MPOOL(3) OpenBSD Programmer's Manual MPOOL(3)
mpool_open, mpool_filter, mpool_new, mpool_delete, mpool_get, mpool_put,
mpool_sync, mpool_close - shared memory buffer pool
mpool_open(void *key, int fd, pgno_t pagesize, pgno_t maxcache);
mpool_filter(MPOOL *mp, void (*pgin)(void *, pgno_t, void *), void
(*pgout)(void *, pgno_t, void *), void *pgcookie);
mpool_new(MPOOL *mp, pgno_t *pgnoaddr, unsigned int flags);
mpool_delete(MPOOL *mp, void *page);
mpool_get(MPOOL *mp, pgno_t pgno, unsigned int flags);
mpool_put(MPOOL *mp, void *pgaddr, unsigned int flags);
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
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
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:
Allocate a new page with a specific page number.
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
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:
The page has been modified and needs to be written to the backing
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.
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
[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
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.
btree(3), dbopen(3), hash(3), recno(3)
OpenBSD 5.4 June 5, 2013 OpenBSD 5.4
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