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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 #include <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〉.