MPOOL(3) UNIX System V (June 4, 1993) MPOOL(3)
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, u_int flags);
int
mpool_put (MPOOL *mp, void *pgaddr, u_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 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 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 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 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).
SEE ALSO
dbopen(3), btree(3), hash(3), recno(3)
Page 3 (printed 4/30/98)