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.\"	$OpenBSD: atexit.3,v 1.12 2019/06/27 16:30:39 schwarze Exp $
.\"
.Dd $Mdocdate: June 27 2019 $
.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
The
.Nm
function returns the value 0 if successful; otherwise a non-zero
value is returned and the global variable
.Va errno
is set to indicate the error.
.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
Setting
.Va errno
on error and the behavior when a shared object is unloaded
are extensions to that standard.