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<!DOCTYPE appref SYSTEM "appref.dtd">
<appref>
<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.
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distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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limitations under the License.
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<title>kernel</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
</header>
<app>kernel</app>
<appsummary>The Kernel application.</appsummary>
<description>
<p>The Kernel application has all the code necessary to run
the Erlang runtime system: file servers, code servers,
and so on.</p>
<p>The Kernel application is the first application started. It is
mandatory in the sense that the minimal system based on
Erlang/OTP consists of Kernel and STDLIB. Kernel
contains the following functional areas:</p>
<list type="bulleted">
<item>Start, stop, supervision, configuration, and distribution of applications</item>
<item>Code loading</item>
<item>Logging</item>
<item>Global name service</item>
<item>Supervision of Erlang/OTP</item>
<item>Communication with sockets</item>
<item>Operating system interface</item>
</list>
</description>
<section>
<title>Logger Handlers</title>
<p>Two standard logger handlers are defined in
the Kernel application. These are described in the
<seealso marker="logger_chapter">Kernel User's Guide</seealso>,
and in <seealso marker="logger_std_h"><c>logger_std_h(3)</c></seealso>
and <seealso marker="logger_disk_log_h"><c>logger_disk_log_h(3)</c>
</seealso>.</p>
</section>
<section>
<marker id="erl_signal_server"/>
<title>OS Signal Event Handler</title>
<p>Asynchronous OS signals may be subscribed to via the Kernel applications event manager
(see <seealso marker="doc/design_principles:des_princ">OTP Design Principles</seealso> and
<seealso marker="stdlib:gen_event"><c>gen_event(3)</c></seealso>) registered as <c>erl_signal_server</c>.
A default signal handler is installed which handles the following signals:</p>
<taglist>
<tag><c>sigusr1</c></tag>
<item><p>The default handler will halt Erlang and produce a crashdump
with slogan "Received SIGUSR1".
This is equivalent to calling <c>erlang:halt("Received SIGUSR1")</c>.
</p></item>
<tag><c>sigquit</c></tag>
<item><p>The default handler will halt Erlang immediately.
This is equivalent to calling <c>erlang:halt()</c>.
</p></item>
<tag><c>sigterm</c></tag>
<item><p>The default handler will terminate Erlang normally.
This is equivalent to calling <c>init:stop()</c>.
</p></item>
</taglist>
<section>
<title>Events</title>
<p>Any event handler added to <c>erl_signal_server</c> must handle the following events.</p>
<taglist>
<tag><c>sighup</c></tag>
<item><p>Hangup detected on controlling terminal or death of controlling process</p></item>
<tag><c>sigquit</c></tag>
<item><p>Quit from keyboard</p></item>
<tag><c>sigabrt</c></tag>
<item><p>Abort signal from abort</p></item>
<tag><c>sigalrm</c></tag>
<item><p>Timer signal from alarm</p></item>
<tag><c>sigterm</c></tag>
<item><p>Termination signal</p></item>
<tag><c>sigusr1</c></tag>
<item><p>User-defined signal 1</p></item>
<tag><c>sigusr2</c></tag>
<item><p>User-defined signal 2</p></item>
<tag><c>sigchld</c></tag>
<item><p>Child process stopped or terminated</p></item>
<tag><c>sigstop</c></tag>
<item><p>Stop process</p></item>
<tag><c>sigtstp</c></tag>
<item><p>Stop typed at terminal</p></item>
</taglist>
<p>Setting OS signals are described in <seealso marker="os#set_signal/2"><c>os:set_signal/2</c></seealso>.</p>
</section>
</section>
<section>
<marker id="configuration"/>
<title>Configuration</title>
<p>The following configuration parameters are defined for the Kernel
application. For more information about configuration parameters,
see file <seealso marker="app"><c>app(4)</c></seealso>.</p>
<taglist>
<tag><c>distributed = [Distrib]</c></tag>
<item>
<p>Specifies which applications that are distributed and on which
nodes they are allowed to execute. In this parameter:</p>
