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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.1‐2017 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 from wait() and
waitpid(); the full value shall be available from waitid() and in the siginfo_t passed to
a signal handler for SIGCHLD.

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 has set its SA_NOCLDWAIT flag or has set
the action for the SIGCHLD signal to SIG_IGN:

-- The process' status information (see Section 2.13, Status Information), if any,
shall be discarded.

-- 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.

-- If a thread in the parent process of the calling process is blocked in wait(),
waitpid(), or waitid(), and the parent process has no remaining child processes in
the set of waited-for children, the wait(), waitid(), or waitpid() function shall
fail and set errno to [ECHILD].

Otherwise:

-- Status information (see Section 2.13, Status Information) shall be generated.

-- The calling process shall be transformed into a zombie process. Its status
information shall be made available to the parent process until the process'
lifetime ends.

-- The process' lifetime shall end once its parent obtains the process' status
information via a currently-blocked or future call to wait(), waitid() (without
WNOWAIT), or waitpid().

-- If one or more threads in the parent process of the calling process is blocked in
a call to wait(), waitid(), or waitpid() awaiting termination of the process, one
(or, if any are calling waitid() with WNOWAIT, possibly more) of these threads
shall obtain the process' status information as specified in Section 2.13, Status
Information and become unblocked.

-- A SIGCHLD shall be sent to the parent process.

* 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.

* 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.1‐2017.

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.1‐2017 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.1‐2017, 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.1‐2017, the signals are sent to the entire process group at the same time. Also, in
this volume of POSIX.1‐2017, 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.1‐2017 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.

COPYRIGHT

       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1-2017,  Standard  for  Information Technology -- Portable Operating System Interface
       (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C)  2018  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 .

       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 .