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<HTML>
<HEAD>
<!-- refpage -->
<TITLE>gen_server</TITLE>
</HEAD>
<BODY BGCOLOR="#FFFFFF">
<CENTER>
<A HREF="http://www.erlang.se"><IMG BORDER=0 ALT="[Erlang Systems]" SRC="min_head.gif"></A>
<H1>gen_server</H1>
</CENTER>
<H3>MODULE</H3>
<UL>
gen_server</UL>
<H3>MODULE SUMMARY</H3>
<UL>
Generic Server Behaviour</UL>
<H3>DESCRIPTION</H3>
<UL>
<P>A behaviour module for implementing the server of a client-server
relation. A generic server process (gen_server) implemented using this
module will have a standard set of interface functions and include
functionality for tracing and error reporting. It will also fit into
an OTP supervision tree. Refer to <STRONG>OTP Design Principles</STRONG> for
more information.<P>A gen_server assumes all specific parts to be located in a callback
module exporting a pre-defined set of functions. The relationship
between the behaviour functions and the callback functions can be
illustrated as follows:<PRE>gen_server module Callback module
----------------- ---------------
gen_server:start -----> Module:init/1
gen_server:call
gen_server:multi_call -----> Module:handle_call/3
gen_server:cast
gen_server:abcast -----> Module:handle_cast/2
- -----> Module:handle_info/2
- -----> Module:terminate/2
- -----> Module:code_change/3</PRE>
<P>If a callback function fails or returns a bad value, the gen_server
will terminate.<P>The <CODE>sys</CODE> module can be used for debugging a gen_server.<P>Note that a gen_server does not trap exit signals automatically,
this must be explicitly initiated in the callback module.<P>Unless otherwise stated, all functions in this module fail if
the specified gen_server does not exist or if bad arguments are given.
</UL>
<H3>EXPORTS</H3>
<P><A NAME="start%3"><STRONG><CODE>start(Module, Args, Options) -> Result</CODE></STRONG></A><BR>
<A NAME="start%4"><STRONG><CODE>start(ServerName, Module, Args, Options) -> Result</CODE></STRONG></A><BR>
<A NAME="start_link%3"><STRONG><CODE>start_link(Module, Args, Options) -> Result</CODE></STRONG></A><BR>
<A NAME="start_link%4"><STRONG><CODE>start_link(ServerName, Module, Args, Options) -> Result</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>ServerName = {local,Name} | {global,Name}</CODE></STRONG><BR>
<STRONG><CODE> Name = atom()</CODE></STRONG><BR>
<STRONG><CODE>Module = atom()</CODE></STRONG><BR>
<STRONG><CODE>Args = term()</CODE></STRONG><BR>
<STRONG><CODE>Options = [Option]</CODE></STRONG><BR>
<STRONG><CODE> Option = {debug,Dbgs} | {timeout,Time}</CODE></STRONG><BR>
<STRONG><CODE>  Dbgs = [Dbg]</CODE></STRONG><BR>
<STRONG><CODE>   Dbg = trace | log | statistics |
{log_to_file,FileName} | {install,{Func,FuncState}}</CODE></STRONG><BR>
<STRONG><CODE>Result = {ok,Pid} | ignore | {error,Error}</CODE></STRONG><BR>
<STRONG><CODE> Pid = pid()</CODE></STRONG><BR>
<STRONG><CODE> Error = {already_started,Pid} | term()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Creates a gen_server process which calls <CODE>Module:init/1</CODE> to
initialize. To ensure a synchronized start-up procedure, this
function does not return until <CODE>Module:init/1</CODE> has returned.
<P>A gen_server started using <CODE>start_link</CODE> is linked to
the calling process, this function must be used if the gen_server
is included in a supervision tree. A gen_server started using
<CODE>start</CODE> is not linked to the calling process.<P>If <CODE>ServerName={local,Name}</CODE> the gen_server is registered
locally as <CODE>Name</CODE> using <CODE>register/2</CODE>.