<list type="bulleted">
<item><c>Distrib = {App,Nodes} | {App,Time,Nodes}</c></item>
<item><c>App = atom()</c></item>
<item><c>Time = integer()>0</c></item>
<item><c>Nodes = [node() | {node(),...,node()}]</c></item>
</list>
<p>The parameter is described in
<seealso marker="application#load/2"><c>application:load/2</c></seealso>.</p>
</item>
<tag><c>dist_auto_connect = Value</c></tag>
<item>
<p>Specifies when nodes are automatically connected. If
this parameter is not specified, a node is always
automatically connected, for example, when a message is to be sent to
that node. <c>Value</c> is one of:</p>
<taglist>
<tag><c>never</c></tag>
<item><p>Connections are never automatically established, they
must be explicitly connected. See
<seealso marker="net_kernel"><c>net_kernel(3)</c></seealso>.</p></item>
<tag><c>once</c></tag>
<item><p>Connections are established automatically, but only
once per node. If a node goes down, it must thereafter be
explicitly connected. See
<seealso marker="net_kernel"><c>net_kernel(3)</c></seealso>.</p></item>
</taglist>
</item>
<tag><c>permissions = [Perm]</c></tag>
<item>
<p>Specifies the default permission for applications when they
are started. In this parameter:</p>
<list type="bulleted">
<item><c>Perm = {ApplName,Bool}</c></item>
<item><c>ApplName = atom()</c></item>
<item><c>Bool = boolean()</c></item>
</list>
<p>Permissions are described in
<seealso marker="application#permit/2"><c>application:permit/2</c></seealso>.</p>
</item>
<tag><marker id="logger"/><c>logger = [Config]</c></tag>
<item>
<p>Specifies how <seealso marker="logger"><c>logger</c></seealso> should be
configured.</p>
<p>For more details and examples, see the <seealso marker="logger_chapter#logger">
Configuration</seealso> section in the <seealso marker="logger_chapter">
Logger User's Guide</seealso>.
</p>
</item>
<tag><marker id="logger_level"/><c>logger_level = Level</c></tag>
<item>
<p><c>Level = emergency | alert | critical | error | warning |
notice | info | debug</c></p>
<p>This parameter specifies which log levels to log. The
specified level, and all levels that are more severe, will
be logged.</p>
<p>The default value is <c>info</c>.</p>
<p>To change the global log level at run-time, use
<seealso marker="logger#set_logger_config/2">
<c>logger:set_logger_config(level, error)</c></seealso>.</p>
</item>
<tag><marker id="logger_sasl_compatible"/>
<c>logger_sasl_compatible = boolean()</c></tag>
<item>
<p>If this parameter is set to true, then the <c>default</c> logger handler
will not log any progress-, crash-, or supervisor reports.
If the SASL application is started,
these log events will be sent to a second handler instance
named <c>sasl</c>, according to values of the SASL
configuration parameter <c>sasl_error_logger</c>
and <c>sasl_errlog_type</c>, see
<seealso marker="sasl:sasl_app#deprecated_error_logger_config">sasl(6)
</seealso></p>
<p>The default value is <c>false</c>.</p>
<p>See chapter <seealso marker="logger_chapter#compatibility">Backwards
compatibility with error_logger</seealso> for more
information about handling of the so called SASL reports.</p>
<note><p>This configuration option only effects the <c>default</c>
and <c>sasl</c> handler. Any other handlers are uneffected.</p></note>
</item>
<tag><marker id="logger_progress_reports"/>
<c>logger_progress_reports = stop | log</c></tag>
<item>
<p>If <c>logger_sasl_compatible = false</c>,
then <c>logger_progress_reports</c> specifies if progress
reports from <c>supervisor</c>
and <c>application_controller</c> shall be logged by the
default logger.</p>
<p>If <c>logger_sasl_compatible = true</c>,
then <c>logger_progress_reports</c> is ignored.</p>
<p>The default value is <c>stop</c></p>
<note><p>This configuration option only effects the <c>default</c>
and <c>sasl</c> handler. Any other handlers are uneffected.</p></note>
</item>
<tag><c>global_groups = [GroupTuple]</c></tag>
<item>
<marker id="global_groups"></marker>
<p>Defines global groups, see
<seealso marker="global_group"><c>global_group(3)</c></seealso>.
