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<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">
<erlref>
<header>
<copyright>
<year>1996</year><year>2018</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
</legalnotice>
<title>gen_server</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
</header>
<module since="">gen_server</module>
<modulesummary>Generic server behavior.</modulesummary>
<description>
<p>This behavior module provides the server of a client-server
relation. A generic server process (<c>gen_server</c>) implemented using
this module has a standard set of interface functions and
includes functionality for tracing and error reporting. It also
fits into an OTP supervision tree. For more information, see section
<seealso marker="doc/design_principles:gen_server_concepts">
gen_server Behaviour</seealso> in OTP Design Principles.</p>
<p>A <c>gen_server</c> process assumes all specific parts to be located in
a callback module exporting a predefined set of functions.
The relationship between the behavior functions and the callback
functions is as follows:</p>
<pre>
gen_server module Callback module
----------------- ---------------
gen_server:start
gen_server:start_link -----> Module:init/1
gen_server:stop -----> Module:terminate/2
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:handle_continue/2
- -----> Module:terminate/2
- -----> Module:code_change/3</pre>
<p>If a callback function fails or returns a bad value, the
<c>gen_server</c> process terminates.</p>
<p>A <c>gen_server</c> process handles system messages as described in
<seealso marker="sys"><c>sys(3)</c></seealso>. The <c>sys</c> module
can be used for debugging a <c>gen_server</c> process.</p>
<p>Notice that a <c>gen_server</c> process does not trap exit signals
automatically, this must be explicitly initiated in the callback
module.</p>
<p>Unless otherwise stated, all functions in this module fail if
the specified <c>gen_server</c> process does not exist or if bad
arguments are specified.</p>
<p>The <c>gen_server</c> process can go into hibernation
(see <seealso marker="erts:erlang#hibernate/3">
<c>erlang:hibernate/3</c></seealso>) if a callback
function specifies <c>'hibernate'</c> instead of a time-out value. This
can be useful if the server is expected to be idle for a long
time. However, use this feature with care, as hibernation
implies at least two garbage collections (when hibernating and
shortly after waking up) and is not something you want to do
between each call to a busy server.</p>
<p>If the <c>gen_server</c> process needs to perform an action
immediately after initialization or to break the execution of a
callback into multiple steps, it can return <c>{continue,Continue}</c>
in place of the time-out or hibernation value, which will immediately
invoke the <c>handle_continue/2</c> callback.</p>
</description>
<funcs>
<func>
<name since="">abcast(Name, Request) -> abcast</name>
<name since="">abcast(Nodes, Name, Request) -> abcast</name>
<fsummary>Send an asynchronous request to many generic servers.</fsummary>
<type>
<v>Nodes = [Node]</v>
<v>&nbsp;Node = atom()</v>
<v>Name = atom()</v>
<v>Request = term()</v>
</type>
<desc>
<p>Sends an asynchronous request to the <c>gen_server</c> processes
locally registered as <c>Name</c> at the specified nodes. The function
returns immediately and ignores nodes that do not exist, or
where the <c>gen_server</c> <c>Name</c> does not exist.
The <c>gen_server</c> processes call
<seealso marker="#Module:handle_cast/2">
<c>Module:handle_cast/2</c></seealso> to handle the request.</p>
<p>For a description of the arguments, see
<seealso marker="#multi_call/2"><c>multi_call/2,3,4</c></seealso>.</p>
</desc>
</func>
<func>
<name since="">call(ServerRef, Request) -> Reply</name>
<name since="">call(ServerRef, Request, Timeout) -> Reply</name>
<fsummary>Make a synchronous call to a generic server.</fsummary>
<type>
<v>ServerRef = Name | {Name,Node} | {global,GlobalName}</v>
<v>&nbsp;&nbsp;| {via,Module,ViaName} | pid()</v>
<v>&nbsp;Node = atom()</v>
<v>&nbsp;GlobalName = ViaName = term()</v>
<v>Request = term()</v>
<v>Timeout = int()>0 | infinity</v>
<v>Reply = term()</v>
</type>
<desc>
<p>Makes a synchronous call to the <c>ServerRef</c> of the
<c>gen_server</c> process
by sending a request and waiting until a reply arrives or a
time-out occurs. The <c>gen_server</c> process calls
<seealso marker="#Module:handle_call/3">
<c>Module:handle_call/3</c></seealso> to handle the request.</p>
<p><c>ServerRef</c> can be any of the following:</p>
<list type="bulleted">
<item>The pid</item>
<item><c>Name</c>, if the <c>gen_server</c> process is locally
registered</item>
<item><c>{Name,Node}</c>, if the <c>gen_server</c> process is locally
registered at another node</item>
<item><c>{global,GlobalName}</c>, if the <c>gen_server</c> process is
globally registered</item>
<item><c>{via,Module,ViaName}</c>, if the <c>gen_server</c> process is
registered through an alternative process registry</item>
</list>
<p><c>Request</c> is any term that is passed as one of
the arguments to <c>Module:handle_call/3</c>.</p>
<p><c>Timeout</c> is an integer greater than zero that
specifies how many milliseconds to wait for a reply, or
the atom <c>infinity</c> to wait indefinitely. Defaults to
5000. If no reply is received within the specified time,
the function call fails. If the caller catches the failure
and continues running, and the server is just late with the reply,
it can arrive at any time later into the message queue of the caller.
