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exit, atexit, _exit - Terminate a process
void (*function)(void) );
int status );
int status );
Standard C Library (libc)
Interfaces documented on this reference page conform to industry standards
exit(), _exit(), atexit(): XSH4.0, XSH4.2, XSH5.0
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
status Indicates the status of the process.
Points to a function that is called at normal process termination
for cleanup processing. The number of exit handlers that can be
specified with the atexit() function is limited by the amount of
available virtual memory.
The atexit() function registers functions to be called at normal process
termination for cleanup processing. The function adds a single exit handler
to a list of handlers to be called at process termination. The system
calls the functions in reverse order, calling the function at the top of
the list first. Any function that is registered more than once will be
The exit() function terminates the calling process after calling the
_cleanup() function to flush any buffered output. Then it calls any
functions registered previously for the process by the atexit() function,
in the reverse order to that in which they were registered. In addition,
the exit() function flushes all open output streams, closes all open
streams, and removes all files created by the tmpfile() function. Finally,
it calls the _exit() function, which completes process termination and does
The _exit() and exit() functions terminate the calling process and cause
the following to occur:
· All of the file descriptors and directory streams open in the calling
process are closed. Since the exit() function terminates the process,
any errors encountered during these close operations go unreported.
Message catalog descriptors and conversion descriptors opened in the
calling process are also closed with no reporting of errors.
· The parent process ID of all the calling process' existing child
processes and zombie processes is reset. The child processes continue
executing; however, their parent process ID is set to the process ID
of the init process. The init process thus adopts each of these
processes, catches the SIGCHLD signals that they generate, and calls
the wait() function for each of them.
· If the parent process of the calling process is running a wait() or
waitpid() function, that parent process is notified that the calling
process is being terminated. The low-order 8 bits (that is, bits 0377
or 0xFF) of the status parameter are made available to the parent
[XSH4.2] [XSH5.0] This behavior also applies if the parent process
is running a wait3() or waitid() function. In addition, this behavior
only applies when the parent process of the calling process has
neither set its SA_NOCLDWAIT flag nor set SIGCHLD to SIG_IGN.
· If the parent process is not running a wait() or waitpid() function
when the child process terminates, the parent process receives a
SIGCHLD signal to notify it that the child process is terminating. The
child process is transformed into a zombie process.
Once the parent process calls the wait() or waitpid() routine, the
child process completes termination and the low-order 8 bits (that is,
bits 0377 or 0xFF) of the status parameter are made available to it.
[XSH4.2] [XSH5.0] This behavior also applies to the wait3() and
waitid() functions. In addition, this behavior only applies when the
parent process has not set its SA_NOCLDWAIT flag or set SIGCHLD to
· The parent process is sent a SIGCHLD signal when a child terminates;
however, since the default action for this signal is to ignore it, the
signal usually is not seen.
· If the process is a controlling process, the system sends a SIGHUP
signal to each process executing in the foreground on the terminal
that belongs to the calling process. The terminal is disassociated
from the session, allowing it to be acquired by a new controlling
· If the termination of a process causes a process group to become
orphaned, and if any member of the newly orphaned process group is
stopped, a SIGHUP signal, followed by a SIGCONT signal, is sent to
each newly orphaned process.
· [XSH4.2] [XSH5.0] If the parent process has set its SA_NOCLDWAIT
flag or set SIGCHLD to SIG_IGN, the status is discarded, and the
lifetime of the calling process ends immediately.
· [XSH4.2] [XSH5.0] Each mapped memory object is unmapped.
· Each attached shared memory segment is detached and the value of
shm_nattach in the data structure associated with its shared memory
identifier is decremented by 1. (See shmget(2) for information about
the data structure.)
· For each semaphore for which the calling process has set a semadj
value, that semadj value is added to the semval of the specified
semaphore. (See semop(2)for information about semaphore operations.)
· [Tru64 UNIX] If the process has a process lock, text lock, or data
lock, an unlock() is performed. (See plock(2) for information on
· [Tru64 UNIX] An accounting record is written to the accounting file
if the system accounting routine is enabled. (See acct(2) for
information about enabling accounting routines.)
· [Tru64 UNIX] Locks set by the fcntl(), flock(), and lockf() functions
[Tru64 UNIX] If a thread calls the _exit() function, the entire process
exits and all threads within the process are terminated.
[XSH4.2] [XSH5.0] An application should call sysconf() to obtain the
value of ATEXIT_MAX, the number of handlers that can be registered. There
is no way for an application to tell how many functions have already been
registered with atexit().
To prematurely terminate atexit() handler processing from within a handler,
_exit() can be called. It is not recommended to call exit() from within an
The exit() function and _exit() function do not return.
The atexit() function returns 0 (zero) if successful. The function fails
if an application attempts to register more process cleanup functions than
available virtual memory allows. In this case, the function returns a
Functions: acct(2), sigaction(2), sigvec(2), wait(2), ldr_atexit(3),
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