.\" Copyright (c) 1990, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" SPDX-License-Identifier: BSD-4-Clause-UC
.\"
.\"	@(#)mpool.3	8.1 (Berkeley) 6/4/93
.\"
.TH mpool 3 2023-03-30 "Linux man-pages 6.04"
.UC 7
.SH NAME
mpool \- shared memory buffer pool
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <db.h>
.B #include <mpool.h>
.PP
.BI "MPOOL *mpool_open(DBT *" key ", int " fd ", pgno_t " pagesize \
", pgno_t " maxcache );
.PP
.BI "void mpool_filter(MPOOL *" mp ", void (*pgin)(void *, pgno_t, void *),"
.BI "                  void (*" pgout ")(void *, pgno_t, void *),"
.BI "                  void *" pgcookie );
.PP
.BI "void *mpool_new(MPOOL *" mp ", pgno_t *" pgnoaddr );
.BI "void *mpool_get(MPOOL *" mp ", pgno_t " pgno ", unsigned int " flags );
.BI "int mpool_put(MPOOL *" mp ", void *" pgaddr ", unsigned int " flags );
.PP
.BI "int mpool_sync(MPOOL *" mp );
.BI "int mpool_close(MPOOL *" mp );
.fi
.SH DESCRIPTION
.IR "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
.I libdb
library instead.
.PP
.I Mpool
is the library interface intended to provide page oriented buffer management
of files.
The buffers may be shared between processes.
.PP
The function
.BR mpool_open ()
initializes a memory pool.
The
.I 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
.I key
is NULL, the buffers are mapped into private memory.
The
.I fd
argument is a file descriptor for the underlying file, which must be seekable.
If
.I key
is non-NULL and matches a file already being mapped, the
.I fd
argument is ignored.
.PP
The
.I pagesize
argument is the size, in bytes, of the pages into which the file is broken up.
The
.I 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.
.PP
The
.BR mpool_filter ()
function is intended to make transparent input and output processing of the
pages possible.
If the
.I pgin
function is specified, it is called each time a buffer is read into the memory
pool from the backing file.
If the
.I pgout
function is specified, it is called each time a buffer is written into the
backing file.
Both functions are called with the
.I pgcookie
pointer, the page number and a pointer to the page to being read or written.
.PP
The function
.BR mpool_new ()
takes an
.I 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
.I pgnoaddr
address.
Otherwise, NULL is returned and
.I errno
is set.
.PP
The function
.BR mpool_get ()
takes an
.I MPOOL
pointer and a page number as arguments.
If the page exists, a pointer to the page is returned.
Otherwise, NULL is returned and
.I errno
is set.
The
.I flags
argument is not currently used.
.PP
The function
.BR mpool_put ()
unpins the page referenced by
.IR pgaddr .
.I pgaddr
must be an address previously returned by
.BR mpool_get ()
or
.BR mpool_new ().
The flag value is specified by ORing
any of the following values:
.TP
.B MPOOL_DIRTY
The page has been modified and needs to be written to the backing file.
.PP
.BR mpool_put ()
returns 0 on success and \-1 if an error occurs.
.PP
The function
.BR mpool_sync ()
writes all modified pages associated with the
.I MPOOL
pointer to the
backing file.
.BR mpool_sync ()
returns 0 on success and \-1 if an error occurs.
.PP
The
.BR mpool_close ()
function free's up any allocated memory associated with the memory pool
cookie.
Modified pages are
.B not
written to the backing file.
.BR mpool_close ()
returns 0 on success and \-1 if an error occurs.
.SH ERRORS
The
.BR mpool_open ()
function may fail and set
.I errno
for any of the errors specified for the library routine
.BR malloc (3).
.PP
The
.BR mpool_get ()
function may fail and set
.I errno
for the following:
.TP 15
.B EINVAL
The requested record doesn't exist.
.PP
The
.BR mpool_new ()
and
.BR mpool_get ()
functions may fail and set
.I errno
for any of the errors specified for the library routines
.BR read (2),
.BR write (2),
and
.BR malloc (3).
.PP
The
.BR mpool_sync ()
function may fail and set
.I errno
for any of the errors specified for the library routine
.BR write (2).
.PP
The
.BR mpool_close ()
function may fail and set
.I errno
for any of the errors specified for the library routine
.BR free (3).
.SH STANDARDS
BSD.
.SH SEE ALSO
.BR btree (3),
.BR dbopen (3),
.BR hash (3),
.BR recno (3)