The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


exit, _Exit, _exit - terminate a process


#include <stdlib.h>

void exit(int
void _Exit(int

#include <unistd.h>
void _exit(int


For exit() and _Exit(): [CX] [Option Start] 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. [Option End]

The value of status may be 0, EXIT_SUCCESS, EXIT_FAILURE, [CX] [Option Start]  or any other value, though only the least significant 8 bits (that is, status & 0377) shall be available to a waiting parent process. [Option End]

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 terminate 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.

[CX] [Option Start] The _Exit() and _exit() functions shall be functionally equivalent. [Option End]

The _Exit() [CX] [Option Start]  and _exit() [Option End] functions shall not call functions registered with atexit() nor any registered signal handlers. Whether open streams are flushed or closed, or temporary files are removed is implementation-defined. Finally, the calling process is terminated with the consequences described below.

These functions shall terminate the calling process [CX] [Option Start]  with the following consequences: [Option End]

These consequences are all extensions to the ISO C standard and are not further CX shaded. However, XSI extensions are shaded.


These functions do not return.


No errors are defined.

The following sections are informative.




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


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 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 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 IEEE Std 1003.1-2001 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 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 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 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 <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 IEEE Std 1003.1-2001, this is equivalent to the _exit() function.




atexit(), 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>


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 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.

End of informative text.

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