.\"	$OpenBSD: refcnt_init.9,v 1.6 2023/07/12 18:14:13 jmc Exp $
.\"
.\" Copyright (c) 2015 David Gwynne <dlg@openbsd.org>
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
.\"
.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.\"
.Dd $Mdocdate: July 12 2023 $
.Dt REFCNT_INIT 9
.Os
.Sh NAME
.Nm refcnt_init ,
.Nm refcnt_init_trace ,
.Nm refcnt_take ,
.Nm refcnt_rele ,
.Nm refcnt_rele_wake ,
.Nm refcnt_finalize ,
.Nm refcnt_shared ,
.Nm refcnt_read ,
.Nm REFCNT_INITIALIZER
.Nd reference count API
.Sh SYNOPSIS
.In sys/refcnt.h
.Ft void
.Fn "refcnt_init" "struct refcnt *r"
.Ft void
.Fn "refcnt_init_trace" "struct refcnt *r" "int idx"
.Ft void
.Fn "refcnt_take" "struct refcnt *r"
.Ft int
.Fn "refcnt_rele" "struct refcnt *r"
.Ft void
.Fn "refcnt_rele_wake" "struct refcnt *r"
.Ft void
.Fn "refcnt_finalize" "struct refcnt *r" "const char *wmesg"
.Ft int
.Fn "refcnt_shared" "struct refcnt *r"
.Ft unsigned int
.Fn "refcnt_read" "struct refcnt *r"
.Fn "REFCNT_INITIALIZER"
.Sh DESCRIPTION
The refcnt API provides simple reference counters that can be used
to manage the lifetime of a shared object.
.Pp
.Fn refcnt_init
sets the initial value of the counter to 1 to account for the caller's
reference to the object.
.Fn refcnt_init_trace
additionally accepts a
.Xr dt 4
static probe index.
.Pp
.Fn refcnt_take
is used to acquire a new reference.
It is the responsibility of the caller to guarantee that it holds
a valid reference before taking a new reference.
.Pp
.Fn refcnt_rele
is used to release an existing reference.
.Pp
.Fn refcnt_rele_wake
is used to release an existing reference and wakes up a process
that is currently waiting in
.Fn refcnt_finalize
for all the references to be released.
.Pp
.Fn refcnt_finalize
releases the caller's reference and sleeps until all the other
references to the relevant object have been released.
There may only be one caller to
.Fn refcnt_finalize
per refcnt
.Fa r .
.Pp
.Fn refcnt_rele ,
.Fn refcnt_rele_wake
and
.Fn refcnt_finalize
order prior memory loads and stores before the release of the reference.
The functions enforce control dependency so that after the final reference
has been released, subsequent loads and stores happen after the release.
These ensure that concurrent accesses cease before the object's destructor
runs and that the destructor sees all updates done during the lifetime
of the object.
.Pp
.Fn refcnt_shared
tests if the object has multiple references.
.Pp
.Fn refcnt_read
returns a snapshot of the counter value.
Its use is discouraged,
code should use
.Fn refcnt_shared
whenever possible.
.Pp
.Fn REFCNT_INITIALIZER
initialises a declaration of a refcnt to 1.
.Sh CONTEXT
.Fn refcnt_init ,
.Fn refcnt_init_trace ,
.Fn refcnt_take ,
.Fn refcnt_rele ,
.Fn refcnt_rele_wake ,
.Fn refcnt_shared
and
.Fn refcnt_read
can be called during autoconf, from process context, or from interrupt
context.
.Pp
.Fn refcnt_finalize
can be called from process context.
.Sh RETURN VALUES
.Fn refcnt_rele
returns a non-zero value if the last reference has been released,
otherwise 0.
.Pp
.Fn refcnt_shared
returns a non-zero value if the object has multiple references,
otherwise 0.
.Pp
.Fn refcnt_read
returns a snapshot of the counter value.