exit, atexit, _exit - Terminate a process
void (*function)(void) ); void exit(
int status ); #include <unistd.h>
int status );
Standard C Library (libc)
Interfaces documented on this reference page conform to industry stan‐
dards as follows:
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.
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 repeated.
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() func‐
tion, 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() func‐
tion. Finally, it calls the _exit() function, which completes process
termination and does not return.
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() func‐
tion 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 pro‐
cesses 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 termi‐
nated. The low-order 8 bits (that is, bits 0377 or 0xFF) of the status
parameter are made available to the parent process.
[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 call‐
ing 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 trans‐
formed 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 SIG_IGN. 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 termi‐
nal that belongs to the calling process. The terminal is disas‐
sociated from the session, allowing it to be acquired by a new
controlling process. 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_NOCLD‐
WAIT 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 infor‐
mation 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 locking.) [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 han‐
dler, _exit() can be called. It is not recommended to call exit() from
within an atexit() handler.
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 func‐
tions than available virtual memory allows. In this case, the function
returns a nonzero value.
Functions: acct(2), sigaction(2), sigvec(2), wait(2), ldr_atexit(3),