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 allo

cates between 16
and 256 physical buffers depending on the amount of memory

in the system.

Each subsystem that allocates buffers via these calls is ex

pected to manage its own percentage free counter. If the value is ini

tialized 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 #include <sys/buf.h> and initialized in cpu_startup(). A recommended initializa

tion 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 return

ing 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@ac

ns.ab.ca>.

BSD July 9, 2001

BSD