MPOOL(3)                  OpenBSD Programmer's Manual                 MPOOL(3)

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, u_int flags);

     int
     mpool_delete(MPOOL *mp, void *page);

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

     int
     mpool_put(MPOOL *mp, void *pgaddr, u_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 pro-
     cesses 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 de-
     scriptor 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 speci-
     fied, 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, an address, and a set of
     flags 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 flags value is formed

     by OR'ing the following values:

     MPOOL_PAGE_REQUEST
             Allocate a new page with a specific page number.

     MPOOL_PAGE_NEXT
             Allocate a new page with the next page number.

     The function mpool_delete() deletes the specified page from a pool and
     frees the page.  It takes an MPOOL pointer and a page as arguments.  The
     page must have been generated by mpool_new().

     The function mpool_get() takes an MPOOL pointer and a page number as ar-
     guments.  If the page exists, a pointer to the page is returned.  Other-
     wise, NULL is returned and errno is set.  The flags parameter is not cur-
     rently 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 flags value is formed by OR'ing 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).

     The mpool_delete() always acts as if it succeeded.

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

OpenBSD 2.6                    February 25, 1999                             2
