mpool, MPOOL, mpool_open, mpool_filter, mpool_new, mpool_get,
mpool_put, mpool_sync, mpool_close - shared memory buffer pool
#include <db.h> #include <mpool.h>
MPOOL * mpool_open(
pgno_t maxcache ); void mpool_filter(
void (*pgin)(void *, pgno_t, void *),
void (*pgout)(void *, pgno_t, void *),
void *pgcookie ); void * mpool_new(
pgno_t *pgnoaddr ); void * mpool_get(
u_int flags ); int mpool_put(
u_int flags ); int mpool_sync(
MPOOL *mp ); int mpool_close(
MPOOL *mp );
The mpool routines are the library interface that provides page-ori‐
ented buffer management of files. The buffers may be shared between
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 point in time. This
value is not relative to the number of processes that share a file's
buffers, but will be the largest value specified by any of the pro‐
cesses sharing the file.
The function mpool_filter() 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 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
arguments. If the page exists, a pointer to the page is returned. Oth‐
erwise, 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 option value is specified by ORing any of the following values: The
page has been modified and needs to be written to the backing file.
The function mpool_put() returns 0 on success and -1 if an error
The function mpool_sync() writes all modified pages associated with the
MPOOL pointer to the backing file. It returns 0 on success and -1 if an
The function mpool_close() frees up any allocated memory associated
with the memory pool cookie. Modified pages are not written to the
backing file. The function returns 0 on success and -1 if an error
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: The
requested record does not 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),
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).
SEE ALSOdbopen(3), btree(3), hash(3), recno(3)mpool(3)