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<A HREF="http://www.erlang.se"><IMG BORDER=0 ALT="[Erlang Systems]" SRC="min_head.gif"></A>
<H1>dets</H1>
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<H3>MODULE</H3>
<UL>
dets</UL>
<H3>MODULE SUMMARY</H3>
<UL>
A Disk Based Term Storage</UL>
<H3>DESCRIPTION</H3>
<UL>
<P>The module <CODE>dets</CODE> provides a term storage on file. The
stored terms, in this module called <STRONG>objects</STRONG>, are tuples
such that one element is defined to be the key. A Dets
<STRONG>table</STRONG> is a collection of objects with the key at the same
position stored on a file.<P>Dets is used by the Mnesia application, and is provided as is
for users who are interested in an efficient storage of Erlang
terms on disk only. Many applications just need to store some
terms in a file. Mnesia adds transactions, queries, and
distribution.<P>There are three types of Dets tables: set, bag and
duplicate_bag. A table of type <STRONG>set</STRONG> has at most one object
with a given key. If an object with a key already present in the
table is inserted, the existing object is overwritten by the new
object. A table of type <STRONG>bag</STRONG> has zero or more different
objects with a given key. A table of type <STRONG>duplicate_bag</STRONG>
has zero or more possibly equal objects with a given key.<P>Dets tables must be opened before they can be updated or read,
and when finished they must be properly closed. If a table has not
been properly closed, Dets will automatically repair the table.
This can take a substantial time if the table is large. A Dets
table is closed when the process which opened the table
terminates. If several Erlang processes (users) open the same Dets
table, they will share the table. The table is properly closed
when all users have either terminated or closed the table. Dets
tables are not properly closed if the Erlang runtime system is
terminated abnormally.<P><TABLE CELLPADDING=4>
<TR>
      <TD VALIGN=TOP><IMG ALT="Note!" SRC="note.gif"></TD>
      <TD>
<P>A ^C command abnormally terminates an Erlang runtime
system in a Unix environment with a break-handler.</TD>
</TR>
      </TABLE>
<P>Since all operations performed by Dets are disk operations, it
is important to realize that a single look-up operation involves a
series of disk seek and read operations. For this reason, the Dets
functions are much slower than the corresponding Ets functions,
although Dets exports a similar interface.<P>Dets organizes data as a linear hash list and the hash list
grows gracefully as more data is inserted into the table. Space
management on the file is performed by what is called a buddy
system. The current implementation keeps the entire buddy system
in RAM, which implies that if the table gets heavily fragmented,
quite some memory can be used up. The only way to defragment a
table is to close it and then open it again with the <CODE>repair</CODE>
option set to <CODE>force</CODE>.<P>It is worth noting that the ordered_set type present in Ets is
not yet implemented by Dets, neither is the limited support for
concurrent updates which makes a sequence of <CODE>first</CODE> and
<CODE>next</CODE> calls safe to use on fixed Ets tables. Both these
features will be implemented by Dets in a future release of
Erlang/OTP. Until then, the Mnesia application (or some user
implemented method for locking) has to be used to implement safe
concurrency. Currently, no library of Erlang/OTP has support for
ordered disk based term storage.<P>Two versions of the format used for storing objects on file are
supported by Dets. The first version, 8, is the format always used
for tables created by OTP R7 and earlier. The second version, 9,
is the default version of tables created by OTP R8 (and later OTP
releases). OTP R8 can create version 8 tables, and convert version
8 tables to version 9, and vice versa, upon request.

<P>All Dets functions return <CODE>{error, Reason}</CODE> if an error
occurs (<CODE>first/1</CODE> and <CODE>next/2</CODE> are exceptions, they exit
the process with the error tuple). If given badly formed
arguments, all functions exit the process with a <CODE>badarg</CODE>
message.<P><STRONG>Types</STRONG>
<PRE>access() = read | read_write
auto_save() = infinity | int()
bindings_cont() = tuple()
bool() = true | false
file() = string()
int() = integer() &#62;= 0
keypos() = integer() &#62;= 1
name() = atom() | ref()
no_slots() = integer() &#62;= 1 | default
object() = tuple()
object_cont() = tuple()
select_cont() = tuple()
type() = bag | duplicate_bag | set
version() = 8 | 9 | default</PRE>
</UL>
<H3>EXPORTS</H3>
<P><A NAME="all%0"><STRONG><CODE>all() -&#62; [Name]</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Returns a list of the names of all open tables on this
node.</UL>
<P><A NAME="close%1"><STRONG><CODE>close(Name) -&#62; ok | {error, Reason} </CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Closes a table. Only processes that have opened a table are
        allowed to close it.

