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NAME
mpool - shared memory buffer pool
LIBRARY
Standard C library (libc, -lc)
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
Note well: This page documents interfaces provided up until glibc 2.1. Since glibc 2.2, glibc no longer
provides these interfaces. Probably, you are looking for the APIs provided by the libdb library instead.
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 ORing 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).
STANDARDS
BSD.
SEE ALSO
btree(3), dbopen(3), hash(3), recno(3)