MPOOL(3)

NAME
mpool - shared memory buffer pool

SYNOPSIS
#include <db.h>
#include <mpool.h>
MPOOL *
mpool_open(void *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
The mpool library interface is intended to provide page oriented buffer management of files.

The mpool_open() function initializes a memory pool. The key argument is currently ignored. The fd argument is a file descriptor for the underlying file, which must be seekable.

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, 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 mpool_get() function takes a 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 specified by or'ing any of the following values:

MPOOL_IGNOREPIN
    The page returned is not pinned; page will otherwise be pinned on return.

The mpool_put() function unpins the page referenced by pgaddr. The pgaddr argument must be an address previously returned by mpool_get() or mpool_new(). The flags argument 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.

The mpool_put() function returns 0 on success and -1 if an error occurs.

The mpool_sync() function writes all modified pages associated with the MPOOL pointer to the backing file. The mpool_sync() function 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. The mpool_close() function 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 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), 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).

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