<P>All open tables must be closed before the system is
stopped. If an attempt is made to open a table which has not
been properly closed, Dets automatically tries to repair the
table.</UL>
<P><A NAME="delete%2"><STRONG><CODE>delete(Name, Key) -&#62; ok | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Deletes all objects with the key <CODE>Key</CODE> from the table
<CODE>Name</CODE>.</UL>
<P><A NAME="delete_all_objects%1"><STRONG><CODE>delete_all_objects(Name) -&#62; ok | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Deletes all objects from a table in almost constant
time.</UL>
<P><A NAME="delete_object%2"><STRONG><CODE>delete_object(Name, Object) -&#62; ok | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>Object = object()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Deletes all instances of a given object from a table. If a
table is of type <CODE>bag</CODE> or <CODE>duplicate_bag</CODE>, the
<CODE>delete/2</CODE> function cannot be used to delete only some of
the objects with a given key. This function makes this
possible.</UL>
<P><A NAME="first%1"><STRONG><CODE>first(Name) -&#62; Key | '$end_of_table'</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Key = term()</CODE></STRONG><BR>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Returns the first key stored in the table <CODE>Name</CODE>
        according to the table's internal order, or
        <CODE>'$end_of_table'</CODE> if the table is empty.<P>Unless the table is protected using <CODE>safe_fixtable/2</CODE>,
        subsequent calls to <CODE>next/2</CODE> may not work as expected if
        concurrent updates are made to the table.<P>Should an error occur, the process is exited with an error
        tuple <CODE>{error, Reason}</CODE>. The reason for not returning the
        error tuple is that it cannot be distinguished from a key.<P>There are two reasons why <CODE>first/1</CODE> and <CODE>next/2</CODE>
should not be used: they are not very efficient, and they
prevent the use of the key <CODE>'$end_of_table'</CODE> since this
atom is used to indicate the end of the table. If possible,
the <CODE>match</CODE>, <CODE>match_object</CODE>, and <CODE>select</CODE>
functions should be used for traversing tables.</UL>
<P><A NAME="foldl%3"><STRONG><CODE>foldl(Function, Acc0, Name) -&#62; Acc1 | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Function = fun(Object, AccIn) -&#62; AccOut</CODE></STRONG><BR>
<STRONG><CODE>Acc0 = Acc1 = AccIn = AccOut = term()</CODE></STRONG><BR>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>Object = object()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Calls <CODE>Function</CODE> on successive elements of the table
<CODE>Name</CODE> together with an extra argument <CODE>AccIn</CODE>. The
order in which the elements of the table are traversed is
unspecified. <CODE>Function</CODE> must return a new accumulator
which is passed to the next call. <CODE>Acc0</CODE> is returned if
the table is empty.
</UL>
<P><A NAME="foldr%3"><STRONG><CODE>foldr(Function, Acc0, Name) -&#62; Acc1 | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Function = fun(Object, AccIn) -&#62; AccOut</CODE></STRONG><BR>
<STRONG><CODE>Acc0 = Acc1 = AccIn = AccOut = term()</CODE></STRONG><BR>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>Object = object()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Calls <CODE>Function</CODE> on successive elements of the table
<CODE>Name</CODE> together with an extra argument <CODE>AccIn</CODE>. The
order in which the elements of the table are traversed is
unspecified. <CODE>Function</CODE> must return a new accumulator
which is passed to the next call. <CODE>Acc0</CODE> is returned if
the table is empty.
</UL>
<P><A NAME="from_ets%2"><STRONG><CODE>from_ets(Name, EtsTab) -&#62; ok | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>EtsTab = -&#160;see ets(3)&#160;-
</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Replaces the objects of the table <CODE>Name</CODE> with the
objects of the Ets table <CODE>EtsTab</CODE>. The order in which the
objects are inserted is not specified. Since
<CODE>ets:safe_fixtable/2</CODE> is called, the Ets table must be
public or owned by the calling process.
</UL>
<P><A NAME="info%1"><STRONG><CODE>info(Name) -&#62; InfoList | undefined</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>InfoList = [{Item, Value}]</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Returns information about the table <CODE>Name</CODE> as a list of
        <CODE>{Item, Value}</CODE> tuples:<P><UL>
<LI><CODE>{file_size, int()}</CODE>, the size of the file in
         bytes.<BR>

</LI><BR>
<LI><CODE>{filename, file()}</CODE>, the name of the file
where objects are stored.<BR>

</LI><BR>
<LI><CODE>{keypos, keypos()}</CODE>, the position of the
key.<BR>

</LI><BR>
<LI><CODE>{size, int()}</CODE>, the number of objects stored
         in the table.<BR>

