.\" Copyright (c) 1990, 1991 The Regents of the University of California.
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.\"	$OpenBSD: atexit.3,v 1.11 2015/05/12 20:14:09 guenther Exp $
.\"
.Dd $Mdocdate: May 12 2015 $
.Dt ATEXIT 3
.Os
.Sh NAME
.Nm atexit
.Nd register a function to be called on exit
.Sh SYNOPSIS
.In stdlib.h
.Ft int
.Fn atexit "void (*function)(void)"
.Sh DESCRIPTION
The
.Fn atexit
function registers the given
.Fa function
to be called at program exit, whether via
.Xr exit 3
or via return from the program's
.Fn main .
Functions so registered are called in reverse order;
no arguments are passed.
At least 32 functions can always be registered,
and more are allowed as long as sufficient memory can be allocated.
.Pp
If a shared object is unloaded from process memory using
.Xr dlclose 3 ,
then any functions registered by calling
.Fn atexit
from that shared object will be called in reverse order and unregistered.
Note that it is the source of the call to
.Fn atexit
that matters, not the source of the function that was registered.
.Pp
.Fn atexit
is very difficult to use correctly without creating
.Xr exit 3 Ns -time
races.
Unless absolutely necessary, please avoid using it.
.Sh RETURN VALUES
.Rv -std atexit
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er ENOMEM
No memory was available to add the function to the list.
The existing list of functions is unmodified.
.El
.Sh SEE ALSO
.Xr dlclose 3 ,
.Xr exit 3
.Sh STANDARDS
The
.Fn atexit
function conforms to
.St -ansiC .
.Pp
The behavior when a shared object is unloaded is an extension to
that standard.