NAME¶
pbuf
,
getpbuf
,
trypbuf
,
relpbuf
—
functions for managing physical buffers
SYNOPSIS¶
#include
<sys/param.h>
#include
<sys/systm.h>
#include
<sys/bio.h>
#include
<sys/buf.h>
struct buf *
getpbuf
(
int
*pfreecnt);
struct buf *
trypbuf
(
int
*pfreecnt);
void
relpbuf
(
struct
buf *bp,
int
*pfreecnt);
DESCRIPTION¶
These functions are used to allocate and release physical buffers.
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.
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
nswbuf which is defined in
<sys/buf.h>
and initialized in
cpu_startup
(). A
recommended initialization value is 1/2
nswbuf.
The
getpbuf
() function returns the first
available buffer to the user. If there are no buffers available,
getpbuf
() will sleep waiting for one to
become available. If
pfreecnt is zero,
getpbuf
() will sleep until it increases.
pfreecnt is decremented prior to returning.
The
trypbuf
() function returns the first
available buffer. If there are no buffers available,
NULL
is returned. As well, if
pfreecnt is zero,
NULL
is returned.
pfreecnt is decremented prior to returning a
valid buffer. If
NULL
is returned,
pfreecnt is not modified.
The
relpbuf
() function releases the buffer
back to the free list. If the buffers
b_rcred
or
b_wcred structures are not
NULL
, they are freed. See
crfree(9).
pfreecnt is incremented prior to returning.
RETURN VALUES¶
getpbuf
() and
trypbuf
() return a pointer to the buffer.
In the case of
trypbuf
(),
NULL
can also be returned indicating that
there are no buffers available.
AUTHORS¶
This manual page was written by
Chad David
⟨davidc@acns.ab.ca⟩.