</LI><BR>
<LI><CODE>{type, type()}</CODE>, the type of the table.<BR>

</LI><BR>
</UL>
</UL>
<P><A NAME="info%2"><STRONG><CODE>info(Name, Item) -&#62; Value | undefined</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Returns the information associated with <CODE>Item</CODE> for the
        table <CODE>Name</CODE>. In addition to the <CODE>{Item, Value}</CODE>
        pairs defined for <CODE>info/1</CODE>, the following items are
        allowed:<P><UL>
<LI>{<CODE>access, access()}</CODE>, the access mode.<BR>

         </LI><BR>
<LI>{<CODE>auto_save, auto_save()}</CODE>, the auto save
         interval.<BR>

         </LI><BR>
<LI>{<CODE>hash,</CODE> Hash<CODE>}</CODE>. Describes which BIF is
         used to calculate the hash values of the objects stored in
         the Dets table. Possible values of Hash are <CODE>hash</CODE>,
         which implies that the <CODE>erlang:hash/2</CODE> BIF is used,
         and <CODE>phash</CODE>, which implies that the
         <CODE>erlang:phash/2</CODE> BIF is used.<BR>

         </LI><BR>
<LI>{<CODE>memory, int()}</CODE>, the size of the file in
         bytes. The same value is associated with the item
         <CODE>file_size</CODE>.
         <BR>
</LI><BR>
<LI><CODE>{no_keys, int()}</CODE>, the number of different
         keys stored in the table. Only available for version 9
         tables.<BR>

</LI><BR>
<LI><CODE>{no_objects, int()}</CODE>, the number of objects
         stored in the table.<BR>

</LI><BR>
<LI><CODE>{no_slots, {Min, Used, Max}}</CODE>, the number of
         slots of the table. <CODE>Min</CODE> is the minimum number of
         slots, <CODE>Used</CODE> is the number of currently used slots,
         and <CODE>Max</CODE> is the maximum number of slots. Only
         available for version 9 tables.<BR>

