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

     pbuf, getpbuf, trypbuf, relpbuf --	functions for managing physical	buf-

     #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 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

     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

     This manual page was written by Chad David	<>.

FreeBSD	13.0			 July 9, 2001			  FreeBSD 13.0


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