between normal and abnormal process termination. Abnormal termination was
       caused only by certain signals and resulted in implementation-defined "actions", as discussed below. Subsequent
       proposals  distinguished three types of termination: normal termination (as in the current specification), sim-
       ple abnormal termination, and abnormal termination with actions. Again the distinction between the two types of
       abnormal  termination  was  that  they were caused by different signals and that implementation-defined actions
       would result in the latter case. Given that these actions were  completely  implementation-defined,  the  early
       proposals  were  only  saying  when the actions could occur and how their occurrence could be detected, but not
       what they were. This was of little or no use to conforming applications, and thus the distinction is  not  made
       in this volume of IEEE Std 1003.1-2001.

       The  implementation-defined actions usually include, in most historical implementations, the creation of a file
       named core in the current working directory of the process. This file contains an image of the  memory  of  the
       process,  together  with descriptive information about the process, perhaps sufficient to reconstruct the state
       of the process at the receipt of the signal.

       There is a potential security problem in creating a core file if the process was set-user-ID  and  the  current
       user  is  not the owner of the program, if the process was set-group-ID and none of the user's groups match the
       group of the program, or if the user does not have permission to write in the current directory. In this situa-
       tion, an implementation either should not create a core file or should make it unreadable by the user.

       Despite  the  silence  of  this volume of IEEE Std 1003.1-2001 on this feature, applications are advised not to
       create files named core because of potential conflicts in many implementations.   Some  implementations  use  a
       name other than core for the file; for example, by appending the process ID to the filename.

   Terminating a Process
       It  is important that the consequences of process termination as described occur regardless of whether the pro-
       cess called _exit() (perhaps indirectly through exit()) or instead was terminated due to a signal or  for  some
       other reason. Note that in the specific case of exit() this means that the status argument to exit() is treated
       in the same way as the status argument to _exit().

       A language other than C may have other termination primitives than the C-language exit() function, and programs
       written in such a language should use its native termination primitives, but those should have as part of their
       function the behavior of _exit() as described. Implementations in languages other than C are outside the  scope
       of this version of this volume of IEEE Std 1003.1-2001, however.

       As  required  by the ISO C standard, using return from main() has the same behavior (other than with respect to
       language scope issues) as calling exit() with the returned value.  Reaching the end of the main() function  has
       the same behavior as calling exit(0).

       A  value  of  zero (or EXIT_SUCCESS, which is required to be zero) for the argument status conventionally indi-
       cates successful termination. This corresponds to the specification for exit() in the ISO C standard. The  con-
       vention  is  followed  by utilities such as make and various shells, which interpret a zero status from a child
       process as success. For this reason, applications should not call  exit(0)  or  _exit(0)  when  they  terminate
       unsuccessfully; for example, in signal-catching functions.

       Historically,  the  implementation-defined process that inherits children whose parents have terminated without
       waiting on them is called init and has a process ID of 1.

       The sending of a SIGHUP to the foreground process group when a controlling process  terminates  corresponds  to
       somewhat different historical implementations. In System V, the kernel sends a SIGHUP on termination of (essen-
       tially) a controlling process. In 4.2 BSD, the kernel does not send SIGHUP in a case like this, but the  termi-
       nation  of  a controlling process is usually noticed by a system daemon, which arranges to send a SIGHUP to the
       foreground process group with the vhangup() function. However, in 4.2 BSD, due to the behavior  of  the  shells
       that  support  job  control,  the controlling process is usually a shell with no other processes in its process
       group. Thus, a change to make _exit() behave this way in such systems should not cause problems  with  existing
       applications.

       The termination of a process may cause a process group to become orphaned in either of two ways. The connection
       of a process group to its parent(s) outside of the group depends on both the parents and their children.  Thus,
       a  process  group may be orphaned by the termination of the last connecting parent process outside of the group
       or by the termination of the last direct descendant of the parent process(es). In either case, if the  termina-
       tion  of  a process causes a process group to become orphaned, processes within the group are disconnected from
       their job control shell, which no longer has any information on the existence of  the  process  group.  Stopped
       processes  within  the  group  would  languish forever.  In order to avoid this problem, newly orphaned process
       groups that contain stopped processes are sent a SIGHUP signal and a SIGCONT signal to indicate that they  have
       been  disconnected  from  their session. The SIGHUP signal causes the process group members to terminate unless
       they are catching or ignoring SIGHUP. Under most circumstances, all of the members of  the  process  group  are
       stopped if any of them are stopped.

       The  action of sending a SIGHUP and a SIGCONT signal to members of a newly orphaned process group is similar to
       the action of 4.2 BSD, which sends SIGHUP and SIGCONT to each stopped child of  an  exiting  process.  If  such
       children exit in response to the SIGHUP, any additional descendants receive similar treatment at that time.  In
       this volume of IEEE Std 1003.1-2001, the signals are sent to the entire process group at the same  time.  Also,
       in  this volume of IEEE Std 1003.1-2001, but not in 4.2 BSD, stopped processes may be orphaned, but may be mem-
       bers of a process group that is not orphaned; therefore, the action taken at _exit()  must  consider  processes
       other than child processes.

       It  is  possible  for  a process group to be orphaned by a call to setpgid() or setsid(), as well as by process
       termination. This volume of IEEE Std 1003.1-2001 does not require sending SIGHUP and SIGCONT  in  those  cases,
       because,  unlike  process termination, those cases are not caused accidentally by applications that are unaware
       of job control. An implementation can choose to send SIGHUP and SIGCONT in those cases as an extension; such an
       extension must be documented as required in <signal.h>.

       The  ISO/IEC 9899:1999 standard adds the _Exit() function that results in immediate program termination without
       triggering signals or atexit()-registered functions. In IEEE Std 1003.1-2001, this is equivalent to the _exit()
       function.

FUTURE DIRECTIONS
       None.

SEE ALSO
       atexit(),  close(),  fclose(),  longjmp(),  posix_trace_shutdown(),  posix_trace_trid_eventid_open(),  semop(),
       shmget(), sigaction(), wait() , waitid(), waitpid(),  the  Base  Definitions  volume  of  IEEE Std 1003.1-2001,
       <stdlib.h>, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard for  Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group  Base
       Specifications  Issue  6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc
       and The Open Group. 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.opengroup.org/unix/online.html .



IEEE/The Open Group                  2003                             EXIT(3P)