# kernel

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Application

kernel

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Application Summary

The Kernel application.

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Description

The Kernel application has all the code necessary to run the Erlang runtime system: file servers, code servers, and so on.

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:

- Start, stop, supervision, configuration, and distribution of applications
- Code loading
- Logging
- Global name service
- Supervision of Erlang/OTP
- Communication with sockets
- Operating system interface

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Logger Handlers

Two standard logger handlers are defined in the Kernel application. These are described in the Kernel User's Guide, and in the logger_std_h(3) and logger_disk_log_h(3) manual pages.

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OS Signal Event Handler

Asynchronous OS signals may be subscribed to via the Kernel applications event manager (see OTP Design Principles and gen_event(3)) registered as erl_signal_server. A default signal handler is installed which handles the following signals:

sigusr1

  The default handler will halt Erlang and produce a crashdump with slogan "Received SIGUSR1". This is equivalent to calling erlang:halt("Received SIGUSR1").

sigquit

  The default handler will halt Erlang immediately. This is equivalent to calling erlang:halt().

sigterm

  The default handler will terminate Erlang normally. This is equivalent to calling init:stop().

#### Events

Any event handler added to erl_signal_server must handle the following events.

sighup

  Hangup detected on controlling terminal or death of controlling process

sigquit

  Quit from keyboard

sigabrt

  Abort signal from abort

sigalrm

  Timer signal from alarm

sigterm

  Termination signal

sigusr1

  User-defined signal 1

sigusr2

  User-defined signal 2

sigchld

  Child process stopped or terminated

sigstop

  Stop process

sigtstp

  Stop typed at terminal

Setting OS signals are described in os:set_signal/2.

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Configuration

The following configuration parameters are defined for the Kernel application. For more information about configuration parameters, see file app(4).

distributed = [Distrib]

  Specifies which applications that are distributed and on which nodes they are allowed to execute. In this parameter:

  - Distrib = {App,Nodes} | {App,Time,Nodes}
  - App = atom()
  - Time = integer()>0
  - Nodes = [node() | {node(),...,node()}]

  The parameter is described in application:load/2.

dist_auto_connect = Value

  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. Value is one of:

  never

    Connections are never automatically established, they must be explicitly connected. See net_kernel(3).

  once

    Connections are established automatically, but only once per node. If a node goes down, it must thereafter be explicitly connected. See net_kernel(3).

dist_listen = boolean()

  Specifies whether this node should be listening for incoming distribution connections. Using this option implies that the node also is -hidden.

permissions = [Perm]

  Specifies the default permission for applications when they are started. In this parameter:

  - Perm = {ApplName,Bool}
  - ApplName = atom()
  - Bool = boolean()

  Permissions are described in application:permit/2.

logger = [Config]

  Specifies the configuration for Logger, except the primary log level, which is specified with logger_level, and the compatibility with SASL Error Logging, which is specified with logger_sasl_compatible.

  The logger parameter is described in section Logging in the Kernel User's Guide.

logger_level = Level

  Specifies the primary log level for Logger. Log events with the same, or a more severe level, pass through the primary log level check. See section Logging in the Kernel User's Guide for more information about Logger and log levels.

  Level = emergency | alert | critical | error | warning | notice | info | debug | all | none

  To change the primary log level at runtime, use logger:set_primary_config(level, Level).

  Defaults to notice.

logger_metadata = Metadata

  Specifies primary metadata for log events.

  Metadata = map()

  Defaults to #{}.

logger_sasl_compatible = true | false

  Specifies if Logger behaves backwards compatible with the SASL error logging functionality from releases prior to Erlang/OTP 21.0.

  If this parameter is set to true, the default Logger handler does not log any progress-, crash-, or supervisor reports. If the SASL application is then started, it adds a Logger handler named sasl, which logs these events according to values of the SASL configuration parameter sasl_error_logger and sasl_errlog_type.

  See section Deprecated Error Logger Event Handlers and Configuration in the sasl(6) manual page for information about the SASL configuration parameters.

  See section SASL Error Logging in the SASL User's Guide, and section Backwards Compatibility with error_logger in the Kernel User's Guide for information about the SASL error logging functionality, and how Logger can be backwards compatible with this.