The caller must in this case be prepared for this
and discard any such garbage messages that are two element
tuples with a reference as the first element.</p>
<p>The return value <c>Reply</c> is defined in the return value
of <c>Module:handle_call/3</c>.</p>
<p>The call can fail for many reasons, including time-out and the
called <c>gen_server</c> process dying before or during the call.</p>
</desc>
</func>
<func>
<name since="">cast(ServerRef, Request) -> ok</name>
<fsummary>Send an asynchronous request to a generic server.</fsummary>
<type>
<v>ServerRef = Name | {Name,Node} | {global,GlobalName}</v>
<v>&nbsp;&nbsp;| {via,Module,ViaName} | pid()</v>
<v>&nbsp;Node = atom()</v>
<v>&nbsp;GlobalName = ViaName = term()</v>
<v>Request = term()</v>
</type>
<desc>
<p>Sends an asynchronous request to the <c>ServerRef</c> of the
<c>gen_server</c> process
and returns <c>ok</c> immediately, ignoring
if the destination node or <c>gen_server</c> process does not exist.
The <c>gen_server</c> process calls
<seealso marker="#Module:handle_cast/2">
<c>Module:handle_cast/2</c></seealso> to handle the request.</p>
<p>For a description of <c>ServerRef</c>, see
<seealso marker="#call/2"><c>call/2,3</c></seealso>.</p>
<p><c>Request</c> is any term that is passed as one
of the arguments to <c>Module:handle_cast/2</c>.</p>
</desc>
</func>
<func>
<name since="">enter_loop(Module, Options, State)</name>
<name since="">enter_loop(Module, Options, State, ServerName)</name>
<name since="">enter_loop(Module, Options, State, Timeout)</name>
<name since="">enter_loop(Module, Options, State, ServerName, Timeout)</name>
<fsummary>Enter the <c>gen_server</c> receive loop.</fsummary>
<type>
<v>Module = atom()</v>
<v>Options = [Option]</v>
<v>&nbsp;Option = {debug,Dbgs} | {hibernate_after,HibernateAfterTimeout}</v>
<v>&nbsp;&nbsp;Dbgs = [Dbg]</v>
<v>&nbsp;&nbsp;&nbsp;Dbg = trace | log | statistics</v>
<v>&nbsp;&nbsp;&nbsp;&nbsp;| {log_to_file,FileName} | {install,{Func,FuncState}}</v>
<v>State = term()</v>
<v>ServerName = {local,Name} | {global,GlobalName}</v>
<v>&nbsp;&nbsp;| {via,Module,ViaName}</v>
<v>&nbsp;Name = atom()</v>
<v>&nbsp;GlobalName = ViaName = term()</v>
<v>Timeout = int() | infinity</v>
</type>
<desc>
<p>Makes an existing process into a <c>gen_server</c> process. Does not
return, instead the calling process enters the <c>gen_server</c>
process receive
loop and becomes a <c>gen_server</c> process. The process
<em>must</em> have been started using one of the start functions in
<seealso marker="proc_lib"><c>proc_lib(3)</c></seealso>. The user is
responsible for any initialization of the process, including
registering a name for it.</p>
<p>This function is useful when a more complex initialization procedure
is needed than the <c>gen_server</c> process behavior provides.</p>
<p><c>Module</c>, <c>Options</c>, and <c>ServerName</c> have
the same meanings as when calling
<seealso marker="#start_link/3"><c>start[_link]/3,4</c></seealso>.
However, if <c>ServerName</c> is specified, the process must
have been registered accordingly <em>before</em> this function
is called.</p>
<p><c>State</c> and <c>Timeout</c> have the same meanings as in
the return value of
<seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>.