If <CODE>ServerName={global,Name}</CODE> the gen_server is registered
globally as <CODE>Name</CODE> using <CODE>global:register_name/2</CODE>.
If no name is provided, the gen_server is not registered.<P><CODE>Module</CODE> is the name of the callback module.<P><CODE>Args</CODE> is an arbitrary term which is passed as the argument
to <CODE>Module:init/1</CODE>.<P>If the option <CODE>{timeout,Time}</CODE> is present, the gen_server is
allowed to spend <CODE>Time</CODE> milliseconds initializing or it will
be terminated and the start function will return
<CODE>{error,timeout}</CODE>.<P>If the option <CODE>{debug,Dbgs}</CODE> is present, the corresponding
<CODE>sys</CODE> function will be called for each item in <CODE>Dbgs</CODE>.
Refer to <CODE>sys(3)</CODE> for more information.<P>If the gen_server is successfully created and initialized
the function returns <CODE>{ok,Pid}</CODE>, where <CODE>Pid</CODE> is the pid
of the gen_server. If there already exists a process with
the specified <CODE>ServerName</CODE> the function returns
<CODE>{error,{already_started,Pid}}</CODE>, where <CODE>Pid</CODE> is the pid
of that process.<P>If <CODE>Module:init/1</CODE> fails with <CODE>Reason</CODE>, the function
returns <CODE>{error,Reason}</CODE>. If <CODE>Module:init/1</CODE> returns
<CODE>{stop,Reason}</CODE> or <CODE>ignore</CODE>, the process is terminated
and the function returns <CODE>{error,Reason}</CODE> or <CODE>ignore</CODE>,
respectively.</UL>
<P><A NAME="call%2"><STRONG><CODE>call(ServerRef, Request) -> Reply</CODE></STRONG></A><BR>
<A NAME="call%3"><STRONG><CODE>call(ServerRef, Request, Timeout) -> Reply</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>ServerRef = Name | {Name,Node} | {global,Name} | pid()</CODE></STRONG><BR>
<STRONG><CODE> Name = Node = atom()</CODE></STRONG><BR>
<STRONG><CODE>Request = term()</CODE></STRONG><BR>
<STRONG><CODE>Timeout = int()>0 | infinity</CODE></STRONG><BR>
<STRONG><CODE>Reply = term()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Makes a synchronous call to the gen_server <CODE>ServerRef</CODE> by
sending a request and waiting until a reply arrives or a timout
occurs. The gen_server will call <CODE>Module:handle_call/3</CODE> to
handle the request.<P><CODE>ServerRef</CODE> can be:<P><UL>
<LI>the pid,</LI><BR>
<LI><CODE>Name</CODE>, if the gen_server is locally registered,</LI><BR>
<LI><CODE>{Name,Node}</CODE>, if the gen_server is locally
registered at another node, or</LI><BR>
<LI><CODE>{global,Name}</CODE>, if the gen_server is globally
registered.</LI><BR>
</UL>
<P><CODE>Request</CODE> is an arbitrary term which is passed as one of
the arguments to <CODE>Module:handle_call/3</CODE>.<P><CODE>Timeout</CODE> is an integer greater than zero which specifies
how many milliseconds to wait for a reply, or the atom
<CODE>infinity</CODE> to wait indefinitely. Default value is 5000.
