'\" t
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified 1993-03-29, David Metcalfe
.\" Modified 1993-07-24, Rik Faith (faith@cs.unc.edu)
.\" Modified 2003-10-25, Walter Harms
.\"
.TH atexit 3 2023-02-05 "Linux man-pages 6.03"
.SH NAME
atexit \- register a function to be called at normal process termination
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.PP
.BI "int atexit(void (*" function )(void));
.fi
.SH DESCRIPTION
The
.BR atexit ()
function registers the given
.I function
to be
called at normal process termination, either via
.BR exit (3)
or via return from the program's
.IR main ().
Functions so registered are called in
the reverse order of their registration; no arguments are passed.
.PP
The same function may be registered multiple times:
it is called once for each registration.
.PP
POSIX.1 requires that an implementation allow at least
.\" POSIX.1-2001, POSIX.1-2008
.B ATEXIT_MAX
(32) such functions to be registered.
The actual limit supported by an implementation can be obtained using
.BR sysconf (3).
.PP
When a child process is created via
.BR fork (2),
it inherits copies of its parent's registrations.
Upon a successful call to one of the
.BR exec (3)
functions,
all registrations are removed.
.SH RETURN VALUE
The
.BR atexit ()
function returns the value 0 if successful; otherwise
it returns a nonzero value.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR atexit ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH STANDARDS
POSIX.1-2001, POSIX.1-2008, C99, SVr4, 4.3BSD.
.SH NOTES
Functions registered using
.BR atexit ()
(and
.BR on_exit (3))
are not called if a process terminates abnormally because
of the delivery of a signal.
.PP
If one of the registered functions calls
.BR _exit (2),
then any remaining functions are not invoked,
and the other process termination steps performed by
.BR exit (3)
are not performed.
.PP
POSIX.1 says that the result of calling
.\" POSIX.1-2001, POSIX.1-2008
.BR exit (3)
more than once (i.e., calling
.BR exit (3)
within a function registered using
.BR atexit ())
is undefined.
On some systems (but not Linux), this can result in an infinite recursion;
.\" This can happen on OpenBSD 4.2 for example, and is documented
.\" as occurring on FreeBSD as well.
.\" glibc does "the Right Thing" -- invocation of the remaining
.\" exit handlers carries on as normal.
portable programs should not invoke
.BR exit (3)
inside a function registered using
.BR atexit ().
.PP
The
.BR atexit ()
and
.BR on_exit (3)
functions register functions on the same list:
at normal process termination,
the registered functions are invoked in reverse order
of their registration by these two functions.
.PP
According to POSIX.1, the result is undefined if
.BR longjmp (3)
is used to terminate execution of one of the functions registered using
.BR atexit ().
.\" In glibc, things seem to be handled okay
.SS Linux notes
Since glibc 2.2.3,
.BR atexit ()
(and
.BR on_exit (3))
can be used within a shared library to establish functions
that are called when the shared library is unloaded.
.SH EXAMPLES
.\" SRC BEGIN (atexit.c)
.EX
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

void
bye(void)
{
    printf("That was all, folks\en");
}

int
main(void)
{
    long a;
    int i;

    a = sysconf(_SC_ATEXIT_MAX);
    printf("ATEXIT_MAX = %ld\en", a);

    i = atexit(bye);
    if (i != 0) {
        fprintf(stderr, "cannot set exit function\en");
        exit(EXIT_FAILURE);
    }

    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR _exit (2),
.BR dlopen (3),
.BR exit (3),
.BR on_exit (3)