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 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>.

Debian                                            July 9, 2001                                           PBUF(9)