</LI><BR>
<LI>{<CODE>owner, pid()}</CODE>, the pid of the process that
         handles requests to the Dets table.
         <BR>
</LI><BR>
<LI>{<CODE>ram_file, bool()}</CODE>, whether the table is
         kept in RAM.
         <BR>
</LI><BR>
<LI>{<CODE>safe_fixed,</CODE> SafeFixed<CODE>}</CODE>. If the table
         is fixed, SafeFixed is a tuple <CODE>{FixedAtTime,
         [{Pid,RefCount}]}</CODE>. <CODE>FixedAtTime</CODE> is the time when
         the table was first fixed, and <CODE>Pid</CODE> is the pid of
         the process that fixes the table <CODE>RefCount</CODE> times.
         There may be any number of processes in the list. If the
         table is not fixed, SafeFixed is the atom <CODE>false</CODE>.
         <BR>
</LI><BR>
<LI>{<CODE>version, int()}</CODE>, the version of the format
         of the table.
         <BR>
</LI><BR>
</UL>
</UL>
<P><A NAME="init_table%2"><STRONG><CODE>init_table(Name, InitFun) -&#62; ok | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = atom()</CODE></STRONG><BR>
<STRONG><CODE>InitFun = fun(Arg) -&#62; Res</CODE></STRONG><BR>
<STRONG><CODE>Arg = read | close</CODE></STRONG><BR>
<STRONG><CODE>Res = end_of_input | {[object()], InitFun} | term()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Replaces the existing objects of the table <CODE>Name</CODE> with
        objects created by calling the input function <CODE>InitFun</CODE>,
        see below. The reason for using this function rather than
        calling <CODE>insert/2</CODE> is that of efficiency.<P>When called with the argument <CODE>read</CODE> the function
<CODE>InitFun</CODE> is assumed to return <CODE>end_of_input</CODE> when
there is no more input, or <CODE>{Objects, Fun}</CODE>, where
<CODE>Objects</CODE> is a list of objects and <CODE>Fun</CODE> is a new
input function. Any other value Value is returned as an error
<CODE>{error, {init_fun, Value}}</CODE>. Each input function will be
called exactly once, and should an error occur, the last
function is called with the argument <CODE>close</CODE>, the reply
of which is ignored.<P>If the type of the table is <CODE>set</CODE> and there is more
        than one object with a given key, one of the objects is
        chosen. This is not necessarily the last object with the given
        key in the sequence of objects returned by the input
        functions. Extra objects should be avoided, or the file will
        be unnecessarily fragmented. This holds also for duplicated
        objects stored in tables of type <CODE>duplicate_bag</CODE>.</UL>
<P><A NAME="insert%2"><STRONG><CODE>insert(Name, Objects) -&#62; ok | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>Objects = object() | [object()]</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Inserts one or more objects into the table <CODE>Name</CODE>. If
there already exists an object with the same key as some of
the given objects and the table type is <CODE>set</CODE>, the old
object will be replaced.</UL>
<P><A NAME="is_dets_file%1"><STRONG><CODE>is_dets_file(FileName) -&#62; Bool | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>FileName = file()</CODE></STRONG><BR>
<STRONG><CODE>Bool = bool()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Returns <CODE>true</CODE> if the file <CODE>FileName</CODE> is a Dets
table, <CODE>false</CODE> otherwise.
</UL>
<P><A NAME="lookup%2"><STRONG><CODE>lookup(Name, Key) -&#62; [Object] | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Key = term()</CODE></STRONG><BR>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>Object = object()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Returns a list of all objects with the key <CODE>Key</CODE>
        stored in the table <CODE>Name</CODE>. For example:<PRE>2&#62; <STRONG>dets:open_file(abc, [{type, bag}]).</STRONG>
{ok,abc}
3&#62; <STRONG>dets:insert(abc, {1,2,3}).</STRONG>
ok
4&#62; <STRONG>dets:insert(abc, {1,3,4}).</STRONG>
ok
5&#62; <STRONG>dets:lookup(abc, 1).</STRONG>
[{1,2,3},{1,3,4}]</PRE>
<P>If the table is of type <CODE>set</CODE>, the function returns
        either the empty list or a list with one object, as there
        cannot be more than one object with a given key. If the table
        is of type <CODE>bag</CODE> or <CODE>duplicate_bag</CODE>, the function
        returns a list of arbitrary length.<P>Note that the order of objects returned is unspecified. In
particular, the order in which objects were inserted is not
reflected.</UL>
<P><A NAME="match%1"><STRONG><CODE>match(Continuation) -&#62; {[Match], Continuation2} | '$end_of_table'
| {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Continuation = Continuation2 = bindings_cont()</CODE></STRONG><BR>
<STRONG><CODE>Match = [term()]</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Matches some objects stored in a table and returns a list
        of the bindings that match a given pattern in some unspecified
        order. The table, the pattern, and the number of objects that
        are matched are all defined by <CODE>Continuation</CODE>, which has
        been returned by a prior call to <CODE>match/1</CODE> or
        <CODE>match/3</CODE>.<P>When all objects of the table have been matched,
        <CODE>'$end_of_table'</CODE> is returned.</UL>
<P><A NAME="match%2"><STRONG><CODE>match(Name, Pattern) -&#62; [Match] | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>Pattern = tuple()</CODE></STRONG><BR>
<STRONG><CODE>Match = [term()]</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Returns for each object of the table <CODE>Name</CODE> that
matches <CODE>Pattern</CODE> a list of bindings in some unspecified
order. See <A HREF="ets.html">ets(3)</A> for a
description of patterns. If the keypos'th element of
<CODE>Pattern</CODE> is unbound, all objects of the table are
matched. If the keypos'th element is bound, only the
objects with the right key are matched.</UL>
<P><A NAME="match%3"><STRONG><CODE>match(Name, Pattern, N) -&#62; {[Match], Continuation} 
| '$end_of_table' | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>Pattern = tuple()</CODE></STRONG><BR>
<STRONG><CODE>N = default | int()</CODE></STRONG><BR>
<STRONG><CODE>Match = [term()]</CODE></STRONG><BR>
<STRONG><CODE>Continuation = bindings_cont()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Matches some or all objects of the table <CODE>Name</CODE> and
        returns a list of the bindings that match <CODE>Pattern</CODE> in
        some unspecified order. See <A HREF="ets.html">ets(3)</A> for a description of
        patterns.<P>A tuple of the bindings and a continuation is returned,
        unless the table is empty, in which case
        <CODE>'$end_of_table'</CODE> is returned. The continuation is to be
        used when matching further objects by calling
        <CODE>match/1</CODE>.<P>If the keypos'th element of <CODE>Pattern</CODE> is bound,
        all objects of the table are matched. If the keypos'th
        element is unbound, all objects of the table are matched,
        <CODE>N</CODE> objects at a time. The default, indicated by giving
        <CODE>N</CODE> the value <CODE>default</CODE>, is to let the number of
        objects vary depending on the sizes of the objects. If
        <CODE>Name</CODE> is a version 9 table, all objects with the same
        key are always matched at the same time which implies that
        more than N objects may sometimes be matched.
        
