PBUF(9) BSD Kernel Developer's Manual PBUF(9)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 man‐
age 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 VALUESgetpbuf() 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⟩.
BSD July 9, 2001 BSD
