April 17, 2003 MPOOL 3

### NAME

mpool, mpool_open, mpool_filter, mpool_new, mpool_get, mpool_put, mpool_sync, mpool_close - shared memory buffer pool

### SYNOPSIS

```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 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 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

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