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, 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 in glibc up until version 2.1.  Since
       version 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).

CONFORMING TO

       Not in POSIX.1.  Present on the BSDs.

SEE ALSO

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

COLOPHON

       This  page  is  part of release 4.15 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at https://www.kernel.org/doc/man-pages/.