In this parameter:</p>
<list type="bulleted">
<item><p><c>GroupTuple = {GroupName, [Node]} | {GroupName, PublishType, [Node]}</c></p></item>
<item><p><c>GroupName = atom()</c></p></item>
<item><p><c>PublishType = normal | hidden</c></p></item>
<item><p><c>Node = node()</c></p></item>
</list>
</item>
<tag><c>inet_default_connect_options = [{Opt, Val}]</c></tag>
<item>
<p>Specifies default options for <c>connect</c> sockets,
see <seealso marker="inet"><c>inet(3)</c></seealso>.</p>
</item>
<tag><c>inet_default_listen_options = [{Opt, Val}]</c></tag>
<item>
<p>Specifies default options for <c>listen</c> (and
<c>accept</c>) sockets, see <seealso marker="inet"><c>inet(3)</c></seealso>.</p>
</item>
<tag><c>{inet_dist_use_interface, ip_address()}</c></tag>
<item>
<p>If the host of an Erlang node has many network interfaces,
this parameter specifies which one to listen on. For the type definition
of <c>ip_address()</c>,
see <seealso marker="inet"><c>inet(3)</c></seealso>.</p>
</item>
<tag><c>{inet_dist_listen_min, First}</c> and <c>{inet_dist_listen_max, Last}</c></tag>
<item>
<p>Defines the <c>First..Last</c> port range for the listener
socket of a distributed Erlang node.</p>
</item>
<tag><c>{inet_dist_listen_options, Opts}</c></tag>
<item>
<marker id="inet_dist_listen_options"></marker>
<p>Defines a list of extra socket options to be used when opening the
listening socket for a distributed Erlang node.
See <seealso marker="gen_tcp#listen/2"><c>gen_tcp:listen/2</c></seealso>.</p>
</item>
<tag><c>{inet_dist_connect_options, Opts}</c></tag>
<item>
<marker id="inet_dist_connect_options"></marker>
<p>Defines a list of extra socket options to be used when connecting to
other distributed Erlang nodes.
See <seealso marker="gen_tcp#connect/4"><c>gen_tcp:connect/4</c></seealso>.</p>
</item>
<tag><c>inet_parse_error_log = silent</c></tag>
<item>
<p>If set, no
<c>logger</c> messages are generated when erroneous
lines are found and skipped in the various Inet configuration
files.</p>
</item>
<tag><c>inetrc = Filename</c></tag>
<item>
<p>The name (string) of an Inet user configuration file. For details,
see section
<seealso marker="erts:inet_cfg"><c>Inet Configuration</c></seealso>
in the ERTS User's Guide.</p>
</item>
<tag><c>net_setuptime = SetupTime</c></tag>
<item>
<marker id="net_setuptime"></marker>
<p><c>SetupTime</c> must be a positive integer or floating point
number, and is interpreted as the maximum allowed time
for each network operation during connection setup to another
Erlang node. The maximum allowed value is <c>120</c>. If higher values
are specified, <c>120</c> is used. Default is 7 seconds if the variable
is not specified, or if the value is incorrect (for example, not a number).</p>
<p>Notice that this value does not limit the total connection
setup time, but rather each individual network operation during
the connection setup and handshake.</p>
</item>
<tag><c>net_ticktime = TickTime</c></tag>
<item>
<marker id="net_ticktime"></marker>
<p>Specifies the <c>net_kernel</c> tick time. <c>TickTime</c>
is specified in seconds. Once every <c>TickTime/4</c> second, all
connected nodes are ticked (if anything else is written
to a node). If nothing is received from another node
within the last four tick times, that node is considered
to be down. This ensures that nodes that are not responding,
for reasons such as hardware errors, are considered to be
down.</p>
<p>The time <c>T</c>, in which a node that is not responding is
detected, is calculated as <c><![CDATA[MinT < T < MaxT]]></c>, where:</p>
<code type="none">
MinT = TickTime - TickTime / 4
MaxT = TickTime + TickTime / 4</code>
<p><c>TickTime</c> defaults to <c>60</c> (seconds). Thus,
<c><![CDATA[45 < T < 75]]></c> seconds.</p>
<p>Notice that <em>all</em> communicating nodes are to have the <em>same</em>
<c>TickTime</c> value specified.</p>
<p>Normally, a terminating node is detected immediately.</p>
</item>
<tag><c>shutdown_timeout = integer() | infinity</c></tag>
<item>
<p>Specifies the time <c>application_controller</c> waits
for an application to terminate during node shutdown. If the
timer expires, <c>application_controller</c> brutally
kills <c>application_master</c> of the hanging
application. If this parameter is undefined, it defaults
to <c>infinity</c>.</p>
</item>
<tag><c>sync_nodes_mandatory = [NodeName]</c></tag>
<item>
<p>Specifies which other nodes that <em>must</em> be alive
for this node to start properly. If some node in the list
does not start within the specified time, this node does not
start either. If this parameter is undefined, it defaults to
<c>[]</c>.</p>
</item>
<tag><c>sync_nodes_optional = [NodeName]</c></tag>
<item>
<p>Specifies which other nodes that <em>can</em> be alive
for this node to start properly. If some node in this list
does not start within the specified time, this node starts
anyway. If this parameter is undefined, it defaults to
the empty list.</p>
</item>
<tag><c>sync_nodes_timeout = integer() | infinity</c></tag>
<item>
<p>Specifies the time (in milliseconds) that this node
waits for the mandatory and optional nodes to start. If
this parameter is undefined, no node synchronization is
performed. This option ensures that <c>global</c> is
synchronized.</p>
</item>
<tag><c>start_dist_ac = true | false</c></tag>
<item>
<p>Starts the <c>dist_ac</c> server if the parameter is
<c>true</c>. This parameter is to be set to <c>true</c> for
systems using distributed applications.</p>
<p>Defaults to <c>false</c>. If this parameter is
undefined, the server is started if parameter
<c>distributed</c> is set.</p>
</item>
<tag><c>start_boot_server = true | false</c></tag>
<item>
<p>Starts the <c>boot_server</c> if the parameter is <c>true</c>
(see <seealso marker="erl_boot_server"><c>erl_boot_server(3)</c></seealso>).
