.\"	$OpenBSD: srpl_rc_init.9,v 1.6 2015/09/18 17:09:11 jmc Exp $
.\"
.\" Copyright (c) 2015 David Gwynne <dlg@openbsd.org>
.\"
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.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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.Dd $Mdocdate: September 18 2015 $
.Dt SRPL_RC_INIT 9
.Os
.Sh NAME
.Nm srpl_rc_init ,
.Nm SRPL_INIT ,
.Nm SRPL_ENTER ,
.Nm SRPL_NEXT ,
.Nm SRPL_FOREACH ,
.Nm SRPL_LEAVE ,
.Nm SRPL_EMPTY_LOCKED ,
.Nm SRPL_FIRST_LOCKED ,
.Nm SRPL_NEXT_LOCKED ,
.Nm SRPL_FOREACH_LOCKED ,
.Nm SRPL_INSERT_HEAD_LOCKED ,
.Nm SRPL_INSERT_AFTER_LOCKED ,
.Nm SRPL_REMOVE_LOCKED ,
.Nm SRPL_RC_INITIALIZER
.Nd singly-linked shared reference pointer list
.Sh SYNOPSIS
.In sys/srp.h
.Vt struct srpl;
.Vt struct srpl_entry;
.Vt struct srpl_iter;
.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_INIT" "struct srpl_head *sl"
.Ft void *
.Fn "SRPL_ENTER" "struct srpl *sl" "struct srpl_iter *si"
.Ft void *
.Fn "SRPL_NEXT" "struct srpl_iter *si" "struct TYPE *listelm" "FIELDNAME"
.Fo "SRPL_FOREACH"
.Fa "VARNAME"
.Fa "struct srpl *sl"
.Fa "struct srpl_iter *si"
.Fa "FIELDNAME"
.Fc
.Fn "SRPL_LEAVE" "struct srpl_iter *si" "struct TYPE *listelm"
.Fn "SRPL_EMPTY_LOCKED" "struct srpl *sl"
.Ft void *
.Fn "SRPL_FIRST_LOCKED" "struct srpl *sl"
.Ft void *
.Fn "SRPL_NEXT_LOCKED" "struct TYPE *listelm" "FIELDNAME"
.Fn "SRPL_FOREACH_LOCKED" "VARNAME" "struct srpl *sl" "FIELDNAME"
.Fo "SRPL_INSERT_HEAD_LOCKED"
.Fa "struct srpl_rc *rc"
.Fa "struct srpl *sl"
.Fa "struct TYPE *elm"
.Fa "FIELDNAME"
.Fc
.Fo "SRPL_INSERT_AFTER_LOCKED"
.Fa "struct srpl_rc *rc"
.Fa "struct TYPE *listelm"
.Fa "struct TYPE *elm"
.Fa "FIELDNAME"
.Fc
.Fo "SRPL_REMOVE_LOCKED"
.Fa "struct srpl_rc *rc"
.Fa "struct srpl *sl"
.Fa "struct TYPE *listelm"
.Fa "TYPE"
.Fa "FIELDNAME"
.Fc
.Fo "SRPL_RC_INITIALIZER"
.Fa "void (*ref)(void *, void *)"
.Fa "void (*unref)(void *, void *)"
.Fa "void *ctx"
.Fc
.Sh DESCRIPTION
The
srpl
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 struct srpl .
The elements inserted into the list must be structures that contain a
.Vt struct 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_ENTER
begins iterating over elements in the SRP list
.Fa sl .
Local state necessary for iterating over the list is stored in the
SRP list iterator structure
.Fa si .
Every call to
.Fn SRPL_ENTER
must have a corresponding call to
.Fn SRPL_LEAVE
to release references to the list and its elements.
.Pp
.Fn SRPL_NEXT
accesses the element in the SRP list after
.Fa listelm .
.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_ENTER
or
.Fn SRPL_FOREACH .
.Pp
.Fn SRPL_EMPTY_LOCKED
tests whether the SRP list
.Fa sl
is empty.
.Pp
.Fn SRPL_FIRST_LOCKED
accesses the first element in the SRP list
.Fa sl .
.Pp
.Fn SRPL_NEXT_LOCKED
accesses the next element in the SRP list after
.Fa listelm .
.Pp
.Fn SRPL_FOREACH_LOCKED
creates a loop for traversing the elements in the SRP list
.Fa sl .
.Pp
.Fn SRPL_INSERT_HEAD_LOCKED
inserts
.Fa elm
into the SRP list
.Fa sl .
Reference counts are adjusted on the list items using the functions
specified by
.Fa rc .
.Pp
.Fn SRPL_INSERT_AFTER_LOCKED
inserts
.Fa elm
into an SRP list after the element
.Fa listelm .
Reference counts are adjusted on the list items using the functions
specified by
.Fa rc .
.Pp
.Fn SRPL_REMOVE_LOCKED
iterates over the SRP list
.Fa sl
until it finds
.Fa listelm
and then removes it.
Reference counts are adjusted on the list items using the functions
specified by
.Fa rc .
.Pp
An srpl_rc declaration can be initialised with the
.Fn SRPL_RC_INITIALIZER
macro.
.Sh CONTEXT
.Fn SRPL_INIT ,
.Fn SRPL_ENTER ,
.Fn SRPL_NEXT ,
.Fn SRPL_FOREACH ,
and
.Fn SRPL_LEAVE
may be called during autoconf, from process context, or from interrupt
context.
.Pp
.Fn srpl_rc_init ,
.Fn SRPL_EMPTY_LOCKED ,
.Fn SRPL_FIRST_LOCKED ,
.Fn SRPL_NEXT_LOCKED ,
.Fn SRPL_FOREACH_LOCKED ,
.Fn SRPL_INSERT_HEAD_LOCKED ,
.Fn SRPL_INSERT_AFTER_LOCKED ,
and
.Fn SRPL_REMOVE_LOCKED
may be called during autoconf or from process context.
An appropriate lock must be held that prevents concurrent modifications
to the list.
.Sh RETURN VALUES
.Fn SRPL_ENTER ,
.Fn SRPL_NEXT ,
.Fn SRPL_FIRST_LOCKED ,
and
.Fn SRPL_NEXT_LOCKED
return a pointer to elements in the SRP list, or
.Dv NULL
if there are no more elements.
.Pp
.Fn SRPL_EMPTY_LOCKED
returns non-zero when the list is empty, otherwise 0.
.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 .