mpool - shared memory buffer pool
mpool_open (DBT *key, int fd, pgno_t pagesize, pgno_t maxcache);
mpool_filter (MPOOL *mp, void (*pgin)(void *, pgno_t, void *),
void (*pgout)(void *, pgno_t, void *), void *pgcookie);
mpool_new (MPOOL *mp, pgno_t *pgnoaddr);
mpool_get (MPOOL *mp, pgno_t pgno, u_int flags);
mpool_put (MPOOL *mp, void *pgaddr, u_int flags);
mpool_sync (MPOOL *mp);
mpool_close (MPOOL *mp);
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
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 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 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 a MPOOL pointer and a page number as argu-
ments. 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
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:
The page has been modified and needs to be written to the back-
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.
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
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).
dbopen(3), btree(3), hash(3), recno(3)
4.4 Berkeley Distribution 1993-06-04 MPOOL(3)