NAME

_Exit, _exit — terminate a process

SYNOPSIS

#include <stdlib.h>

_Noreturn void _Exit(int status);


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


DESCRIPTION

For _Exit(): ^[CX]  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.1-2024 defers to the ISO C standard.

The value of status may be 0, EXIT_SUCCESS, EXIT_FAILURE, ^[CX]  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.

^[CX] The _Exit() and _exit() functions shall be functionally equivalent.

The _Exit() ^[CX]  and _exit() functions shall not call functions registered with atexit() nor at_quick_exit(), nor any registered signal handlers. ^[CX]  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

^[CX] 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.

• ^[XSI] 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 2.12 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 2.12 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 2.12 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.

• ^[XSI] 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.

• ^[XSI] 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(). All open unnamed semaphores in the calling process shall be destroyed as if by appropriate calls to sem_destroy().

• ^[ML] 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.

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

• ^[MSG] 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.

RETURN VALUE

These functions do not return.

ERRORS

No errors are defined.

EXAMPLES

None.

APPLICATION USAGE

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

Exit statuses of 126 and greater are ambiguous in certain circumstances because they have special meanings in the shell (see 2.8.2 Exit Status for Commands and the EXIT STATUS section of sh).

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.1-2024.

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.1-2024 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.1-2024, 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.1-2024, the signals are sent to the entire process group at the same time. Also, in this volume of POSIX.1-2024, 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.1-2024 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 added the _Exit() function that results in immediate program termination without triggering signals or atexit()-registered functions. In POSIX.1-2024, this is equivalent to the _exit() function.

FUTURE DIRECTIONS

None.

SEE ALSO

at_quick_exit, atexit, exit, mlock, mlockall, mq_close, munmap, quick_exit, sem_close, semop, setpgid, setsid, shmget, wait, waitid

XBD <stdlib.h>, <unistd.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

The DESCRIPTION is updated for alignment with the POSIX Realtime Extension and the POSIX Threads Extension.

Interactions with the SA_NOCLDWAIT flag and SIGCHLD signal are further clarified.

The values of status from exit() are better described.

Issue 6

Extensions beyond the ISO C standard are marked.

The DESCRIPTION is updated for alignment with IEEE Std 1003.1j-2000 by adding semantics for typed memory.

The following changes are made for alignment with the ISO/IEC 9899:1999 standard:

• The _Exit() function is included.

• The DESCRIPTION is updated.

The description of tracing semantics is added for alignment with IEEE Std 1003.1q-2000.

References to the wait3() function are removed.

IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/16 is applied, correcting grammar in the DESCRIPTION.

Issue 7

Austin Group Interpretation 1003.1-2001 #031 is applied, separating these functions from the exit() function.

Austin Group Interpretation 1003.1-2001 #085 is applied, clarifying the text regarding flushing of streams and closing of temporary files.

Functionality relating to the Asynchronous Input and Output, Memory Mapped Files, and Semaphores options is moved to the Base.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0033 [594] and XSH/TC2-2008/0034 [594,690] are applied.

Issue 8

Austin Group Defect 368 is applied, adding a requirement for unnamed semaphores to be destroyed.

Austin Group Defect 1302 is applied, aligning these functions with the ISO/IEC 9899:2018 standard.

Austin Group Defect 1330 is applied, removing obsolescent interfaces.

Austin Group Defect 1629 is applied, changing the APPLICATION USAGE section.