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PBUF(9) Kernel Developer's Manual PBUF(9)


pbuf, getpbuf, trypbuf, relpbuffunctions for managing physical buffers


#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);

relpbuf(struct buf *bp, int *pfreecnt);


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 (). A recommended initialization value is 1/2 nswbuf.

The () 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 () 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 () 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.


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.


This manual page was written by Chad David <>.

July 9, 2001 Debian