  Defaults to false.

  Note

  If this parameter is set to true, sasl_errlog_type indicates that progress reports shall be logged, and the configured primary log level is notice or more severe, then SASL automatically sets the primary log level to info. That is, this setting can potentially overwrite the value of the Kernel configuration parameter logger_level. This is to allow progress reports, which have log level info, to be forwarded to the handlers.

global_groups = [GroupTuple]

  Defines global groups, see global_group(3). In this parameter:

  - GroupTuple = {GroupName, [Node]} | {GroupName, PublishType, [Node]}

  - GroupName = atom()

  - PublishType = normal | hidden

  - Node = node()

inet_default_connect_options = [{Opt, Val}]

  Specifies default options for connect sockets, see inet(3).

inet_default_listen_options = [{Opt, Val}]

  Specifies default options for listen (and accept) sockets, see inet(3).

{inet_dist_use_interface, ip_address()}

  If the host of an Erlang node has many network interfaces, this parameter specifies which one to listen on. For the type definition of ip_address(), see inet(3).

{inet_dist_listen_min, First} and {inet_dist_listen_max, Last}

  Defines the First..Last port range for the listener socket of a distributed Erlang node.

{inet_dist_listen_options, Opts}

  Defines a list of extra socket options to be used when opening the listening socket for a distributed Erlang node. See gen_tcp:listen/2.

{inet_dist_connect_options, Opts}

  Defines a list of extra socket options to be used when connecting to other distributed Erlang nodes. See gen_tcp:connect/4.

inet_parse_error_log = silent

  If set, no log events are issued when erroneous lines are found and skipped in the various Inet configuration files.

inetrc = Filename

  The name (string) of an Inet user configuration file. For details, see section Inet Configuration in the ERTS User's Guide.

net_setuptime = SetupTime

  SetupTime 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 120. If higher values are specified, 120 is used. Default is 7 seconds if the variable is not specified, or if the value is incorrect (for example, not a number).

  Notice that this value does not limit the total connection setup time, but rather each individual network operation during the connection setup and handshake.

net_ticktime = TickTime

  Specifies the net_kernel tick time in seconds. This is the approximate time a connected node may be unresponsive until it is considered down and thereby disconnected.

  Once every TickTime/4 seconds, each connected node is ticked if nothing has been sent to it during that last TickTime/4 interval. A tick is a small package sent on the connection. A connected node is considered to be down if no ticks or payload packages have been received during the last four TickTime/4 intervals. This ensures that nodes that are not responding, for reasons such as hardware errors, are considered to be down.

  As the availability is only checked every TickTime/4 seconds, the actual time T a node have been unresponsive when detected may vary between MinT and MaxT, where:

      MinT = TickTime - TickTime / 4
      MaxT = TickTime + TickTime / 4

  TickTime defaults to 60 seconds. Thus, 45 < T < 75 seconds.

  Notice that all communicating nodes are to have the same TickTime value specified, as it determines both the frequency of outgoing ticks and the expected frequency of incominging ticks.

  Normally, a terminating node is detected immediately by the transport protocol (like TCP/IP).

shutdown_timeout = integer() | infinity

  Specifies the time application_controller waits for an application to terminate during node shutdown. If the timer expires, application_controller brutally kills application_master of the hanging application. If this parameter is undefined, it defaults to infinity.

sync_nodes_mandatory = [NodeName]

  Specifies which other nodes that must 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 [].

sync_nodes_optional = [NodeName]

  Specifies which other nodes that can 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.

sync_nodes_timeout = integer() | infinity

  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 global is synchronized.

start_distribution = true | false

  Starts all distribution services, such as rpc, global, and net_kernel if the parameter is true. This parameter is to be set to false for systems who want to disable all distribution functionality.

  Defaults to true.

start_dist_ac = true | false

  Starts the dist_ac server if the parameter is true. This parameter is to be set to true for systems using distributed applications.

  Defaults to false. If this parameter is undefined, the server is started if parameter distributed is set.

start_boot_server = true | false

  Starts the boot_server if the parameter is true (see erl_boot_server(3)). This parameter is to be set to true in an embedded system using this service.