This parameter is to be set to <c>true</c> in an embedded system
using this service.</p>
<p>Defaults to <c>false</c>.</p>
</item>
<tag><c>boot_server_slaves = [SlaveIP]</c></tag>
<item>
<p>If configuration parameter <c>start_boot_server</c> is
<c>true</c>, this parameter can be used to initialize
<c>boot_server</c> with a list of slave IP addresses:</p>
<p>
<c>SlaveIP = string() | atom | {integer(),integer(),integer(),integer()}</c>,</p>
<p>where <c><![CDATA[0 <= integer() <=255]]></c>.</p>
<p>Examples of <c>SlaveIP</c> in atom, string, and tuple form:</p>
<p><c>'150.236.16.70', "150,236,16,70", {150,236,16,70}</c>.</p>
<p>Defaults to <c>[]</c>.</p>
</item>
<tag><c>start_disk_log = true | false</c></tag>
<item>
<p>Starts the <c>disk_log_server</c> if the parameter is
<c>true</c> (see <seealso marker="disk_log"><c>disk_log(3)</c></seealso>).
This parameter is to be set to <c>true</c> in an embedded system
using this service.</p>
<p>Defaults to <c>false</c>.</p>
</item>
<tag><c>start_pg2 = true | false</c></tag>
<item>
<marker id="start_pg2"></marker>
<p>Starts the <c>pg2</c> server (see
<seealso marker="pg2"><c>pg2(3)</c></seealso>) if
the parameter is <c>true</c>. This parameter is to be set to
<c>true</c> in an embedded system that uses this service.</p>
<p>Defaults to <c>false</c>.</p>
</item>
<tag><c>start_timer = true | false</c></tag>
<item>
<p>Starts the <c>timer_server</c> if the parameter is
<c>true</c> (see <seealso marker="stdlib:timer"><c>timer(3)</c></seealso>).
This parameter is to be set to <c>true</c> in an embedded system
using this service.</p>
<p>Defaults to <c>false</c>.</p>
</item>
<tag><c>shell_history = enabled | disabled </c></tag>
<item>
<p>Specifies whether shell history should be logged to disk
between usages of <c>erl</c>.</p>
</item>
<tag><c>shell_history_drop = [string()]</c></tag>
<item>
<p>Specific log lines that should not be persisted. For
example <c>["q().", "init:stop()."]</c> will allow to
ignore commands that shut the node down. Defaults to
<c>[]</c>.</p>
</item>
<tag><c>shell_history_file_bytes = integer()</c></tag>
<item>
<p>how many bytes the shell should remember. By default, the
value is set to 512kb, and the minimal value is 50kb.</p>
</item>
<tag><c>shell_history_path = string()</c></tag>
<item>
<p>Specifies where the shell history files will be stored.
