.\"	$OpenBSD: srpl_rc_init.9,v 1.14 2016/11/21 07:11:13 dlg 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: November 21 2016 $
.Dt SRPL_RC_INIT 9
.Os
.Sh NAME
.Nm srpl_rc_init ,
.Nm SRPL_HEAD ,
.Nm SRPL_ENTRY ,
.Nm SRPL_INIT ,
.Nm SRPL_FIRST ,
.Nm SRPL_NEXT ,
.Nm SRPL_FOLLOW ,
.Nm SRPL_FOREACH ,
.Nm SRPL_LEAVE ,
.Nm SRPL_RC_INITIALIZER
.Nd singly-linked shared reference pointer list
.Sh SYNOPSIS
.In sys/srp.h
.Vt struct srpl_rc;
.Ft void
.Fo "srpl_rc_init"
.Fa "struct srpl_rc *rc"
.Fa "void (*ref)(void *, void *)"
.Fa "void (*unref)(void *, void *)"
.Fa "void *ctx"
.Fc
.Fn SRPL_HEAD "HEADNAME" "TYPE"
.Fn SRPL_ENTRY "TYPE"
.Ft void
.Fn "SRPL_INIT" "SRPL_HEAD *sl"
.Ft void *
.Fn "SRPL_FIRST" "struct srp_ref *sr" "SRPL_HEAD *sl"
.Ft void *
.Fn "SRPL_NEXT" "struct srp_ref *sr" "struct TYPE *listelm" "FIELDNAME"
.Ft void *
.Fn "SRPL_FOLLOW" "struct srp_ref *sr" "struct TYPE *listelm" "FIELDNAME"
.Fo "SRPL_FOREACH"
.Fa "VARNAME"
.Fa "struct srp_ref *sr"
.Fa "SRPL_HEAD *sl"
.Fa "FIELDNAME"
.Fc
.Ft void
.Fn "SRPL_LEAVE" "struct srp_ref *sr"
.Fo "SRPL_RC_INITIALIZER"
.Fa "void (*ref)(void *, void *)"
.Fa "void (*unref)(void *, void *)"
.Fa "void *ctx"
.Fc
.Sh DESCRIPTION
The SRPL list
macros build a linked list on top of shared reference pointers.
This allows concurrent traversal of a linked list and access to the
items on the list.
.Pp
SRP lists are a generic type represented by a
.Vt SRPL_HEAD .
The elements inserted into the list must be structures that contain a
.Vt SRPL_ENTRY
field.
Further, the elements must also support reference counting as
insertion and removal operations can cause items to be temporarily
referenced by multiple SRPs within the list at the same time.
.Pp
.Fn srpl_rc_init
initialises the SRP list refcounts
.Fa rc
structure so it can be used to manage the reference counts on
elements in the list.
The insertion or removal of an element in an SRP list will increment
the reference counts on elements within the list via calls to
.Fa ref .
As these references are released by the SRP infrastructure, the
reference counts will be decremented by calls to
.Fa unref .
.Fa unref
is also responsible for freeing the element when the reference count
reaches 0.
Both
.Fa ref
and
.Fa unref
will be called with
.Fa ctx
as their first argument and a pointer to the element as their second
argument.
.Pp
.Fn SRPL_INIT
initialises the SRP list
.Fa sl
to an empty state.
.Pp
.Fn SRPL_FIRST
accesses the first element in the SRP list
.Fa sl
and holds its reference via
.Fa sr .
.Pp
.Fn SRPL_NEXT
accesses the element in the SRP list after
.Fa listelm
and holds its reference via
.Fa sr .
.\".Pp
.\"Every call to
.\".Fn SRPL_FIRST
.\"must have a corresponding call to
.\".Fn SRPL_LEAVE
.\"to release references to the list and its elements.
.Pp
.Fn SRPL_FOLLOW
accesses the element in the SRP list after
.Fa listelm
and swaps the previous reference held via
.Fa sr
for the reference of the newly accessed item.
.Pp
.Fn SRPL_FOREACH
creates a loop for traversing the list.
Every call to
.Fn SRPL_FOREACH
must have a corresponding call to
.Fn SRPL_LEAVE
to release references to the list and its elements.
.Pp
.Fn SRPL_LEAVE
releases references to the list and its elements held by previous
calls to
.Fn SRPL_FIRST ,
.Fn SRPL_NEXT ,
.Fn SRPL_FOLLOW ,
or
.Fn SRPL_FOREACH .
.Pp
An srpl_rc declaration can be initialised with the
.Fn SRPL_RC_INITIALIZER
macro.
.Sh CONTEXT
.Fn SRPL_INIT ,
.Fn SRPL_FIRST ,
.Fn SRPL_NEXT ,
.Fn SRPL_FOLLOW ,
.Fn SRPL_FOREACH ,
and
.Fn SRPL_LEAVE
may be called during autoconf, from process context, or from interrupt
context.
.Pp
.Fn srpl_rc_init ,
may be called during autoconf or from process context.
.Sh RETURN VALUES
.Fn SRPL_FIRST ,
.Fn SRPL_NEXT ,
and
.Fn SRPL_FOLLOW
return a pointer to elements in the SRP list, or
.Dv NULL
if there are no more elements.
.Sh SEE ALSO
.Xr SRPL_FIRST_LOCKED 9
.Sh HISTORY
The srp API was originally written by
.An Jonathan Matthew Aq Mt jmatthew@openbsd.org
and
.An David Gwynne Aq Mt dlg@openbsd.org .
The SRP list API first appeared in
.Ox 5.9 .