# mpool

## 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_**,**
                             **typeof(void (void *, pgno_t, void *)) ***_pgin_**,**
                             **typeof(void (void *, pgno_t, void *)) ***_pgout_**,**
                             **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.
    


* * *