<P>The table should always be protected using
<CODE>safe_fixtable/2</CODE> before calling <CODE>match/3</CODE>, or
errors may occur when calling <CODE>match/1</CODE>.</UL>
<P><A NAME="match_delete%2"><STRONG><CODE>match_delete(Name, Pattern) -&#62; N | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>N = int()</CODE></STRONG><BR>
<STRONG><CODE>Pattern = tuple()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Deletes all objects that match <CODE>Pattern</CODE> from the
         table <CODE>Name</CODE>, and returns the number of deleted
         objects. See <A HREF="ets.html">ets(3)</A> for a
         description of patterns.<P>If the keypos'th element of <CODE>Pattern</CODE> is bound,
only the objects with the right key are matched.</UL>
<P><A NAME="match_object%1"><STRONG><CODE>match_object(Continuation) -&#62; {[Object], Continuation2} 
| '$end_of_table' | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Continuation = Continuation2 = object_cont()</CODE></STRONG><BR>
<STRONG><CODE>Object = object()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Returns a list of some objects stored in a table that match
        a given pattern in some unspecified order. The table, the
        pattern, and the number of objects that are matched are all
        defined by <CODE>Continuation</CODE>, which has been returned by a
        prior call to <CODE>match_object/1</CODE> or
        <CODE>match_object/3</CODE>.<P>When all objects of the table have been matched,
        <CODE>'$end_of_table'</CODE> is returned.</UL>
<P><A NAME="match_object%2"><STRONG><CODE>match_object(Name, Pattern) -&#62; [Object] | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>Pattern = tuple()</CODE></STRONG><BR>
<STRONG><CODE>Object = object()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Returns a list of all objects of the table <CODE>Name</CODE> that
        match <CODE>Pattern</CODE> in some unspecified order. See <A HREF="ets.html">ets(3)</A> for a description of patterns.

<P>If the keypos'th element of <CODE>Pattern</CODE> is
        unbound, all objects of the table are matched. If the
        keypos'th element of <CODE>Pattern</CODE> is bound, only the
        objects with the right key are matched.<P>Using the <CODE>match_object</CODE> functions for traversing all
        objects of a table is more efficient than calling
        <CODE>first/1</CODE> and <CODE>next/2</CODE> or <CODE>slot/2</CODE>.
</UL>
<P><A NAME="match_object%3"><STRONG><CODE>match_object(Name, Pattern, N) -&#62; {[Object], Continuation} 
| '$end_of_table' | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>Pattern = tuple()</CODE></STRONG><BR>
<STRONG><CODE>N = default | int()</CODE></STRONG><BR>
<STRONG><CODE>Object = object()</CODE></STRONG><BR>
<STRONG><CODE>Continuation = object_cont()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Matches some or all objects stored in the table <CODE>Name</CODE>
        and returns a list of the objects that match <CODE>Pattern</CODE> in
        some unspecified order. See <A HREF="ets.html">ets(3)</A> for a description of
        patterns.<P>A list of objects and a continuation is returned, unless
        the table is empty, in which case <CODE>'$end_of_table'</CODE>
        is returned. The continuation is to be used when matching
        further objects by calling <CODE>match_object/1</CODE>.<P>If the keypos'th element of <CODE>Pattern</CODE> is bound,
        all objects of the table are matched. If the keypos'th
        element is unbound, all objects of the table are matched,
        <CODE>N</CODE> objects at a time. The default, indicated by giving
        <CODE>N</CODE> the value <CODE>default</CODE>, is to let the number of
        objects vary depending on the sizes of the objects. If
        <CODE>Name</CODE> is a version 9 table, all matching objects with
        the same key are always returned in the same reply which
        implies that more than N objects may sometimes be returned.
        
<P>The table should always be protected using
<CODE>safe_fixtable/2</CODE> before calling <CODE>match_object/3</CODE>,
or errors may occur when calling <CODE>match_object/1</CODE>.</UL>
<P><A NAME="member%2"><STRONG><CODE>member(Name, Key) -&#62; Bool | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>Key = term()</CODE></STRONG><BR>
<STRONG><CODE>Bool = bool()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Works like <CODE>lookup/2</CODE>, but does not return the
        objects. The function returns <CODE>true</CODE> if one or more
        elements of the table has the key <CODE>Key</CODE>, <CODE>false</CODE>
        otherwise.</UL>
<P><A NAME="next%2"><STRONG><CODE>next(Name, Key1) -&#62; Key2 | '$end_of_table'</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>Key1 = Key2 = term()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Returns the key following <CODE>Key1</CODE> in the table
        <CODE>Name</CODE> according to the table's internal order, or
        <CODE>'$end_of_table'</CODE> if there is no next key.<P>Should an error occur, the process is exited with an error
        tuple <CODE>{error, Reason}</CODE>.<P>Use <CODE>first/1</CODE> to find the first key in the table.</UL>
<P><A NAME="open_file%1"><STRONG><CODE>open_file(Filename) -&#62; {ok, Reference} | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>FileName = file()</CODE></STRONG><BR>
<STRONG><CODE>Reference = ref()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Opens an existing table. If the table has not been properly
        closed, the error <CODE>{error, need_repair}</CODE> is returned. The
        returned reference is to be used as the name of the table.
        This function is most useful for debugging purposes.</UL>
<P><A NAME="open_file%2"><STRONG><CODE>open_file(Name, Args) -&#62; {ok, Name} | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = atom()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Opens a table. An empty Dets table is created if no file
exists.<P>The atom <CODE>Name</CODE> is the name of the table. The table
        name must be provided in all subsequent operations on the
        table. The name can be used by other processes as well, and
        several process can share one table.