defaults to the user's cache directory as returned by
<c>filename:basedir(user_cache, "erlang-history")</c>.</p>
</item>
<tag><c>shutdown_func = {Mod, Func}</c></tag>
<item>
<p>Where:</p>
<list type="bulleted">
<item><c>Mod = atom()</c></item>
<item><c>Func = atom()</c></item>
</list>
<p>Sets a function that <c>application_controller</c> calls
when it starts to terminate. The function is called as
<c>Mod:Func(Reason)</c>, where <c>Reason</c> is the terminate
reason for <c>application_controller</c>, and it must
return as soon as possible for <c>application_controller</c>
to terminate properly.</p>
</item>
<tag><c>source_search_rules = [DirRule] | [SuffixRule] </c></tag>
<item>
<marker id="source_search_rules"></marker>
<p>Where:</p>
<list type="bulleted">
<item><c>DirRule = {ObjDirSuffix,SrcDirSuffix}</c></item>
<item><c>SuffixRule = {ObjSuffix,SrcSuffix,[DirRule]}</c></item>
<item><c>ObjDirSuffix = string()</c></item>
<item><c>SrcDirSuffix = string()</c></item>
<item><c>ObjSuffix = string()</c></item>
<item><c>SrcSuffix = string()</c></item>
</list>
<p>Specifies a list of rules for use by
<seealso marker="stdlib:filelib#find_file/2">
<c>filelib:find_file/2</c></seealso>
<seealso marker="stdlib:filelib#find_source/2">
<c>filelib:find_source/2</c></seealso>
If this is set to some other value
than the empty list, it replaces the default rules. Rules can be
simple pairs of directory suffixes, such as <c>{"ebin",
"src"}</c>, which are used by <c>filelib:find_file/2</c>, or
triples specifying separate directory suffix rules depending on
file name extensions, for example <c>[{".beam", ".erl", [{"ebin",
"src"}]}</c>, which are used by <c>filelib:find_source/2</c>. Both
kinds of rules can be mixed in the list.</p>
<p>The interpretation of <c>ObjDirSuffix</c> and <c>SrcDirSuffix</c>
is as follows: if the end of the directory name where an
object is located matches <c>ObjDirSuffix</c>, then the
name created by replacing <c>ObjDirSuffix</c> with
<c>SrcDirSuffix</c> is expanded by calling
<seealso marker="stdlib:filelib#wildcard/1">
<c>filelib:wildcard/1</c></seealso>, and the first regular
file found among the matches is the source file.
</p>
</item>
</taglist>
</section>
<section>
<title>Deprecated Configuration Parameters</title>
<p>In OTP-21, a new API for logging was added to Erlang/OTP. The
old <c>error_logger</c> event manager, and event handlers
running on this manager, will still work, but they are not used
by default.</p>
<p>The following application configuration parameters can still be
set, but they will only be used if the corresponding new logger
variables are not set.</p>
<taglist>
<tag><c>error_logger</c></tag>
<item>Replaced by setting the type of the default
<seealso marker="logger_std_h#type"><c>logger_std_h</c></seealso>
to the same value. Example:
<code type="none">
erl -kernel logger '[{handler,default,logger_std_h,#{logger_std_h=>#{type=>{file,"/tmp/erlang.log"}}}}]'
</code>
</item>
<tag><c>error_logger_format_depth</c></tag>
<item>Replaced by setting the <seealso marker="logger_formatter#depth"><c>depth</c></seealso>
parameter of the default handlers formatter. Example:
<code type="none">
erl -kernel logger '[{handler,default,logger_std_h,#{formatter=>{logger_formatter,#{legacy_header=>true,template=>[{logger_formatter,header},"\n",msg,"\n"],depth=>10}}}]'
</code>
</item>
</taglist>
<p>See <seealso marker="logger_chapter#compatibility">Backwards
compatibility with error_logger</seealso> for more
information.</p>
</section>
<section>
<title>See Also</title>
<p><seealso marker="app"><c>app(4)</c></seealso>,
<seealso marker="application"><c>application(3)</c></seealso>,
<seealso marker="code"><c>code(3)</c></seealso>,
<seealso marker="disk_log"><c>disk_log(3)</c></seealso>,
<seealso marker="erl_boot_server"><c>erl_boot_server(3)</c></seealso>,
<seealso marker="erl_ddll"><c>erl_ddll(3)</c></seealso>,
<seealso marker="file"><c>file(3)</c></seealso>,
<seealso marker="global"><c>global(3)</c></seealso>,
<seealso marker="global_group"><c>global_group(3)</c></seealso>,
<seealso marker="heart"><c>heart(3)</c></seealso>,
<seealso marker="inet"><c>inet(3)</c></seealso>,
<seealso marker="logger"><c>logger(3)</c></seealso>,
<seealso marker="net_kernel"><c>net_kernel(3)</c></seealso>,
<seealso marker="os"><c>os(3)</c></seealso>,
<seealso marker="pg2"><c>pg2(3)</c></seealso>,
<seealso marker="rpc"><c>rpc(3)</c></seealso>,
<seealso marker="seq_trace"><c>seq_trace(3)</c></seealso>,
<seealso marker="user"><c>user(3)</c></seealso>,
<seealso marker="stdlib:timer"><c>timer(3)</c></seealso></p>
</section>
</appref>
