FreeBSD 7.0 manual page repository

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



      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 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 #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
      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 〈〉.


Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.