  Defaults to false.

boot_server_slaves = [SlaveIP]

  If configuration parameter start_boot_server is true, this parameter can be used to initialize boot_server with a list of slave IP addresses:

  SlaveIP = string() | atom | {integer(),integer(),integer(),integer()},

  where 0 <= integer() <=255.

  Examples of SlaveIP in atom, string, and tuple form:

  '150.236.16.70', "150,236,16,70", {150,236,16,70}.

  Defaults to [].

start_disk_log = true | false

  Starts the disk_log_server if the parameter is true (see disk_log(3)). This parameter is to be set to true in an embedded system using this service.

  Defaults to false.

start_pg = true | false

  Starts the default pg scope server (see pg(3)) if the parameter is true. This parameter is to be set to true in an embedded system that uses this service.

  Defaults to false.

start_timer = true | false

  Starts the timer_server if the parameter is true (see timer(3)). This parameter is to be set to true in an embedded system using this service.

  Defaults to false.

shell_history = enabled | disabled | module()

  Specifies whether shell history should be logged to disk between usages of erl (enabled), not logged at all (disabled), or a user-specified module will be used to log shell history. This module should export load() -> [string()] returning a list of strings to load in the shell when it starts, and add(iodata()) -> ok. called every time new line is entered in the shell. By default logging is disabled.

shell_history_drop = [string()]

  Specific log lines that should not be persisted. For example ["q().", "init:stop()."] will allow to ignore commands that shut the node down. Defaults to [].

shell_history_file_bytes = integer()

  How many bytes the shell should remember. By default, the value is set to 512kb, and the minimal value is 50kb.

shell_history_path = string()

  Specifies where the shell history files will be stored. defaults to the user's cache directory as returned by filename:basedir(user_cache, "erlang-history").

shutdown_func = {Mod, Func}

  Where:

  - Mod = atom()
  - Func = atom()

  Sets a function that application_controller calls when it starts to terminate. The function is called as Mod:Func(Reason), where Reason is the terminate reason for application_controller, and it must return as soon as possible for application_controller to terminate properly.

source_search_rules = [DirRule] | [SuffixRule]

  Where:

  - DirRule = {ObjDirSuffix,SrcDirSuffix}
  - SuffixRule = {ObjSuffix,SrcSuffix,[DirRule]}
  - ObjDirSuffix = string()
  - SrcDirSuffix = string()
  - ObjSuffix = string()
  - SrcSuffix = string()

  Specifies a list of rules for use by filelib:find_file/2 filelib:find_source/2 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 {"ebin", "src"}, which are used by filelib:find_file/2, or triples specifying separate directory suffix rules depending on file name extensions, for example [{".beam", ".erl", [{"ebin", "src"}]}, which are used by filelib:find_source/2. Both kinds of rules can be mixed in the list.

  The interpretation of ObjDirSuffix and SrcDirSuffix is as follows: if the end of the directory name where an object is located matches ObjDirSuffix, then the name created by replacing ObjDirSuffix with SrcDirSuffix is expanded by calling filelib:wildcard/1, and the first regular file found among the matches is the source file.

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Deprecated Configuration Parameters

In Erlang/OTP 21.0, a new API for logging was added. The old error_logger event manager, and event handlers running on this manager, still work, but they are no longer used by default.

The following application configuration parameters can still be set, but they are only used if the corresponding configuration parameters for Logger are not set.

error_logger
  Replaced by setting the type, and possibly file and modes parameters of the default logger_std_h handler. Example:

      erl -kernel logger '[{handler,default,logger_std_h,#{config=>#{file=>"/tmp/erlang.log"}}}]'

error_logger_format_depth
  Replaced by setting the depth parameter of the default handlers formatter. Example:

      erl -kernel logger '[{handler,default,logger_std_h,#{formatter=>{logger_formatter,#{legacy_header=>true,template=>[{logger_formatter,header},"\n",msg,"\n"],depth=>10}}}]'

See Backwards compatibility with error_logger for more information.

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See Also

app(4), application(3), code(3), disk_log(3), erl_boot_server(3), erl_ddll(3), file(3), global(3), global_group(3), heart(3), inet(3), logger(3), net_kernel(3), os(3), pg(3), rpc(3), seq_trace(3), user(3), timer(3)