The callback module <c>Module</c> does not need to
export an <c>init/1</c> function.</p>
<p>The function fails if the calling process was not started by a
<c>proc_lib</c> start function, or if it is not registered
according to <c>ServerName</c>.</p>
</desc>
</func>
<func>
<name since="">multi_call(Name, Request) -> Result</name>
<name since="">multi_call(Nodes, Name, Request) -> Result</name>
<name since="">multi_call(Nodes, Name, Request, Timeout) -> Result</name>
<fsummary>Make a synchronous call to many generic servers.</fsummary>
<type>
<v>Nodes = [Node]</v>
<v>&nbsp;Node = atom()</v>
<v>Name = atom()</v>
<v>Request = term()</v>
<v>Timeout = int()>=0 | infinity</v>
<v>Result = {Replies,BadNodes}</v>
<v>&nbsp;Replies = [{Node,Reply}]</v>
<v>&nbsp;&nbsp;Reply = term()</v>
<v>BadNodes = [Node]</v>
</type>
<desc>
<p>Makes a synchronous call to all <c>gen_server</c> processes locally
registered as <c>Name</c> at the specified nodes by first
sending a request to every node and then waits for
the replies. The <c>gen_server</c> process calls
<seealso marker="#Module:handle_call/3">
<c>Module:handle_call/3</c></seealso> to handle the request.</p>
<p>The function returns a tuple <c>{Replies,BadNodes}</c>, where
<c>Replies</c> is a list of <c>{Node,Reply}</c> and
<c>BadNodes</c> is a list of node that either did not exist,
or where the <c>gen_server</c> <c>Name</c> did not exist or did not
reply.</p>
<p><c>Nodes</c> is a list of node names to which the request
is to be sent. Default value is the list of all known nodes
<c>[node()|nodes()]</c>.</p>
<p><c>Name</c> is the locally registered name of each
<c>gen_server</c> process.</p>
<p><c>Request</c> is any term that is passed as one of
the arguments to <c>Module:handle_call/3</c>.</p>
<p><c>Timeout</c> is an integer greater than zero that
specifies how many milliseconds to wait for each reply, or
the atom <c>infinity</c> to wait indefinitely. Defaults
to <c>infinity</c>. If no reply is received from a node within
the specified time, the node is added to <c>BadNodes</c>.</p>
<p>When a reply <c>Reply</c> is received from the <c>gen_server</c>
process at a node <c>Node</c>, <c>{Node,Reply}</c> is added to
<c>Replies</c>. <c>Reply</c> is defined in the return value of
<c>Module:handle_call/3</c>.</p>
<warning>
<p>If one of the nodes cannot process monitors, for example,
C or Java nodes, and the <c>gen_server</c> process is not started
when the requests are sent, but starts within 2 seconds,
this function waits the whole <c>Timeout</c>,
which may be infinity.</p>
<p>This problem does not exist if all nodes are Erlang nodes.</p>
</warning>
<p>To prevent late answers (after the time-out) from polluting
the message queue of the caller, a middleman process is used to
do the calls. Late answers are then discarded
when they arrive to a terminated process.</p>
</desc>
</func>
<func>
<name since="">reply(Client, Reply) -> Result</name>
<fsummary>Send a reply to a client.</fsummary>
<type>
<v>Client - see below</v>
<v>Reply = term()</v>
<v>Result = term()</v>
</type>
<desc>
<p>This function can be used by a <c>gen_server</c> process to
explicitly send a reply to a client that called
<seealso marker="#call/2"><c>call/2,3</c></seealso> or
<seealso marker="#multi_call/2"><c>multi_call/2,3,4</c></seealso>,
when the reply cannot be defined in the return value of
<seealso marker="#Module:handle_call/3">
<c>Module:handle_call/3</c></seealso>.</p>
<p><c>Client</c> must be the <c>From</c> argument provided to
the callback function. <c>Reply</c> is any term
given back to the client as the return value of
<c>call/2,3</c> or <c>multi_call/2,3,4</c>.</p>
<p>The return value <c>Result</c> is not further defined, and
is always to be ignored.</p>
</desc>
</func>
<func>
<name since="">start(Module, Args, Options) -> Result</name>
<name since="">start(ServerName, Module, Args, Options) -> Result</name>
<fsummary>Create a standalone <c>gen_server</c> process.</fsummary>
<type>
<v>ServerName = {local,Name} | {global,GlobalName}</v>
<v>&nbsp;&nbsp;| {via,Module,ViaName}</v>
<v>&nbsp;Name = atom()</v>
<v>&nbsp;GlobalName = ViaName = term()</v>
<v>Module = atom()</v>
<v>Args = term()</v>
<v>Options = [Option]</v>
<v>&nbsp;Option = {debug,Dbgs} | {timeout,Time} | {hibernate_after,HibernateAfterTimeout} | {spawn_opt,SOpts}</v>
<v>&nbsp;&nbsp;Dbgs = [Dbg]</v>
<v>&nbsp;&nbsp;&nbsp;Dbg = trace | log | statistics | {log_to_file,FileName} | {install,{Func,FuncState}}</v>
<v>&nbsp;&nbsp;SOpts = [term()]</v>
<v>Result = {ok,Pid} | ignore | {error,Error}</v>
<v>&nbsp;Pid = pid()</v>
<v>&nbsp;Error = {already_started,Pid} | term()</v>
</type>
<desc>
<p>Creates a standalone <c>gen_server</c> process, that is, a
<c>gen_server</c> process that is not part of a supervision tree
and thus has no supervisor.</p>
<p>For a description of arguments and return values, see
<seealso marker="#start_link/3"><c>start_link/3,4</c></seealso>.</p>
</desc>
</func>
<func>
<name since="">start_link(Module, Args, Options) -> Result</name>
<name since="">start_link(ServerName, Module, Args, Options) -> Result</name>
<fsummary>Create a <c>gen_server</c> process in a supervision tree.
