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, 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

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   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/.

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