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MPOOL(3) Linux Programmer's Manual 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  inter-
     faces.   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 spec-
     ified,  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 func-
     tions 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 argu-
     ments.  If a new page can be  allocated,  a  pointer  to  the  page  is
     returned and the page number is stored into the pgnoaddr address.  Oth-
     erwise, 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 back-
            ing 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.