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The dbm library: access to NDBM databases
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<H1>Chapter 23: The dbm library: access to NDBM databases</H1>
The <TT>dbm</TT> library provides access to NDBM databases under Unix.
NDBM databases maintain key/data associations, where both the key and
the data are arbitrary strings. They support fairly large databases
(several gigabytes) and can retrieve a keyed item in one or two file
system accesses. Refer to the Unix manual pages for more information.<BR>
<BR>
<FONT COLOR=purple>Unix:</FONT>
<BLOCKQUOTE>
Programs that use the <TT>dbm</TT> library must be linked in ``custom
runtime'' mode, as follows:
<PRE>
ocamlc -custom <I>other options</I> dbm.cma <I>other files</I> -cclib -lmldbm -cclib -lndbm
ocamlopt <I>other options</I> dbm.cmxa <I>other files</I> -cclib -lmldbm -cclib -lndbm
</PRE>
For interactive use of the <TT>dbm</TT> library, do:
<PRE>
ocamlmktop -custom -o mytop dbm.cma -cclib -lmldbm -cclib -lndbm
./mytop
</PRE>
Depending on the Unix system used, the <TT>-cclib -lndbm</TT> option is not
always necessary, or the library may have another name than <TT>-lndbm</TT>.
</BLOCKQUOTE>
<FONT COLOR=purple>Windows:</FONT>
<BLOCKQUOTE>
This library is not available.
</BLOCKQUOTE>
<H2>23.1 Module <TT>Dbm</TT>: interface to the NDBM databases</H2><A NAME="s:Dbm"></A>
<A NAME="@manual879"></A><PRE>
type t
</PRE>
<BLOCKQUOTE>
The type of file descriptors opened on NDBM databases.
</BLOCKQUOTE>
<PRE>
type open_flag =
Dbm_rdonly | Dbm_wronly | Dbm_rdwr | Dbm_create
</PRE>
<BLOCKQUOTE>
Flags for opening a database (see <CODE>opendbm</CODE>).
</BLOCKQUOTE>
<PRE>
exception Dbm_error of string
</PRE>
<A NAME="@manual880"></A><BLOCKQUOTE>
Raised by the following functions when an error is encountered.
</BLOCKQUOTE>
<PRE>
val opendbm : string -> open_flag list -> int -> t
</PRE>
<A NAME="@manual881"></A><BLOCKQUOTE>
Open a descriptor on an NDBM database. The first argument is
the name of the database (without the <CODE>.dir</CODE> and <CODE>.pag</CODE> suffixes).
The second argument is a list of flags: <CODE>Dbm_rdonly</CODE> opens
the database for reading only, <CODE>Dbm_wronly</CODE> for writing only,
<CODE>Dbm_rdwr</CODE> for reading and writing; <CODE>Dbm_create</CODE> causes the
database to be created if it does not already exist.
The third argument is the permissions to give to the database
files, if the database is created.
</BLOCKQUOTE>
<PRE>
val close : t -> unit
</PRE>
<A NAME="@manual882"></A><BLOCKQUOTE>
Close the given descriptor.
</BLOCKQUOTE>
<PRE>
val find : t -> string -> string
</PRE>
<A NAME="@manual883"></A><BLOCKQUOTE>
<CODE>find db key</CODE> returns the data associated with the given
<CODE>key</CODE> in the database opened for the descriptor <CODE>db</CODE>.
Raise <CODE>Not_found</CODE> if the <CODE>key</CODE> has no associated data.
</BLOCKQUOTE>
<PRE>
val add : t -> string -> string -> unit
</PRE>
<A NAME="@manual884"></A><BLOCKQUOTE>
<CODE>add db key data</CODE> inserts the pair (<CODE>key</CODE>, <CODE>data</CODE>) in
the database <CODE>db</CODE>. If the database already contains data
associated with <CODE>key</CODE>, raise <CODE>Dbm_error "Entry already exists"</CODE>.
</BLOCKQUOTE>
<PRE>
val replace : t -> string -> string -> unit
</PRE>
<A NAME="@manual885"></A><BLOCKQUOTE>
<CODE>replace db key data</CODE> inserts the pair (<CODE>key</CODE>, <CODE>data</CODE>) in
the database <CODE>db</CODE>. If the database already contains data
associated with <CODE>key</CODE>, that data is discarded and silently
replaced by the new <CODE>data</CODE>.
</BLOCKQUOTE>
<PRE>
val remove : t -> string -> unit
</PRE>
<A NAME="@manual886"></A><BLOCKQUOTE>
<CODE>remove db key data</CODE> removes the data associated with <CODE>key</CODE>
in <CODE>db</CODE>. If <CODE>key</CODE> has no associated data, raise
<CODE>Dbm_error "dbm_delete"</CODE>.
</BLOCKQUOTE>
<PRE>
val firstkey : t -> string
val nextkey : t -> string
</PRE>
<A NAME="@manual887"></A><A NAME="@manual888"></A><BLOCKQUOTE>
Enumerate all keys in the given database, in an unspecified order.
<CODE>firstkey db</CODE> returns the first key, and repeated calls
to <CODE>nextkey db</CODE> return the remaining keys. <CODE>Not_found</CODE> is raised
when all keys have been enumerated.
</BLOCKQUOTE>
<PRE>
val iter : (string -> string -> 'a) -> t -> unit
</PRE>
<A NAME="@manual889"></A><BLOCKQUOTE>
<CODE>iter f db</CODE> applies <CODE>f</CODE> to each (<CODE>key</CODE>, <CODE>data</CODE>) pair in
the database <CODE>db</CODE>. <CODE>f</CODE> receives <CODE>key</CODE> as first argument
and <CODE>data</CODE> as second argument.
</BLOCKQUOTE>
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