table of contents
PBUF(9) | Kernel Developer's Manual | PBUF(9) |
NAME¶
pbuf
, getpbuf
,
trypbuf
, relpbuf
—
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>.July 9, 2001 | Linux 4.19.0-10-amd64 |