If no reply is received within the specified time, the function
call fails.<P>The return value <CODE>Reply</CODE> is defined in the return value of
<CODE>Module:handle_call/3</CODE>.<P>In the case where the gen_server terminates during the handling of
the request and the client is linked to the gen_server and trapping
exits, the exit message is removed from the client's receive
queue before the function call fails.<BR>
This behaviour is retained for backwards compatibility only and may
change in the future. Note that if the gen_server crashes in
between calls, the client must take care of the exit message
anyway.<BR>
Warning: Under certain circumstances
(e.g. <CODE>ServerRef = {Name,Node}</CODE>, and <CODE>Node</CODE> goes down)
the exit message cannot be removed.</UL>
<P><A NAME="multi_call%2"><STRONG><CODE>multi_call(Name, Request) -> Result</CODE></STRONG></A><BR>
<A NAME="multi_call%3"><STRONG><CODE>multi_call(Nodes, Name, Request) -> Result</CODE></STRONG></A><BR>
<A NAME="multi_call%4"><STRONG><CODE>multi_call(Nodes, Name, Request, Timeout) -> Result</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Nodes = [Node]</CODE></STRONG><BR>
<STRONG><CODE> Node = atom()</CODE></STRONG><BR>
<STRONG><CODE>Name = atom()</CODE></STRONG><BR>
<STRONG><CODE>Request = term()</CODE></STRONG><BR>
<STRONG><CODE>Timeout = int()>=0 | infinity</CODE></STRONG><BR>
<STRONG><CODE>Result = {Replies,BadNodes}</CODE></STRONG><BR>
<STRONG><CODE> Replies = [{Node,Reply}]</CODE></STRONG><BR>
<STRONG><CODE>  Reply = term()</CODE></STRONG><BR>
<STRONG><CODE>BadNodes = [Node]</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Makes a synchronous call to all gen_servers locally registered as
<CODE>Name</CODE> at the specified nodes by first sending a request to
every node and then waiting for the replies. The gen_servers will
call <CODE>Module:handle_call/3</CODE> to handle the request.<P>The function returns a tuple <CODE>{Replies,BadNodes}</CODE> where
<CODE>Replies</CODE> is a list of <CODE>{Node,Reply}</CODE> and <CODE>BadNodes</CODE>
is a list of node that either did not exist, or where
the gen_server <CODE>Name</CODE> did not exist or did not reply.<P><CODE>Nodes</CODE> is a list of node names to which the request
should be sent. Default value is the list of all known nodes
<CODE>[node()|nodes()]</CODE>.<P><CODE>Name</CODE> is the locally registered name of each gen_server.<P><CODE>Request</CODE> is an arbitrary term which is passed as one of
the arguments to <CODE>Module:handle_call/3</CODE>.<P><CODE>Timeout</CODE> is an integer greater than zero which specifies
how many milliseconds to wait for each reply, or the atom
<CODE>infinity</CODE> to wait indefinitely. Default value is
<CODE>infinity</CODE>. If no reply is received from a node within
the specified time, the node is added to <CODE>BadNodes</CODE>.<P>When a reply <CODE>Reply</CODE> is received from the gen_server at a
node <CODE>Node</CODE>, <CODE>{Node,Reply}</CODE> is added to <CODE>Replies</CODE>.
<CODE>Reply</CODE> is defined in the return value of
<CODE>Module:handle_call/3</CODE>.<P><TABLE CELLPADDING=4><TR>
<TD VALIGN=TOP><IMG ALT="Warning!" SRC="warning.gif"></TD>
<TD>
<P>If one of the nodes is running Erlang/OTP R6B or older, and
the gen_server is not started when the requests are sent, but
starts within 2 seconds, this function waits the whole
<CODE>Timeout</CODE>, which may be infinity.<P>This problem does not exist if all nodes are running Erlang/OTP
R7B or later.</TD></TR>
</TABLE>
<P>This function does <STRONG>not</STRONG> read out any exit messages
like <CODE>call/2,3</CODE> does.<P>The previously undocumented functions <CODE>safe_multi_call/2,3,4</CODE>
were removed in OTP R7B/Erlang 5.0 since this function is now safe,
