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
This page is part of release 3.24 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/.