1.\" Copyright (c) 1990, 1991 The Regents of the University of California. 2.\" All rights reserved. 3.\" 4.\" This code is derived from software contributed to Berkeley by 5.\" Chris Torek and the American National Standards Committee X3, 6.\" on Information Processing Systems. 7.\" 8.\" Redistribution and use in source and binary forms, with or without 9.\" modification, are permitted provided that the following conditions 10.\" are met: 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 2. Redistributions in binary form must reproduce the above copyright 14.\" notice, this list of conditions and the following disclaimer in the 15.\" documentation and/or other materials provided with the distribution. 16.\" 3. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" $OpenBSD: atexit.3,v 1.13 2022/02/06 00:29:02 jsg Exp $ 33.\" 34.Dd $Mdocdate: February 6 2022 $ 35.Dt ATEXIT 3 36.Os 37.Sh NAME 38.Nm atexit 39.Nd register a function to be called on exit 40.Sh SYNOPSIS 41.In stdlib.h 42.Ft int 43.Fn atexit "void (*function)(void)" 44.Sh DESCRIPTION 45The 46.Fn atexit 47function registers the given 48.Fa function 49to be called at program exit, whether via 50.Xr exit 3 51or via return from the program's 52.Fn main . 53Functions so registered are called in reverse order; 54no arguments are passed. 55At least 32 functions can always be registered, 56and more are allowed as long as sufficient memory can be allocated. 57.Pp 58If a shared object is unloaded from process memory using 59.Xr dlclose 3 , 60then any functions registered by calling 61.Fn atexit 62from that shared object will be called in reverse order and unregistered. 63Note that it is the source of the call to 64.Fn atexit 65that matters, not the source of the function that was registered. 66.Pp 67.Fn atexit 68is very difficult to use correctly without creating 69.Xr exit 3 Ns -time 70races. 71Unless absolutely necessary, avoid using it. 72.Sh RETURN VALUES 73The 74.Nm 75function returns the value 0 if successful; otherwise a non-zero 76value is returned and the global variable 77.Va errno 78is set to indicate the error. 79.Sh ERRORS 80.Bl -tag -width Er 81.It Bq Er ENOMEM 82No memory was available to add the function to the list. 83The existing list of functions is unmodified. 84.El 85.Sh SEE ALSO 86.Xr dlclose 3 , 87.Xr exit 3 88.Sh STANDARDS 89The 90.Fn atexit 91function conforms to 92.St -ansiC . 93.Pp 94Setting 95.Va errno 96on error and the behavior when a shared object is unloaded 97are extensions to that standard. 98