except in the case mentioned above.</UL>
<P><A NAME="cast%2"><STRONG><CODE>cast(ServerRef, Request) -> ok</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>ServerRef = Name | {Name,Node} | {global,Name} | pid()</CODE></STRONG><BR>
<STRONG><CODE> Name = Node = atom()</CODE></STRONG><BR>
<STRONG><CODE>Request = term()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Sends an asynchronous request to the gen_server <CODE>ServerRef</CODE>
and returns <CODE>ok</CODE> immediately. The gen_server will call
<CODE>Module:handle_cast/2</CODE> to handle the request.<P>See <CODE>call/2,3</CODE> for a description of <CODE>ServerRef</CODE>.<P><CODE>Request</CODE> is an arbitrary term which is passed as one
of the arguments to <CODE>Module:handle_cast/2</CODE>.</UL>
<P><A NAME="abcast%2"><STRONG><CODE>abcast(Name, Request) -> abcast</CODE></STRONG></A><BR>
<A NAME="abcast%3"><STRONG><CODE>abcast(Nodes, Name, Request) -> abcast</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Nodes = [Node]</CODE></STRONG><BR>
<STRONG><CODE> Node = atom()</CODE></STRONG><BR>
<STRONG><CODE>Name = atom()</CODE></STRONG><BR>
<STRONG><CODE>Request = term()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Sends an asynchronous request to the gen_servers locally
registered as <CODE>Name</CODE> at the specified nodes. The function
returns immediately and ignores nodes that does not exist, or
where the gen_server <CODE>Name</CODE> does not exist. The gen_servers
will call <CODE>Module:handle_cast/2</CODE> to handle the request.<P>See <CODE>multi_call/2,3,4</CODE> for a description of the arguments.
</UL>
<P><A NAME="reply%2"><STRONG><CODE>reply(Client, Reply) -> true</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Client - see below</CODE></STRONG><BR>
<STRONG><CODE>Reply = term()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>This function can be used by a gen_server to explicitly send a
reply to a client that called <CODE>call</CODE> or <CODE>multi_call</CODE>,
when the reply cannot be defined in the return value of
<CODE>Module:handle_call/3</CODE>.<P><CODE>Client</CODE> must be the <CODE>From</CODE> argument provided to
the callback function. <CODE>Reply</CODE> is an arbitrary term, which
will be given back to the client as the return value of <CODE>call</CODE>
or <CODE>multi_call</CODE>.</UL>
<H3>CALLBACK FUNCTIONS</H3>
<UL>
<P>The following functions should be exported from a <CODE>gen_server</CODE>
callback module.</UL>
<H3>EXPORTS</H3>
<P><A NAME="Module:init%1"><STRONG><CODE>Module:init(Args) -> Result</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Args = term()</CODE></STRONG><BR>
<STRONG><CODE>Result = {ok,State} | {ok,State,Timeout}</CODE></STRONG><BR>
<STRONG><CODE> | {stop,Reason} | ignore</CODE></STRONG><BR>
<STRONG><CODE> State = term()</CODE></STRONG><BR>
<STRONG><CODE> Timeout = int()>=0 | infinity</CODE></STRONG><BR>
<STRONG><CODE> Reason = term()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Whenever a gen_server is started using <CODE>gen_server:start/3,4</CODE>
or <CODE>gen_server:start_link/3.4</CODE>, this function is called by
the new process to initialize.<P><CODE>Args</CODE> is the <CODE>Args</CODE> argument provided to the start
function.<P>If the initialization is successful, the function should return
<CODE>{ok,State}</CODE> or <CODE>{ok,State,Timout}</CODE>, where <CODE>State</CODE>
is the internal state of the gen_server.<P>If an integer timout value is provided, a timout will occur
unless a request or a message is received within <CODE>Timeout</CODE>
milliseconds. A timout is represented by the atom <CODE>timeout</CODE>
which should be handled by the <CODE>handle_info/2</CODE> callback
