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.\"	@(#)mpool.3	8.1 (Berkeley) 6/4/93
.\"
.Dd February 25, 1999
.Dt MPOOL 3
.Os
.Sh NAME
.Nm mpool_open ,
.Nm mpool_filter ,
.Nm mpool_new ,
.Nm mpool_delete ,
.Nm mpool_get ,
.Nm mpool_put ,
.Nm mpool_sync ,
.Nm mpool_close
.Nd shared memory buffer pool
.Sh SYNOPSIS
.Fd #include <db.h>
.Fd #include <mpool.h>
.Ft MPOOL *
.Fn mpool_open "DBT *key" "int fd" "pgno_t pagesize" "pgno_t maxcache"
.Ft void
.Fn mpool_filter "MPOOL *mp" "void (*pgin)(void *, pgno_t, void *)" "void (*pgout)(void *, pgno_t, void *)" "void *pgcookie"
.Ft void *
.Fn mpool_new "MPOOL *mp" "pgno_t *pgnoaddr" "u_int flags"
.Ft int
.Fn mpool_delete "MPOOL *mp" "void *page"
.Ft void *
.Fn mpool_get "MPOOL *mp" "pgno_t pgno" "u_int flags"
.Ft int
.Fn mpool_put "MPOOL *mp" "void *pgaddr" "u_int flags"
.Ft int
.Fn mpool_sync "MPOOL *mp"
.Ft int
.Fn mpool_close "MPOOL *mp"
.Sh DESCRIPTION
.Nm mpool
is the library interface intended to provide page oriented buffer management
of files.
The buffers may be shared between processes.
.Pp
The function
.Fn mpool_open
initializes a memory pool.
The
.Fa 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
.Fa key
is
.Dv NULL ,
the buffers are mapped into private memory.
The
.Fa fd
argument is a file descriptor for the underlying file, which must be seekable.
If
.Fa key
is non-NULL and matches a file already being mapped, the
.Fa fd
argument is ignored.
.Pp
The
.Fa pagesize
argument is the size, in bytes, of the pages into which the file is broken up.
The
.Fa 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
.Fn mpool_filter
function is intended to make transparent input and output processing of the
pages possible.
If the
.Fa pgin
function is specified, it is called each time a buffer is read into the memory
pool from the backing file.
If the
.Fa pgout
function is specified, it is called each time a buffer is written into the
backing file.
Both functions are called with the
.Fa pgcookie
pointer, the page number and a pointer to the page to being read or written.
.Pp
The function
.Fn mpool_new
takes an
.Dv MPOOL
pointer, an address, and a set of flags as arguments.
If a new page can be allocated, a pointer to the page is returned and
the page number is stored into the
.Fa pgnoaddr
address.
Otherwise,
.Dv NULL
is returned and
.Va errno
is set.
The flags value is formed by
.Tn OR Ns 'ing
the following values:
.Bl -tag -width Ds
.It Dv MPOOL_PAGE_REQUEST
Allocate a new page with a specific page number.
.It Dv MPOOL_PAGE_NEXT
Allocate a new page with the next page number.
.El
.Pp
The function
.Fn mpool_delete
deletes the specified page from a pool and frees the page.
It takes an
.Dv MPOOL
pointer and a page as arguments.
The page must have been generated by
.Fn mpool_new .
.Pp
The function
.Fn mpool_get
takes an
.Dv MPOOL
pointer and a page number as arguments.
If the page exists, a pointer to the page is returned.
Otherwise,
.Dv NULL
is returned and
.Va errno
is set.
The flags parameter is not currently used.
.Pp
The function
.Fn mpool_put
unpins the page referenced by
.Fa pgaddr .
.Fa pgaddr
must be an address previously returned by
.Fn mpool_get
or
.Fn mpool_new .
The flags value is formed by
.Tn OR Ns 'ing
the following values:
.Bl -tag -width Ds
.It Dv MPOOL_DIRTY
The page has been modified and needs to be written to the backing file.
.El
.Pp
.Fn mpool_put
returns 0 on success and \-1 if an error occurs.
.Pp
The function
.Fn mpool_sync
writes all modified pages associated with the
.Dv MPOOL
pointer to the backing file.
.Fn mpool_sync
returns 0 on success and \-1 if an error occurs.
.Pp
The
.Fn mpool_close
function free's up any allocated memory associated with the memory pool
cookie.
Modified pages are
.Em not
written to the backing file.
.Fn mpool_close
returns 0 on success and \-1 if an error occurs.
.Sh ERRORS
The
.Fn mpool_open
function may fail and set
.Va errno
for any of the errors specified for the library routine
.Xr malloc 3 .
.Pp
The
.Fn mpool_get
function may fail and set
.Va errno
for the following:
.Bl -tag -width Er
.It Bq Er EINVAL
The requested record doesn't exist.
.El
.Pp
The
.Fn mpool_new
and
.Fn mpool_get
functions may fail and set
.Va errno
for any of the errors specified for the library routines
.Xr read 2 ,
.Xr write 2 ,
and
.Xr malloc 3 .
.Pp
The
.Fn mpool_sync
function may fail and set
.Va errno
for any of the errors specified for the library routine
.Xr write 2 .
.Pp
The
.Fn mpool_close
function may fail and set
.Va errno
for any of the errors specified for the library routine
.Xr free 3 .
.Pp
The
.Fn mpool_delete
always acts as if it succeeded.
.Sh SEE ALSO
.Xr btree 3 ,
.Xr dbopen 3 ,
.Xr hash 3 ,
.Xr recno 3