        <P>If two processes open the same table by giving the same
        name and arguments, then the table will have two users. If one
        user closes the table, it still remains open until the second
        user closes the table.<P>The <CODE>Args</CODE> argument is a list of <CODE>{Key, Val}</CODE>
        tuples where the following values are allowed:<P><UL>
<LI>     <CODE>{access, access()}</CODE>. It is possible to open
         existing tables in read-only mode. A table which is opened
         in read-only mode is not subjected to the automatic file
         reparation algorithm if it is later opened after a crash.
         The default value is <CODE>read_write</CODE>.<BR>

</LI><BR>
<LI><CODE>{auto_save, auto_save()}</CODE>, the auto save
         interval. If the interval is an integer <CODE>Time</CODE>, the
         table is flushed to disk whenever it is not accessed for
         <CODE>Time</CODE> milliseconds. A table that has been flushed
         will require no reparation when reopened after an
         uncontrolled emulator halt. If the interval is the atom
         <CODE>infinity</CODE>, auto save is disabled. The default value
         is 180000 (3 minutes).<BR>

         </LI><BR>
<LI>     <CODE>{estimated_no_objects, int()}</CODE>. Equivalent to the
         <CODE>min_no_slots</CODE> option.
         <BR>
</LI><BR>
<LI> <CODE>{file, file()}</CODE>, the name of the file to be
opened. The default value is the name of the table.<BR>

</LI><BR>
<LI>     <CODE>{max_no_slots, no_slots()}</CODE>, the maximum number
         of slots that will be used. The default value is 2 M, and
         the maximal value is 32 M. Note that a higher value may
         increase the fragmentation of the table, and conversely,
         that a smaller value may decrease the fragmentation, at
         the expense of execution time. Only available for version
         9 tables.<BR>

         </LI><BR>
<LI>     <CODE>{min_no_slots, no_slots()}</CODE>. Application
performance can be enhanced with this flag by specifying,
when the table is created, the estimated number of
different keys that will be stored in the table. The
default value as well as the minimum value is 256.<BR>

         </LI><BR>
<LI><CODE>{keypos, keypos()}</CODE>, the position of the
         element of each object to be used as key. The default
         value is 1. The ability to explicitly state the key
         position is most convenient when we want to store Erlang
         records in which the first position of the record is the
         name of the record type.<BR>

         </LI><BR>
<LI><CODE>{ram_file, bool()}</CODE>, whether the table is to
         be kept in RAM. Keeping the table in RAM may sound like an
         anomaly, but can enhance the performance of applications
         which open a table, insert a set of objects, and then
         close the table. When the table is closed, its contents
         are written to the disk file. The default value is
         <CODE>false</CODE>.<BR>

</LI><BR>
<LI><CODE>{repair, Value}</CODE>. <CODE>Value</CODE> can be either
         a <CODE>bool()</CODE> or the atom <CODE>force</CODE>. The flag
         specifies whether the Dets server should invoke the
         automatic file reparation algorithm. The default is
         <CODE>true</CODE>. If <CODE>false</CODE> is specified, there is no
         attempt to repair the file and <CODE>{error, need_repair}</CODE>
         is returned if the table needs to be repaired.<BR>


         The value <CODE>force</CODE> means that a reparation will
         take place even if the table has been properly closed.
         This is how to convert tables created by older versions of
         STDLIB. An example is tables hashed with the deprecated
         <CODE>erlang:hash/2</CODE> BIF. Tables created with Dets from a
         STDLIB version of 1.8.2 and later use the new
         <CODE>erlang:phash/2</CODE> function, which is preferred.<BR>

</LI><BR>
<LI>     <CODE>{type, type()}</CODE>, the type of the table. The
         default value is <CODE>set</CODE>.<BR>

</LI><BR>
<LI>     <CODE>{version, version()}</CODE>, the version of the format
         used for the table. The default value is <CODE>9</CODE>. Tables
         on the format used before OTP R8 can be created by giving
         the value <CODE>8</CODE>. A version 8 table can be converted to
         a version 9 table by giving the options <CODE>{version,9}</CODE>
         and <CODE>{repair,force}</CODE>.<BR>

