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NAME

       mpool - shared memory buffer pool

SYNOPSIS

       #include <db.h>
       #include <mpool.h>

       MPOOL *mpool_open(DBT *key, int fd, pgno_t pagesize, pgno_t maxcache);

       void mpool_filter(MPOOL *mp, void (*pgin)(void *, pgno_t, void *),
                         void (*pgout)(void *, pgno_t, void *),
                         void *pgcookie);

       void *mpool_new(MPOOL *mp, pgno_t *pgnoaddr);

       void *mpool_get(MPOOL *mp, pgno_t pgno, unsigned int flags);

       int mpool_put(MPOOL *mp, void *pgaddr, unsigned int flags);

       int mpool_sync(MPOOL *mp);

       int mpool_close(MPOOL *mp);

DESCRIPTION

       Mpool is the library interface intended to provide page oriented buffer
       management of files.  The buffers may be shared between processes.

       The function mpool_open() initializes a memory pool.  The key  argument
       is the byte string used to negotiate between multiple processes wishing
       to share buffers.  If the file buffers are mapped in shared memory, all
       processes  using  the same key will share the buffers.  If key is NULL,
       the buffers are mapped into private memory.  The fd argument is a  file
       descriptor  for the underlying file, which must be seekable.  If key is
       non-NULL and matches a file already being mapped, the  fd  argument  is
       ignored.

       The  pagesize  argument  is the size, in bytes, of the pages into which
       the file is broken up.  The maxcache argument is the maximum number  of
       pages from the underlying file to cache at any one time.  This value is
       not relative to the number of processes which share a  file’s  buffers,
       but will be the largest value specified by any of the processes sharing
       the file.

       The mpool_filter() function is intended to make transparent  input  and
       output  processing  of  the  pages  possible.   If the pgin function is
       specified, it is called each time a buffer is read into the memory pool
       from  the  backing  file.   If  the  pgout function is specified, it is
       called each time a buffer is  written  into  the  backing  file.   Both
       functions  are  called with the pgcookie pointer, the page number and a
       pointer to the page to being read or written.

       The function mpool_new() takes an  MPOOL  pointer  and  an  address  as
       arguments.   If  a  new page can be allocated, a pointer to the page is
       returned and the page number  is  stored  into  the  pgnoaddr  address.
       Otherwise, NULL is returned and errno is set.

       The  function  mpool_get()  takes an MPOOL pointer and a page number as
       arguments.  If the page exists, a pointer  to  the  page  is  returned.
       Otherwise,  NULL  is  returned and errno is set.  The flags argument is
       not currently used.

       The function mpool_put() unpins the page referenced by pgaddr.   pgaddr
       must  be  an address previously returned by mpool_get() or mpool_new().
       The flag value is specified by or’ing any of the following values:

       MPOOL_DIRTY
              The page has been modified  and  needs  to  be  written  to  the
              backing file.

       mpool_put() returns 0 on success and -1 if an error occurs.

       The function mpool_sync() writes all modified pages associated with the
       MPOOL pointer to the backing file.  mpool_sync() returns 0  on  success
       and -1 if an error occurs.

       The  mpool_close()  function  free’s up any allocated memory associated
       with the memory pool cookie.  Modified pages are  not  written  to  the
       backing  file.   mpool_close()  returns 0 on success and -1 if an error
       occurs.

ERRORS

       The mpool_open() function may fail and set errno for any of the  errors
       specified for the library routine malloc(3).

       The mpool_get() function may fail and set errno for the following:

       EINVAL         The requested record doesn’t exist.

       The  mpool_new()  and  mpool_get() functions may fail and set errno for
       any of the errors specified for the library routines read(2), write(2),
       and malloc(3).

       The  mpool_sync() function may fail and set errno for any of the errors
       specified for the library routine write(2).

       The mpool_close() function may fail and set errno for any of the errors
       specified for the library routine free(3).

CONFORMING TO

       Not in POSIX.1-2001.  Present on the BSDs.

SEE ALSO

       btree(3), dbopen(3), hash(3), recno(3)

COLOPHON

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       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.