</fsummary>
<type>
<v>ServerName = {local,Name} | {global,GlobalName}</v>
<v>&nbsp;&nbsp;| {via,Module,ViaName}</v>
<v>&nbsp;Name = atom()</v>
<v>&nbsp;GlobalName = ViaName = term()</v>
<v>Module = atom()</v>
<v>Args = term()</v>
<v>Options = [Option]</v>
<v>&nbsp;Option = {debug,Dbgs} | {timeout,Time} | {hibernate_after,HibernateAfterTimeout} | {spawn_opt,SOpts}</v>
<v>&nbsp;&nbsp;Dbgs = [Dbg]</v>
<v>&nbsp;&nbsp;&nbsp;Dbg = trace | log | statistics | {log_to_file,FileName} | {install,{Func,FuncState}}</v>
<v>&nbsp;&nbsp;SOpts = [term()]</v>
<v>Result = {ok,Pid} | ignore | {error,Error}</v>
<v>&nbsp;Pid = pid()</v>
<v>&nbsp;Error = {already_started,Pid} | term()</v>
</type>
<desc>
<p>Creates a <c>gen_server</c> process as part of a supervision tree.
This function is to be called, directly or indirectly, by
the supervisor. For example, it ensures that
the <c>gen_server</c> process is linked to the supervisor.</p>
<p>The <c>gen_server</c> process calls
<seealso marker="#Module:init/1"><c>Module:init/1</c></seealso> to
initialize. To ensure a synchronized startup procedure,
<c>start_link/3,4</c> does not return until
<c>Module:init/1</c> has returned.</p>
<list type="bulleted">
<item>
<p>If <c>ServerName={local,Name}</c>, the <c>gen_server</c> process
is registered locally as <c>Name</c> using <c>register/2</c>.</p>
</item>
<item>
<p>If <c>ServerName={global,GlobalName}</c>, the <c>gen_server</c>
process id registered globally as <c>GlobalName</c> using
<seealso marker="kernel:global#register_name/2">
<c>global:register_name/2</c></seealso> If no name is
provided, the <c>gen_server</c> process is not registered.</p>
</item>
<item>
<p>If <c>ServerName={via,Module,ViaName}</c>, the <c>gen_server</c>
process registers with the registry represented by <c>Module</c>.
The <c>Module</c> callback is to export the functions
<c>register_name/2</c>, <c>unregister_name/1</c>,
<c>whereis_name/1</c>, and <c>send/2</c>, which are to behave
like the corresponding functions in
<seealso marker="kernel:global"><c>global</c></seealso>.
Thus, <c>{via,global,GlobalName}</c> is a valid reference.</p>
</item>
</list>
<p><c>Module</c> is the name of the callback module.</p>
<p><c>Args</c> is any term that is passed as
the argument to
<seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>.</p>
<list type="bulleted">
<item>
<p>If option <c>{timeout,Time}</c> is present, the <c>gen_server</c>
process is allowed to spend <c>Time</c> milliseconds
initializing or it is terminated and the start function
returns <c>{error,timeout}</c>.</p>
</item>
<item>
<p>If option <c>{hibernate_after,HibernateAfterTimeout}</c> is present, the <c>gen_server</c>
process awaits any message for <c>HibernateAfterTimeout</c> milliseconds and
if no message is received, the process goes into hibernation automatically
(by calling <seealso marker="proc_lib#hibernate/3"><c>proc_lib:hibernate/3</c></seealso>).</p>
</item>
<item>
<p>If option <c>{debug,Dbgs}</c> is present,
the corresponding <c>sys</c> function is called for each
item in <c>Dbgs</c>; see
<seealso marker="sys"><c>sys(3)</c></seealso>.</p>
</item>
<item>
<p>If option <c>{spawn_opt,SOpts}</c> is present,
<c>SOpts</c> is passed as option list to
the <c>spawn_opt</c> BIF, which is used to spawn
the <c>gen_server</c> process; see
<seealso marker="erts:erlang#spawn_opt/2">
<c>spawn_opt/2</c></seealso>.</p>
</item>
</list>
<note>
<p>Using spawn option <c>monitor</c> is not
allowed, it causes the function to fail with reason
<c>badarg</c>.</p>
</note>
<p>If the <c>gen_server</c> process is successfully created and
initialized, the function returns <c>{ok,Pid}</c>, where <c>Pid</c>
is the pid of the <c>gen_server</c> process. If a process with the
specified <c>ServerName</c> exists already, the function returns
<c>{error,{already_started,Pid}}</c>, where <c>Pid</c> is
the pid of that process.</p>
<p>If <c>Module:init/1</c> fails with <c>Reason</c>,
the function returns <c>{error,Reason}</c>. If
<c>Module:init/1</c> returns <c>{stop,Reason}</c> or
<c>ignore</c>, the process is terminated and the function
returns <c>{error,Reason}</c> or <c>ignore</c>, respectively.</p>
