EXIT(P) POSIX Programmer's Manual EXIT(P)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 standard. 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 significant 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 registration, 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 ter‐
minate 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 regis‐
tered with atexit() nor any registered signal handlers. Whether open
streams are flushed or closed, or temporary files are removed is imple‐
mentation-defined. Finally, the calling process is terminated with the
consequences described below.
These functions shall terminate the calling process with the follow‐
ing 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 trans‐
formed 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 termi‐
nates 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 implemen‐
tation-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 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 control‐
ling 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 can‐
celed. 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 sup‐
pressed. 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 identi‐
fiers 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 termina‐
tion, 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 applica‐
tions, 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 work‐
ing 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 IEEE Std 1003.1-2001 on this fea‐
ture, 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() (per‐
haps indirectly through exit()) or instead was terminated due to a sig‐
nal 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. Implemen‐
tations 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 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 ter‐
minate 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 control‐
ling 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 pro‐
cesses in its process group. Thus, a change to make _exit() behave this
way in such systems should not cause problems with existing applica‐
tions.
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 termina‐
tion 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 dis‐
connected from their job control shell, which no longer has any infor‐
mation 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 dis‐
connected 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 descen‐
dants 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 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 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 <sig‐
nal.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 equiva‐
lent to the _exit() function.
FUTURE DIRECTIONS
None.
SEE ALSOatexit() , 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, Standard 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(P)
