MPOOL(3)

NAME
mpool_open, mpool_filter, mpool_new, mpool_delete, mpool_get, mpool_put, mpool_sync, mpool_close - 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, unsigned int flags);
int
mpool_delete(MPOOL *mp, void *page);
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 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 arguments. If the page exists, a pointer to the page is returned. Otherwise, NULL is returned and errno is set. The flags parameter 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 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 frees 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 routines fstat(2) and malloc(3), or the following:

[ESPIPE]
    The given file descriptor is a pipe.

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 pread(2), pwrite(2), and malloc(3).

The mpool_sync() function may fail and set errno for any of the errors specified for the library routine pwrite(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)