</LI><BR>
</UL>
</UL>
<P><A NAME="pid2name%1"><STRONG><CODE>pid2name(Pid) -&#62; {ok, Name} | undefined</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>Pid = pid()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Returns the name of the table given the pid of a process
that handles requests to a table, or <CODE>undefined</CODE> if
there is no such table.<P>This function is meant to be used for debugging only.</UL>
<P><A NAME="safe_fixtable%2"><STRONG><CODE>safe_fixtable(Name, Fix)</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>Fix = bool()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>If <CODE>Fix</CODE> is <CODE>true</CODE>, the table <CODE>Name</CODE> is
fixed (once more) by the calling process, otherwise the table
is released. The table is also released when a fixing process
terminates.

        <P>If several processes fix a table, the table will remain
        fixed until all processes have released it or terminated. A
        reference counter is kept on a per process basis, and N
        consecutive fixes require N releases to release the table.<P>It is not guaranteed that calls to <CODE>first/1</CODE>,
<CODE>next/2</CODE>, select and match functions work as expected
even if the table has been fixed; the limited support for
concurrency implemented in Ets has not yet been implementeded
in Dets. Fixing a table currently only disables resizing of
the hash list of the table.</UL>
<P><A NAME="select%1"><STRONG><CODE>select(Continuation) -&#62; {Selection, Continuation2} 
| '$end_of_table' | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Continuation = Continuation2 = select_cont()</CODE></STRONG><BR>
<STRONG><CODE>Selection = [term()]</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Returns the results of applying a match specification to
some objects stored in a table. The table, the match
specification, and the number of objects that are matched
are all defined by <CODE>Continuation</CODE>, which has been
returned by a prior call to <CODE>select/1</CODE> or
<CODE>select/3</CODE>.<P>When all objects of the table have been matched,
        <CODE>'$end_of_table'</CODE> is returned.</UL>
<P><A NAME="select%2"><STRONG><CODE>select(Name, MatchSpec) -&#62; Selection | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>MatchSpec = match_spec()</CODE></STRONG><BR>
<STRONG><CODE>Selection = [term()]</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Returns the results of applying the match specification
<CODE>MatchSpec</CODE> to all or some objects stored in the table
<CODE>Name</CODE>. The order of the objects is not specified. See
the ERTS User's Guide for a description of match
specifications.<P>If the keypos'th element of <CODE>MatchSpec</CODE> is
unbound, the match specification is applied to all objects of
the table. If the keypos'th element is bound, the match
specification is applied to the objects with the right key(s)
only.<P>Using the <CODE>select</CODE> functions for traversing all
        objects of a table is more efficient than calling
        <CODE>first/1</CODE> and <CODE>next/2</CODE> or <CODE>slot/2</CODE>.

</UL>
<P><A NAME="select%3"><STRONG><CODE>select(Name, MatchSpec, N) -&#62; {Selection, Continuation} 
| '$end_of_table' | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>MatchSpec = match_spec()</CODE></STRONG><BR>
<STRONG><CODE>N = default | int()</CODE></STRONG><BR>
<STRONG><CODE>Selection = [term()]</CODE></STRONG><BR>
<STRONG><CODE>Continuation = select_cont()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Returns the results of applying the match specification
<CODE>MatchSpec</CODE> to some or all objects stored in the table
<CODE>Name</CODE>. The order of the objects is not specified. See
the ERTS User's Guide for a description of match
specifications.<P>A tuple of the results of applying the match specification
        and a continuation is returned, unless the table is empty,
        in which case <CODE>'$end_of_table'</CODE> is returned. The
        continuation is to be used when matching further objects by
        calling <CODE>select/1</CODE>.<P>If the keypos'th element of <CODE>MatchSpec</CODE> is
        bound, the match specification is applied to all objects of
        the table with the right key(s). If the keypos'th
        element of <CODE>MatchSpec</CODE> is unbound, the match
        specification is applied to all objects of the table, <CODE>N</CODE>
        objects at a time. The default, indicated by giving <CODE>N</CODE>
        the value <CODE>default</CODE>, is to let the number of objects vary
        depending on the sizes of the objects. If <CODE>Name</CODE> is a
        version 9 table, all objects with the same key are always
        handled at the same time which implies that the match
        specification may be applied to more than N objects.