function. The atom <CODE>inifinity</CODE> can be used to wait
indefinitely, this is the default value.<P>If something goes wrong during the initialization the function
should return <CODE>{stop,Reason}</CODE> where <CODE>Reason</CODE> is any
term, or <CODE>ignore</CODE>.</UL>
<P><A NAME="Module:handle_call%3"><STRONG><CODE>Module:handle_call(Request, From, State) -> Result</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Request = term()</CODE></STRONG><BR>
<STRONG><CODE>From = {pid(),Tag}</CODE></STRONG><BR>
<STRONG><CODE>State = term()</CODE></STRONG><BR>
<STRONG><CODE>Result = {reply,Reply,NewState} | {reply,Reply,NewState,Timeout}
</CODE></STRONG><BR>
<STRONG><CODE> | {noreply,NewState} | {noreply,NewState,Timeout}</CODE></STRONG><BR>
<STRONG><CODE> | {stop,Reason,Reply,NewState} | {stop,Reason,NewState}
</CODE></STRONG><BR>
<STRONG><CODE> Reply = term()</CODE></STRONG><BR>
<STRONG><CODE> NewState = term()</CODE></STRONG><BR>
<STRONG><CODE> Timeout = int()>=0 | infinity</CODE></STRONG><BR>
<STRONG><CODE> Reason = term()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Whenever a gen_server receives a request sent using
<CODE>gen_server:call/2,3</CODE> or <CODE>gen_server:multi_call/2,3,4</CODE>,
this function is called to handle the request.<P><CODE>Request</CODE> is the <CODE>Request</CODE> argument provided
to <CODE>call</CODE> or <CODE>multi_call</CODE>.<P><CODE>From</CODE> is a tuple <CODE>{Pid,Tag}</CODE> where <CODE>Pid</CODE> is
the pid of the client and <CODE>Tag</CODE> is a unique tag.<P><CODE>State</CODE> is the internal state of the gen_server.<P>If the function returns <CODE>{reply,Reply,NewState}</CODE> or
<CODE>{reply,Reply,NewState,Timout}</CODE>, <CODE>Reply</CODE> will be given
back to <CODE>From</CODE> as the return value of <CODE>call</CODE> or included
in the return value of <CODE>multi_call</CODE>. The gen_server then
continues executing with the possibly updated internal state
<CODE>NewState</CODE>.
See <CODE>Module:init/1</CODE> for a description of <CODE>Timeout</CODE>.<P>If the functions returns <CODE>{noreply,NewState}</CODE> or
<CODE>{noreply,NewState,Timeout}</CODE>, the gen_server will continue
executing with <CODE>NewState</CODE>. Any reply to <CODE>From</CODE> must
be given explicitly using <CODE>gen_server:reply/2</CODE>.<P>If the function returns <CODE>{stop,Reason,Reply,NewState}</CODE>,
<CODE>Reply</CODE> will be given back to <CODE>From</CODE>. If the function
returns <CODE>{stop,Reason,NewState}</CODE>, any reply to <CODE>From</CODE>
must be given explicitly using <CODE>gen_server:reply/2</CODE>.
The gen_server will then call
<CODE>Module:terminate(Reason,NewState)</CODE> and terminate.</UL>
<P><A NAME="Module:handle_cast%2"><STRONG><CODE>Module:handle_cast(Request, State) -> Result</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Request = term()</CODE></STRONG><BR>
<STRONG><CODE>State = term()</CODE></STRONG><BR>
<STRONG><CODE>Result = {noreply,NewState} | {noreply,NewState,Timeout}</CODE></STRONG><BR>
<STRONG><CODE> | {stop,Reason,NewState}</CODE></STRONG><BR>
<STRONG><CODE> NewState = term()</CODE></STRONG><BR>
<STRONG><CODE> Timeout = int()>=0 | infinity</CODE></STRONG><BR>
<STRONG><CODE> Reason = term()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>Whenever a gen_server receives a request sent using
<CODE>gen_server:cast/2</CODE> or <CODE>gen_server:abcast/2,3</CODE>, this
function is called to handle the request.<P>See <CODE>Module:handle_call/3</CODE> for a description of
the arguments and possible return values.</UL>
<P><A NAME="Module:handle_info%2"><STRONG><CODE>Module:handle_info(Info, State) -> Result</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Info = timeout | term()</CODE></STRONG><BR>
