bionic (9) getpbuf.9freebsd.gz

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>.