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_EXIT(3POSIX) POSIX Programmer's Manual _EXIT(3POSIX)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
NAME
_Exit, _exit — terminate a process
SYNOPSIS
#include <stdlib.h>
void _Exit(int status);
#include <unistd.h>
void _exit(int status);
DESCRIPTION
For _Exit(): The functionality described on this reference page is aligned with the ISO C standard. Any
conflict between the requirements described here and the ISO C standard is unintentional. This volume of
POSIX.1‐2008 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
significant 8 bits (that is, status & 0377) shall be available to a waiting parent process.
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. Open streams shall not be flushed. Whether open streams are closed (without flushing)
is implementation-defined. Finally, the calling process shall be terminated with the consequences
described below.
Consequences of Process Termination
Process termination caused by any reason shall have the following consequences:
Note: These consequences are all extensions to the ISO C standard and are not further CX shaded.
However, functionality relating to the XSI option is 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(), waitid(), or waitpid(), and has
neither set its SA_NOCLDWAIT flag nor set SIGCHLD to SIG_IGN, it shall be notified of termination of
the calling process 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(), waitid(), 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(), waitid(), 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(), waitid(), 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 existing child processes and zombie processes of the calling
process shall be set to the process ID of an implementation-defined system process. That is, these
processes shall be inherited 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 disassociated 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 mapping
of trace event names to trace event type identifiers of any process 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), simple 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 POSIX.1‐2008.
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 situation, an implementation either should not create a core file or should make it
unreadable by the user.
Despite the silence of this volume of POSIX.1‐2008 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
process 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 POSIX.1‐2008, 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
indicates successful termination. This corresponds to the specification for exit() in the ISO C standard.
The convention 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 (essentially) a controlling process. In 4.2 BSD, the kernel does not send SIGHUP in a case like this,
but the termination 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 termination 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 POSIX.1‐2008, the signals are sent to the entire process group
at the same time. Also, in this volume of POSIX.1‐2008, but not in 4.2 BSD, stopped processes may be
orphaned, but may be members 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 POSIX.1‐2008 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 POSIX.1‐2008, this is equivalent to the
_exit() function.
FUTURE DIRECTIONS
None.
SEE ALSO
atexit(), exit(), mlock(), mlockall(), mq_close(), munmap(), posix_trace_create(), sem_close(), semop(),
setpgid(), setsid(), shmget(), wait(), waitid()
The Base Definitions volume of POSIX.1‐2008, <stdlib.h>, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc
and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 _EXIT(3POSIX)