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kernel The Kernel application.

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
Logger Handlers

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

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.

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).

 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 how logger should be configured.

For more details and examples, see the Configuration section in the Logger User's Guide.

logger_level = Level

Level = emergency | alert | critical | error | warning | notice | info | debug

This parameter specifies which log levels to log. The specified level, and all levels that are more severe, will be logged.

The default value is info.

To change the global log level at run-time, use logger:set_logger_config(level, error).

 logger_sasl_compatible = boolean()

If this parameter is set to true, then the default 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 sasl, according to values of the SASL configuration parameter sasl_error_logger and sasl_errlog_type, see sasl(6)

The default value is false.

See chapter Backwards compatibility with error_logger for more information about handling of the so called SASL reports.

This configuration option only effects the default and sasl handler. Any other handlers are uneffected.

 logger_progress_reports = stop | log

If logger_sasl_compatible = false, then logger_progress_reports specifies if progress reports from supervisor and application_controller shall be logged by the default logger.

If logger_sasl_compatible = true, then logger_progress_reports is ignored.

The default value is stop

This configuration option only effects the default and sasl handler. Any other handlers are uneffected.

 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 logger messages are generated 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. TickTime is specified in seconds. Once every TickTime/4 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.

The time T, in which a node that is not responding is detected, is calculated as , where:

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

TickTime defaults to 60 (seconds). Thus, seconds.

Notice that all communicating nodes are to have the same TickTime value specified.

Normally, a terminating node is detected immediately.

 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_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 .

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_pg2 = true | false

Starts the pg2 server (see pg2(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

Specifies whether shell history should be logged to disk between usages of erl.

 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.

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 of the default logger_std_h to the same value. Example: erl -kernel logger '[{handler,default,logger_std_h,#{logger_std_h=>#{type=>{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.

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), pg2(3), rpc(3), seq_trace(3), user(3), timer(3)