</desc>
</func>
<func>
<name since="OTP 18.0">stop(ServerRef) -> ok</name>
<name since="OTP 18.0">stop(ServerRef, Reason, Timeout) -> ok</name>
<fsummary>Synchronously stop a generic server.</fsummary>
<type>
<v>ServerRef = Name | {Name,Node} | {global,GlobalName}</v>
<v>&nbsp;&nbsp;| {via,Module,ViaName} | pid()</v>
<v>&nbsp;Node = atom()</v>
<v>&nbsp;GlobalName = ViaName = term()</v>
<v>Reason = term()</v>
<v>Timeout = int()>0 | infinity</v>
</type>
<desc>
<p>Orders a generic server to exit with the specified <c>Reason</c>
and waits for it to terminate. The <c>gen_server</c> process calls
<seealso marker="#Module:terminate/2">
<c>Module:terminate/2</c></seealso> before exiting.</p>
<p>The function returns <c>ok</c> if the server terminates
with the expected reason. Any other reason than <c>normal</c>,
<c>shutdown</c>, or <c>{shutdown,Term}</c> causes an
error report to be issued using
<seealso marker="kernel:logger"><c>logger(3)</c></seealso>.
The default <c>Reason</c> is <c>normal</c>.</p>
<p><c>Timeout</c> is an integer greater than zero that
specifies how many milliseconds to wait for the server to
terminate, or the atom <c>infinity</c> to wait
indefinitely. Defaults to <c>infinity</c>. If the
server has not terminated within the specified time, a
<c>timeout</c> exception is raised.</p>
<p>If the process does not exist, a <c>noproc</c> exception
is raised.</p>
</desc>
</func>
</funcs>
<section>
<title>Callback Functions</title>
<p>The following functions
are to be exported from a <c>gen_server</c> callback module.</p>
</section>
<funcs>
<func>
<name since="">Module:code_change(OldVsn, State, Extra) -> {ok, NewState} | {error, Reason}</name>
<fsummary>Update the internal state during upgrade/downgrade.</fsummary>
<type>
<v>OldVsn = Vsn | {down, Vsn}</v>
<v>&nbsp;&nbsp;Vsn = term()</v>
<v>State = NewState = term()</v>
<v>Extra = term()</v>
<v>Reason = term()</v>
</type>
<desc>
<note>
<p>This callback is optional, so callback modules need not export it.
If a release upgrade/downgrade with <c>Change={advanced,Extra}</c>
specified in the <c>appup</c> file is made when <c>code_change/3</c>
isn't implemented the process will crash with an <c>undef</c> exit
reason.</p>
</note>
<p>This function is called by a <c>gen_server</c> process when it is
to update its internal state during a release upgrade/downgrade,
that is, when the instruction <c>{update,Module,Change,...}</c>,
where <c>Change={advanced,Extra}</c>, is specifed in
the <c>appup</c> file. For more information, see section
<seealso marker="doc/design_principles:release_handling#instr">
Release Handling Instructions</seealso> in OTP Design Principles.</p>
<p>For an upgrade, <c>OldVsn</c> is <c>Vsn</c>, and
for a downgrade, <c>OldVsn</c> is
<c>{down,Vsn}</c>. <c>Vsn</c> is defined by the <c>vsn</c>
attribute(s) of the old version of the callback module
<c>Module</c>. If no such attribute is defined, the version
is the checksum of the Beam file.</p>
<p><c>State</c> is the internal state of the <c>gen_server</c>
process.</p>
<p><c>Extra</c> is passed "as is" from the <c>{advanced,Extra}</c>
part of the update instruction.</p>
<p>If successful, the function must return the updated
internal state.</p>
<p>If the function returns <c>{error,Reason}</c>, the ongoing
upgrade fails and rolls back to the old release.</p>
</desc>
</func>
<func>
<name since="OTP R13B04">Module:format_status(Opt, [PDict, State]) -> Status</name>
<fsummary>Optional function for providing a term describing the
current <c>gen_server</c> status.</fsummary>
<type>
<v>Opt = normal | terminate</v>
<v>PDict = [{Key, Value}]</v>
<v>State = term()</v>
<v>Status = term()</v>
</type>
<desc>
<note>
<p>This callback is optional, so callback modules need not
export it. The <c>gen_server</c> module provides a default
implementation of this function that returns the callback
module state.</p>
</note>
<p>This function is called by a <c>gen_server</c> process in the
following situations:</p>
<list type="bulleted">
<item>
<p>One of <seealso marker="sys#get_status/1">
<c>sys:get_status/1,2</c></seealso>
