Kernel

Reference Manual

Version 6.0

Table of Contents

kernel

Application

kernel

Application Summary

The Kernel application.

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
  • Error 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).

browser_cmd = string() | {M,F,A}

When pressing the Help button in a tool such as Debugger, the help text (an HTML file File) is by default displayed in a Netscape browser, which is required to be operational. This parameter can be used to change the command for how to display the help text if another browser than Netscape is preferred, or if another platform than Unix or Windows is used.

If set to a string Command, the command "Command File" is evaluated using os:cmd/1.

If set to a module-function-args tuple, {M,F,A}, the call apply(M,F,[File|A]) is evaluated.

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_dest = Value

Value is one of:

tty

Installs the standard handler, logger_std_h(3), with type set to standard_io. This is the default option.

{file, FileName}

Installs the standard handler, logger_std_h(3), with type set to {file, FileName}, where FileName is a string. The file is opened with encoding UTF-8.

{disk_log, FileName}

Installs the disk_log handler, logger_disk_log_h(3), with file set to FileName (a string), and possibly other disk_log parameters set by the environment variables logger_disk_log_type, logger_disk_log_maxfiles and logger_disk_log_maxbytes, see below. The file is opened with encoding UTF-8.

false

No standard handler is installed, but the initial, primitive handler is kept, printing raw event messages to tty.

silent

No standard handler is started, and the initial, primitive handler is removed.

logger_level = Level

Value = 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.

This configuration parameter is used both for the global logger level, and for the standard handler started by the Kernel application (see logger_dest variable above).

The default value is info

logger_disk_log_type = halt | wrap
logger_disk_log_maxfiles = integer()
logger_disk_log_maxbytes = integer()

If logger_dest is set to {disk_log,File}, then these parameters specify the configuration to use when opening the disk log file. They specify the type of disk log, the maximum number of files (if the type is wrap) and the maximum size of each file, respectively.

The default values are:

logger_disk_log_type = wrap
logger_disk_log_maxfiles = 10
logger_disk_log_maxbytes = 1048576
logger_sasl_compatible = boolean()

If this parameter is set to true, then the logger handler started by kernel will not log any progress-, crash-, or supervisor reports. If the SASL application is starated, these log events will be sent to a second handler instance named sasl_h, according to values of the SASL environment variables 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.

logger_log_progress = boolean()

If logger_sasl_compatible = false, then logger_log_progress specifies if progress reports from supervisor and application_controller shall be logged or not.

If logger_sasl_compatible = false, then logger_log_progress is ignored.

logger_format_depth = Depth

Can be used to limit the size of the formatted output from the logger handlers.

Note

This configuration parameter was introduced in OTP 18.1 and is experimental. Based on user feedback, it can be changed or improved in future releases, for example, to gain better control over how to limit the size of the formatted output. We have no plans to remove this new feature entirely, unless it turns out to be useless.

Depth is a positive integer representing the maximum depth to which terms are printed by the logger handlers included in OTP. This configuration parameter is used by the default formatter, logger_formatter(3), unless the formatter's depth parameter is explicitly set. (If you have implemented your own formatter, this configuration parameter has no effect on that.)

Depth is used as follows: Format strings received by the formatter are rewritten. The format controls ~p and ~w are replaced with ~P and ~W, respectively, and Depth is used as the depth parameter. For details, see io:format/2 in STDLIB.

Note

A reasonable starting value for Depth is 30. We recommend to test crashing various processes in your application, examine the logs from the crashes, and then increase or decrease the value.

logger_max_size = integer() | unlimited

This parameter specifies the maximum size (bytes) each log event can have when printed by the standard logger handler. If the resulting string after formatting an event is bigger than this, it will be truncated before printed to the handler's destination.

logger_utc = boolean()

If set to true, the default formatter will display all dates in Universal Coordinated Time.

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 error_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 MinT < T < 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.

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 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_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 OTP-21, a new API for logging was added to Erlang/OTP. The old error_logger event manager, and event handlers running on this manager, will still work, but they are not used by default.

The following application environment variables can still be set, but they will only be used if the corresponding new logger variables are not set.

error_logger
Replaced by logger_dest
error_logger_format_depth
Replaced by logger_format_depth

See Backwards compatibility with error_logger for more information.

See Also