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