is invoked to get the <c>gen_server</c> status. <c>Opt</c> is set
to the atom <c>normal</c>.</p>
</item>
<item>
<p>The <c>gen_server</c> process terminates abnormally and logs an
error. <c>Opt</c> is set to the atom <c>terminate</c>.</p>
</item>
</list>
<p>This function is useful for changing the form and
appearance of the <c>gen_server</c> status for these cases. A
callback module wishing to change
the <c>sys:get_status/1,2</c> return value, as well as how
its status appears in termination error logs, exports an
instance of <c>format_status/2</c> that returns a term
describing the current status of the <c>gen_server</c> process.</p>
<p><c>PDict</c> is the current value of the process dictionary of
the <c>gen_server</c> process..</p>
<p><c>State</c> is the internal state of the <c>gen_server</c>
process.</p>
<p>The function is to return <c>Status</c>, a term that
changes the details of the current state and status of
the <c>gen_server</c> process. There are no restrictions on the
form <c>Status</c> can take, but for
the <c>sys:get_status/1,2</c> case (when <c>Opt</c>
is <c>normal</c>), the recommended form for
the <c>Status</c> value is <c>[{data, [{"State",
Term}]}]</c>, where <c>Term</c> provides relevant details of
the <c>gen_server</c> state. Following this recommendation is not
required, but it makes the callback module status
consistent with the rest of the <c>sys:get_status/1,2</c>
return value.</p>
<p>One use for this function is to return compact alternative
state representations to avoid that large state terms are
printed in log files.</p>
</desc>
</func>
<func>
<name since="">Module:handle_call(Request, From, State) -> Result</name>
<fsummary>Handle a synchronous request.</fsummary>
<type>
<v>Request = term()</v>
<v>From = {pid(),Tag}</v>
<v>State = term()</v>
<v>Result = {reply,Reply,NewState} | {reply,Reply,NewState,Timeout}</v>
<v>&nbsp;&nbsp;| {reply,Reply,NewState,hibernate}</v>
<v>&nbsp;&nbsp;| {reply,Reply,NewState,{continue,Continue}}</v>
<v>&nbsp;&nbsp;| {noreply,NewState} | {noreply,NewState,Timeout}</v>
<v>&nbsp;&nbsp;| {noreply,NewState,hibernate}</v>
<v>&nbsp;&nbsp;| {noreply,NewState,{continue,Continue}}</v>
<v>&nbsp;&nbsp;| {stop,Reason,Reply,NewState} | {stop,Reason,NewState}</v>
<v>&nbsp;Reply = term()</v>
<v>&nbsp;NewState = term()</v>
<v>&nbsp;Timeout = int()>=0 | infinity</v>
<v>&nbsp;Continue = term()</v>
<v>&nbsp;Reason = term()</v>
</type>
<desc>
<p>Whenever a <c>gen_server</c> process receives a request sent using
<seealso marker="#call/2"><c>call/2,3</c></seealso> or
<seealso marker="#multi_call/2"><c>multi_call/2,3,4</c></seealso>,
this function is called to handle the request.</p>
<p><c>Request</c> is the <c>Request</c> argument provided
to <c>call</c> or <c>multi_call</c>.</p>
<p><c>From</c> is a tuple <c>{Pid,Tag}</c>, where <c>Pid</c> is
the pid of the client and <c>Tag</c> is a unique tag.</p>
<p><c>State</c> is the internal state of the <c>gen_server</c>
process.</p>
<list type="bulleted">
<item>
<p>If <c>{reply,Reply,NewState}</c> is returned,
<c>{reply,Reply,NewState,Timeout}</c> or
<c>{reply,Reply,NewState,hibernate}</c>, <c>Reply</c> is
given back to <c>From</c> as the return value of
<c>call/2,3</c> or included in the return value of
<c>multi_call/2,3,4</c>. The <c>gen_server</c> process then
continues executing with the possibly updated internal state
<c>NewState</c>.</p>
<p>For a description of <c>Timeout</c> and <c>hibernate</c>, see
<seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>.</p>
</item>
<item>
<p>If <c>{noreply,NewState}</c> is returned,
<c>{noreply,NewState,Timeout}</c>, or
<c>{noreply,NewState,hibernate}</c>, the <c>gen_server</c>
process continues executing with <c>NewState</c>. Any reply to
<c>From</c> must be specified explicitly using
<seealso marker="#reply/2"><c>reply/2</c></seealso>.</p>
</item>
<item>
<p>If <c>{stop,Reason,Reply,NewState}</c> is returned,
<c>Reply</c> is given back to <c>From</c>.</p>
</item>
<item>
<p>If <c>{stop,Reason,NewState}</c> is returned, any reply
to <c>From</c> must be specified explicitly using
<seealso marker="#reply/2"><c>reply/2</c></seealso>.
