завершить процесс (terminate a process)
Пролог (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.
Имя (Name)
_Exit, _exit — terminate a process
Синопсис (Synopsis)
#include <stdlib.h>
void _Exit(int status);
#include <unistd.h>
void _exit(int status);
Описание (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.
Возвращаемое значение (Return value)
These functions do not return.
Ошибки (Error)
No errors are defined.
The following sections are informative.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
Normally applications should use exit() rather than _Exit() or
_exit().
Обоснование (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.
Будущие направления (Future directions)
None.
Смотри также (See also)
atexit(3p), exit(3p), mlock(3p), mlockall(3p), mq_close(3p),
munmap(3p), posix_trace_create(3p), sem_close(3p), semop(3p),
setpgid(3p), setsid(3p), shmget(3p), wait(3p), waitid(3p)
The Base Definitions volume of POSIX.1‐2017, stdlib.h(0p),
unistd.h(0p)