<STRONG><CODE>State = term()</CODE></STRONG><BR>
<STRONG><CODE>Result = {noreply,NewState} | {noreply,NewState,Timeout}</CODE></STRONG><BR>
<STRONG><CODE> | {stop,Reason,NewState}</CODE></STRONG><BR>
<STRONG><CODE> NewState = term()</CODE></STRONG><BR>
<STRONG><CODE> Timeout = int()>=0 | infinity</CODE></STRONG><BR>
<STRONG><CODE> Reason = normal | term()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>This function is called by a gen_server when a timeout occurs or
when it receives any other message than a synchronous or
asynchronous request (or a system message).<P><CODE>Info</CODE> is either the atom <CODE>timeout</CODE>, if a timeout has
occured, or the received message.<P>See <CODE>Module:handle_call/3</CODE> for a description of the other
arguments and possible return values.</UL>
<P><A NAME="Module:terminate%2"><STRONG><CODE>Module:terminate(Reason, State)</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>Reason = normal | shutdown | term()</CODE></STRONG><BR>
<STRONG><CODE>State = term()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>This function is called by a gen_server when it is about to
terminate. It should be the opposite of <CODE>Module:init/1</CODE> and
do any necessary cleaning up. When it returns, the gen_server
terminates with <CODE>Reason</CODE>. The return value is ignored.<P><CODE>Reason</CODE> is a term denoting the stop reason and <CODE>State</CODE>
is the internal state of the gen_server.<P><CODE>Reason</CODE> depends on why the gen_server is terminating. If it
is because another callback function has returned a stop tuple
<CODE>{stop,..}</CODE>, <CODE>Reason</CODE> will have the value specified in
that tuple. If it is due to a failure, <CODE>Reason</CODE> is the error
reason.<P>If the gen_server is part of a supervision tree and is ordered
by its supervisor to terminate, this function will be called with
<CODE>Reason=shutdown</CODE> if the following conditions apply:<P><UL>
<LI>the gen_server has been set to trap exit signals, and</LI><BR>
<LI>the shutdown strategy as defined in the supervisor's child
specification is an integer timeout value, not
<CODE>brutal_kill</CODE>.
</LI><BR>
</UL>
<P>Otherwise, the gen_server will be immediately terminated.<P>Note that for any other reason than <CODE>normal</CODE> or
<CODE>shutdown</CODE>, the gen_server is assumed to terminate due to an
error and an error report is issued using
<CODE>error_logger:format/2</CODE>.</UL>
<P><A NAME="Module:code_change%3"><STRONG><CODE>Module:code_change(OldVsn, State, Extra) -> {ok, NewState}</CODE></STRONG></A><BR>
<P><UL>Types:
<UL>
<STRONG><CODE>OldVsn = undefined | term()</CODE></STRONG><BR>
<STRONG><CODE>State = NewState = term()</CODE></STRONG><BR>
<STRONG><CODE>Extra = term()</CODE></STRONG><BR>
</UL>
</UL>
<UL>
<P>This function is called by a gen_server when it should update its
internal state due to code replacement, i.e. when the instruction
<CODE>{update,Module,Change,PrePurge,PostPurge,Modules}</CODE> where
<CODE>Change={advanced,Extra}</CODE> has been given to the release
handler. See <STRONG>SASL User's Guide</STRONG> for more information.<P><CODE>OldVsn</CODE> is the <CODE>vsn</CODE> attribute of the old version of
the callback module <CODE>Module</CODE>, or <CODE>undefined</CODE> if no
such attribute is defined.<P><CODE>State</CODE> is the internal state of the gen_server.<P><CODE>Extra</CODE> is the same as in the <CODE>{advanced,Extra}</CODE> part
of the update instruction.<P>The function should return the updated internal state.</UL>
<H3>SEE ALSO</H3>
<UL>
<P>supervisor(3), sys(3)</UL>
<H3>AUTHORS</H3>
<UL>
Gunilla Hugosson - support@erlang.ericsson.se<BR>
</UL>
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