The <c>gen_server</c> process then calls
<c>Module:terminate(Reason,NewState)</c> and terminates.</p>
</item>
</list>
</desc>
</func>
<func>
<name since="">Module:handle_cast(Request, State) -> Result</name>
<fsummary>Handle an asynchronous request.</fsummary>
<type>
<v>Request = term()</v>
<v>State = term()</v>
<v>Result = {noreply,NewState} | {noreply,NewState,Timeout}</v>
<v>&nbsp;&nbsp;| {noreply,NewState,hibernate}</v>
<v>&nbsp;&nbsp;| {noreply,NewState,{continue,Continue}}</v>
<v>&nbsp;&nbsp;| {stop,Reason,NewState}</v>
<v>&nbsp;NewState = term()</v>
<v>&nbsp;Timeout = int()>=0 | infinity</v>
<v>&nbsp;Continue = term()</v>
<v>&nbsp;Reason = term()</v>
</type>
<desc>
<p>Whenever a <c>gen_server</c> process receives a request sent using
<seealso marker="#cast/2"><c>cast/2</c></seealso> or
<seealso marker="#abcast/2"><c>abcast/2,3</c></seealso>,
this function is called to handle the request.</p>
<p>For a description of the arguments and possible return values, see
<seealso marker="#Module:handle_call/3">
<c>Module:handle_call/3</c></seealso>.</p>
</desc>
</func>
<func>
<name since="OTP 21.0">Module:handle_continue(Continue, State) -> Result</name>
<fsummary>Handle a continue instruction.</fsummary>
<type>
<v>Continue = term()</v>
<v>State = term()</v>
<v>Result = {noreply,NewState} | {noreply,NewState,Timeout}</v>
<v>&nbsp;&nbsp;| {noreply,NewState,hibernate}</v>
<v>&nbsp;&nbsp;| {noreply,NewState,{continue,Continue}}</v>
<v>&nbsp;&nbsp;| {stop,Reason,NewState}</v>
<v>&nbsp;NewState = term()</v>
<v>&nbsp;Timeout = int()>=0 | infinity</v>
<v>&nbsp;Continue = term()</v>
<v>&nbsp;Reason = normal | term()</v>
</type>
<desc>
<note>
<p>This callback is optional, so callback modules need to
export it only if they return <c>{continue,Continue}</c>
from another callback. If continue is used and the callback
is not implemented, the process will exit with <c>undef</c>
error.</p>
</note>
<p>This function is called by a <c>gen_server</c> process whenever
a previous callback returns <c>{continue, Continue}</c>.
<c>handle_continue/2</c> is invoked immediately after the previous
callback, which makes it useful for performing work after
initialization or for splitting the work in a callback in
multiple steps, updating the process state along the way.</p>
<p>For a description of the other arguments and possible return values,
see <seealso marker="#Module:handle_call/3">
<c>Module:handle_call/3</c></seealso>.</p>
</desc>
</func>
<func>
<name since="">Module:handle_info(Info, State) -> Result</name>
<fsummary>Handle an incoming message.</fsummary>
<type>
<v>Info = timeout | term()</v>
<v>State = term()</v>
<v>Result = {noreply,NewState} | {noreply,NewState,Timeout}</v>
<v>&nbsp;&nbsp;| {noreply,NewState,hibernate}</v>
<v>&nbsp;&nbsp;| {noreply,NewState,{continue,Continue}}</v>
<v>&nbsp;&nbsp;| {stop,Reason,NewState}</v>
<v>&nbsp;NewState = term()</v>
<v>&nbsp;Timeout = int()>=0 | infinity</v>
<v>&nbsp;Reason = normal | term()</v>
</type>
<desc>
<note>
<p>This callback is optional, so callback modules need not
export it. The <c>gen_server</c> module provides a default
implementation of this function that logs about the unexpected
<c>Info</c> message, drops it and returns <c>{noreply, State}</c>.</p>
</note>
<p>This function is called by a <c>gen_server</c> process when a
time-out occurs or when it receives any other message than a
synchronous or asynchronous request (or a system message).</p>
<p><c>Info</c> is either the atom <c>timeout</c>, if a time-out
has occurred, or the received message.</p>
<p>For a description of the other arguments and possible return values,
see <seealso marker="#Module:handle_call/3">
<c>Module:handle_call/3</c></seealso>.</p>
</desc>
</func>
<func>
<name since="">Module:init(Args) -> Result</name>
<fsummary>Initialize process and internal state.</fsummary>
<type>
<v>Args = term()</v>
<v>Result = {ok,State} | {ok,State,Timeout} | {ok,State,hibernate}</v>
<v>&nbsp;| {ok,State,{continue,Continue}} | {stop,Reason} | ignore</v>
