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     pbuf, getpbuf, trypbuf, relpbuf — functions 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

     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

     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.


     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 <>.