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+.\"
+.\" Copyright (C) 2001 Chad David <davidc@acns.ab.ca>. All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice(s), this list of conditions and the following disclaimer as
+.\"    the first lines of this file unmodified other than the possible
+.\"    addition of one or more copyright notices.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice(s), this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
+.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+.\" DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
+.\" DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+.\" CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
+.\" DAMAGE.
+.\"
+.\" $FreeBSD$
+.\"
+.Dd July 9, 2001
+.Dt PBUF 9
+.Os
+.Sh NAME
+.Nm pbuf ,
+.Nm getpbuf ,
+.Nm trypbuf ,
+.Nm relpbuf
+.Nd "functions for managing physical buffers"
+.Sh SYNOPSIS
+.In sys/param.h
+.In sys/systm.h
+.In sys/bio.h
+.In sys/buf.h
+.Ft "struct buf *"
+.Fn getpbuf "int *pfreecnt"
+.Ft "struct buf *"
+.Fn trypbuf "int *pfreecnt"
+.Ft void
+.Fn relpbuf "struct buf *bp" "int *pfreecnt"
+.Sh DESCRIPTION
+These functions are used to allocate and release physical buffers.
+.Pp
+The physical buffers are allocated at system startup and are
+maintained in a separate pool from the main system buffers.
+They are intended for use by subsystems that cannot or should not be
+reliant on the main pool of buffers (for example the swap pager).
+The system allocates between 16 and 256 physical buffers depending
+on the amount of memory in the system.
+.Pp
+Each subsystem that allocates buffers via these calls is expected
+to manage its own percentage free counter.
+If the value is initialized to \-1 the number of buffers available
+to the subsystem is limited only by the number of physical buffers
+available.
+The number of buffers is stored in
+.Va nswbuf
+which is defined in
+.In sys/buf.h
+and initialized in
+.Fn cpu_startup .
+A recommended initialization value is 1/2
+.Va nswbuf .
+.Pp
+The
+.Fn getpbuf
+function returns the first available buffer to the user.
+If there are no buffers available,
+.Fn getpbuf
+will sleep waiting for one to become available.
+If
+.Fa pfreecnt
+is zero,
+.Fn getpbuf
+will sleep until it increases.
+.Fa pfreecnt
+is decremented prior to returning.
+.Pp
+The
+.Fn trypbuf
+function returns the first available buffer.
+If there are no buffers available,
+.Dv NULL
+is returned.
+As well, if
+.Fa pfreecnt
+is zero,
+.Dv NULL
+is returned.
+.Fa pfreecnt
+is decremented prior to returning a valid buffer.
+If
+.Dv NULL
+is returned,
+.Fa pfreecnt
+is not modified.
+.Pp
+The
+.Fn relpbuf
+function releases the buffer back to the free list.
+If the buffers
+.Va b_rcred
+or
+.Va b_wcred
+structures are not
+.Dv NULL ,
+they are freed.
+See
+.Xr crfree 9 .
+.Pp
+.Fa pfreecnt
+is incremented prior to returning.
+.Sh RETURN VALUES
+.Fn getpbuf
+and
+.Fn trypbuf
+return a pointer to the buffer.
+In the case of
+.Fn trypbuf ,
+.Dv NULL
+can also be returned indicating that there are no buffers available.
+.Sh AUTHORS
+This manual page was written by
+.An Chad David Aq Mt davidc@acns.ab.ca .