<v>&nbsp;State = term()</v>
<v>&nbsp;Timeout = int()>=0 | infinity</v>
<v>&nbsp;Reason = term()</v>
</type>
<desc>
<p>Whenever a <c>gen_server</c> process is started using
<seealso marker="#start/3"><c>start/3,4</c></seealso> or
<seealso marker="#start_link/3"><c>start_link/3,4</c></seealso>,
this function is called by the new process to initialize.</p>
<p><c>Args</c> is the <c>Args</c> argument provided to the start
function.</p>
<p>If the initialization is successful, the function is to
return <c>{ok,State}</c>, <c>{ok,State,Timeout}</c>, or
<c>{ok,State,hibernate}</c>, where <c>State</c> is the internal
state of the <c>gen_server</c> process.</p>
<p>If an integer time-out value is provided, a time-out occurs
unless a request or a message is received within
<c>Timeout</c> milliseconds. A time-out is represented by
the atom <c>timeout</c>, which is to be handled by the
<seealso marker="#Module:handle_info/2">
<c>Module:handle_info/2</c></seealso> callback function. The atom
<c>infinity</c> can be used to wait indefinitely, this is
the default value.</p>
<p>If <c>hibernate</c> is specified instead of a time-out value,
the process goes into
hibernation when waiting for the next message to arrive (by calling
<seealso marker="proc_lib#hibernate/3">
<c>proc_lib:hibernate/3</c></seealso>).</p>
<p>If the initialization fails, the function is to return
<c>{stop,Reason}</c>, where <c>Reason</c> is any term, or
<c>ignore</c>.</p>
</desc>
</func>
<func>
<name since="">Module:terminate(Reason, State)</name>
<fsummary>Clean up before termination.</fsummary>
<type>
<v>Reason = normal | shutdown | {shutdown,term()} | term()</v>
<v>State = term()</v>
</type>
<desc>
<note>
<p>This callback is optional, so callback modules need not
export it. The <c>gen_server</c> module provides a default
implementation without cleanup.</p>
</note>
<p>This function is called by a <c>gen_server</c> process when it is
about to terminate. It is to be the opposite of
<seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
and do any necessary cleaning up. When it returns,
the <c>gen_server</c> process terminates with <c>Reason</c>.
The return value is ignored.</p>
<p><c>Reason</c> is a term denoting the stop reason and <c>State</c>
is the internal state of the <c>gen_server</c> process.</p>
<p><c>Reason</c> depends on why the <c>gen_server</c> process is
terminating. If it is because another callback function has returned
a stop tuple <c>{stop,..}</c>, <c>Reason</c> has
the value specified in that tuple. If it is because of a failure,
<c>Reason</c> is the error reason.</p>
<p>If the <c>gen_server</c> process is part of a supervision tree and
is ordered by its supervisor to terminate, this function is
called with <c>Reason=shutdown</c> if the following
conditions apply:</p>
<list type="bulleted">
<item>
<p>The <c>gen_server</c> process has been set to trap exit
signals.</p>
</item>
<item>
<p>The shutdown strategy as defined in the child specification
of the supervisor is an integer time-out value, not
<c>brutal_kill</c>.</p>
</item>
</list>
<p>Even if the <c>gen_server</c> process is <em>not</em> part of a
supervision tree, this function is called if it receives an
<c>'EXIT'</c> message from its parent. <c>Reason</c> is the same
as in the <c>'EXIT'</c> message.</p>
<p>Otherwise, the <c>gen_server</c> process terminates immediately.</p>
<p>Notice that for any other reason than <c>normal</c>,
<c>shutdown</c>, or <c>{shutdown,Term}</c>, the <c>gen_server</c>
process is assumed to terminate because of an error and
an error report is issued using
<seealso marker="kernel:logger"><c>logger(3)</c></seealso>.</p>
</desc>
</func>
</funcs>
<section>
<title>See Also</title>
<p><seealso marker="gen_event"><c>gen_event(3)</c></seealso>,
<seealso marker="gen_statem"><c>gen_statem(3)</c></seealso>,
<seealso marker="proc_lib"><c>proc_lib(3)</c></seealso>,
<seealso marker="supervisor"><c>supervisor(3)</c></seealso>,
<seealso marker="sys"><c>sys(3)</c></seealso></p>
</section>
</erlref>