<P>The table should always be protected using
<CODE>safe_fixtable/2</CODE> before calling <CODE>select/3</CODE>, or
errors may occur when calling <CODE>select/1</CODE>.</UL>
<P><A NAME="select_delete%2"><STRONG><CODE>select_delete(Name, MatchSpec) -&#62; N | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>MatchSpec = match_spec()</CODE></STRONG><BR>
<STRONG><CODE>N = int()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Deletes each object from the table <CODE>Name</CODE> such that
applying the match specification <CODE>MatchSpec</CODE> to the
object returns the value <CODE>true</CODE>. See the ERTS
User's Guide for a description of match
specifications. Returns the number of deleted objects.<P>If the keypos'th element of <CODE>MatchSpec</CODE> is
         bound, the match specification is applied to the objects
         with the right key(s) only.</UL>
<P><A NAME="slot%2"><STRONG><CODE>slot(Name, I) -&#62; '$end_of_table' | [Object] | {error, Reason}
</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>I = int()</CODE></STRONG><BR>
<STRONG><CODE>Object = object()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>The objects of a table are distributed among slots,
starting with slot <CODE>0</CODE> and ending with slot n. This
function returns the list of objects associated with slot
<CODE>I</CODE>. If <CODE>I</CODE> is greater than n <CODE>'$end_of_table'</CODE>
is returned.</UL>
<P><A NAME="sync%1"><STRONG><CODE>sync(Name) -&#62; ok | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Ensures that all updates made to the table <CODE>Name</CODE> are
        written to disk. This also applies to tables which have been
        opened with the <CODE>ram_file</CODE> flag set to <CODE>true</CODE>. In
        this case, the contents of the RAM file are flushed to
        disk.<P>Note that the space management data structures kept in RAM,
the buddy system, is also written to the disk. This may take
some time if the table is fragmented.</UL>
<P><A NAME="to_ets%2"><STRONG><CODE>to_ets(Name, EtsTab) -&#62; EtsTab | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>EtsTab = -&#160;see ets(3)&#160;-
</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Inserts the objects of the Dets table <CODE>Name</CODE> into the
Ets table <CODE>EtsTab</CODE>. The order in which the objects are
inserted is not specified. The existing objects of the Ets
table are kept unless overwritten.
</UL>
<P><A NAME="traverse%2"><STRONG><CODE>traverse(Name, Fun) -&#62; Return | {error, Reason}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Fun = fun(Object) -&#62; FunReturn</CODE></STRONG><BR>
<STRONG><CODE>FunReturn = continue | {continue, Val} | {done, Value}</CODE></STRONG><BR>
<STRONG><CODE>Val = Value = term()</CODE></STRONG><BR>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>Object = object()</CODE></STRONG><BR>
<STRONG><CODE>Return = [term()]</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Applies <CODE>Fun</CODE> to each object stored in the table
<CODE>Name</CODE> in some unspecified order. Different actions are
taken depending on the return value of <CODE>Fun</CODE>. The
following <CODE>Fun</CODE> return values are allowed:<P><DL>
<DT><CODE>continue</CODE></DT>
<DD>     Continue to perform the traversal. For example, the
         following function can be used to print out the contents
         of a table:<BR>

<PRE>fun(X) -&#62; io:format(&#34;~p~n&#34;, [X]), continue end.</PRE>

         </DD>
<DT><CODE>{continue, Val}</CODE></DT>
<DD>     Continue the traversal and accumulate <CODE>Val</CODE>. The
         following function is supplied in order to collect all
         objects of a table in a list: <BR>

<PRE>fun(X) -&#62; {continue, X} end.</PRE>

         </DD>
<DT><CODE>{done, Value}</CODE></DT>
<DD>     Terminate the traversal and return <CODE>[Value | Acc]</CODE>.<BR>

         </DD>
</DL>
</UL>
<P><A NAME="update_counter%3"><STRONG><CODE>update_counter(Name, Key, Increment) -&#62; Result</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Name = name()</CODE></STRONG><BR>
<STRONG><CODE>Key = term()</CODE></STRONG><BR>
<STRONG><CODE>Increment = {Pos, Incr} | Incr</CODE></STRONG><BR>
<STRONG><CODE>Pos = Incr = Result = integer()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Updates the object with key <CODE>Key</CODE> stored in the table
        <CODE>Name</CODE> of type <CODE>set</CODE> by adding <CODE>Incr</CODE> to the
        element at the <CODE>Pos</CODE>:th position. The new counter value
        is returned. If no position is specified, the element directly
        following the key is updated.<P>This functions provides a way of updating a counter,
        without having to look up an object, update the object by
        incrementing an element and insert the resulting object into
        the table again.</UL>
<H3>See Also</H3>
<UL>
<P><A HREF="ets.html">ets(3)</A>, mnesia(3)
</UL>
<H3>AUTHORS</H3>
<UL>
Claes Wikstr&#246;m - support@erlang.ericsson.se<BR>
Hans Bolinder - support@erlang.ericsson.se<BR>
</UL>
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<A HREF="http://www.erlang.se">Ericsson Utvecklings AB</A><BR>
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