freebsd-dev/share/man/man9/pbuf.9

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.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 davidc@acns.ab.ca .