MPOOL(3)		   Linux Programmers Manual		     MPOOL(3)



NAME
       mpool - shared memory buffer pool

SYNOPSIS
       #include 
       #include 

       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  files  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 spec
       ified, 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 func
       tions  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 argu
       ments.	If  a  new  page  can  be  allocated, a pointer to the page is
       returned and the page number is stored into the pgnoaddr address.  Oth
       erwise, 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 oring any of the following values:

       MPOOL_DIRTY
	      The page has been modified and needs to be written to the  back
	      ing 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  frees 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 doesnt 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
       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



4.4 Berkeley Distribution	  1993-06-04			      MPOOL(3)