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NAME

       kernel - 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

         * 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 the logger_std_h(3erl) and logger_disk_log_h(3erl) manual
       pages.

OS SIGNAL EVENT HANDLER

       Asynchronous OS signals may be subscribed to via the  Kernel  applications  event  manager
       (see OTP Design Principles and gen_event(3erl)) 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(5).

         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(3erl).

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

         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_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(7)
           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(3erl). 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(3erl).

         inet_default_listen_options = [{Opt, Val}]:
           Specifies default options for listen (and accept) sockets, see inet(3erl).

         {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(3erl).

         {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(3erl)). 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(3erl)). 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(3erl)) 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(3erl)). 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, 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.

SEE ALSO

       app(5),    application(3erl),    code(3erl),    disk_log(3erl),     erl_boot_server(3erl),
       erl_ddll(3erl),  file(3erl),  global(3erl),  global_group(3erl),  heart(3erl), inet(3erl),
       logger(3erl),   net_kernel(3erl),   os(3erl),   pg2(3erl),   rpc(3erl),   seq_trace(3erl),
       user(3erl), timer(3erl)