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<img alt="Erlang logo" src="../../../../doc/erlang-logo.png"><br><small><a href="index.html">Reference Manual</a><br><a href="release_notes.html">Release Notes</a><br><a href="../pdf/kernel-2.15.3.pdf">PDF</a><br><a href="../../../../doc/index.html">Top</a></small><p><strong>Kernel</strong><br><strong>Reference Manual</strong><br><small>Version 2.15.3</small></p>
<br><a href="javascript:openAllFlips()">Expand All</a><br><a href="javascript:closeAllFlips()">Contract All</a><p><small><strong>Table of Contents</strong></small></p>
<ul class="flipMenu">
<li title="kernel (App)"><a href="kernel_app.html">kernel (App)
</a></li>
<li id="loadscrollpos" title="application " expanded="true">application<ul>
<li><a href="application.html">
Top of manual page
</a></li>
<li title="get_all_env-0"><a href="application.html#get_all_env-0">get_all_env/0</a></li>
<li title="get_all_env-1"><a href="application.html#get_all_env-1">get_all_env/1</a></li>
<li title="get_all_key-0"><a href="application.html#get_all_key-0">get_all_key/0</a></li>
<li title="get_all_key-1"><a href="application.html#get_all_key-1">get_all_key/1</a></li>
<li title="get_application-0"><a href="application.html#get_application-0">get_application/0</a></li>
<li title="get_application-1"><a href="application.html#get_application-1">get_application/1</a></li>
<li title="get_env-1"><a href="application.html#get_env-1">get_env/1</a></li>
<li title="get_env-2"><a href="application.html#get_env-2">get_env/2</a></li>
<li title="get_key-1"><a href="application.html#get_key-1">get_key/1</a></li>
<li title="get_key-2"><a href="application.html#get_key-2">get_key/2</a></li>
<li title="load-1"><a href="application.html#load-1">load/1</a></li>
<li title="load-2"><a href="application.html#load-2">load/2</a></li>
<li title="loaded_applications-0"><a href="application.html#loaded_applications-0">loaded_applications/0</a></li>
<li title="permit-2"><a href="application.html#permit-2">permit/2</a></li>
<li title="set_env-3"><a href="application.html#set_env-3">set_env/3</a></li>
<li title="set_env-4"><a href="application.html#set_env-4">set_env/4</a></li>
<li title="start-1"><a href="application.html#start-1">start/1</a></li>
<li title="start-2"><a href="application.html#start-2">start/2</a></li>
<li title="start_type-0"><a href="application.html#start_type-0">start_type/0</a></li>
<li title="stop-1"><a href="application.html#stop-1">stop/1</a></li>
<li title="takeover-2"><a href="application.html#takeover-2">takeover/2</a></li>
<li title="unload-1"><a href="application.html#unload-1">unload/1</a></li>
<li title="unset_env-2"><a href="application.html#unset_env-2">unset_env/2</a></li>
<li title="unset_env-3"><a href="application.html#unset_env-3">unset_env/3</a></li>
<li title="which_applications-0"><a href="application.html#which_applications-0">which_applications/0</a></li>
<li title="which_applications-1"><a href="application.html#which_applications-1">which_applications/1</a></li>
<li title="Module:start-2"><a href="application.html#Module:start-2">Module:start/2</a></li>
<li title="Module:start_phase-3"><a href="application.html#Module:start_phase-3">Module:start_phase/3</a></li>
<li title="Module:prep_stop-1"><a href="application.html#Module:prep_stop-1">Module:prep_stop/1</a></li>
<li title="Module:stop-1"><a href="application.html#Module:stop-1">Module:stop/1</a></li>
<li title="Module:config_change-3"><a href="application.html#Module:config_change-3">Module:config_change/3</a></li>
</ul>
</li>
<li id="no" title="auth " expanded="false">auth<ul>
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<li title="is_auth-1"><a href="auth.html#is_auth-1">is_auth/1</a></li>
<li title="cookie-0"><a href="auth.html#cookie-0">cookie/0</a></li>
<li title="cookie-1"><a href="auth.html#cookie-1">cookie/1</a></li>
<li title="node_cookie-1"><a href="auth.html#node_cookie-1">node_cookie/1</a></li>
<li title="node_cookie-2"><a href="auth.html#node_cookie-2">node_cookie/2</a></li>
</ul>
</li>
<li id="no" title="code " expanded="false">code<ul>
<li><a href="code.html">
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</a></li>
<li title="set_path-1"><a href="code.html#set_path-1">set_path/1</a></li>
<li title="get_path-0"><a href="code.html#get_path-0">get_path/0</a></li>
<li title="add_path-1"><a href="code.html#add_path-1">add_path/1</a></li>
<li title="add_pathz-1"><a href="code.html#add_pathz-1">add_pathz/1</a></li>
<li title="add_patha-1"><a href="code.html#add_patha-1">add_patha/1</a></li>
<li title="add_paths-1"><a href="code.html#add_paths-1">add_paths/1</a></li>
<li title="add_pathsz-1"><a href="code.html#add_pathsz-1">add_pathsz/1</a></li>
<li title="add_pathsa-1"><a href="code.html#add_pathsa-1">add_pathsa/1</a></li>
<li title="del_path-1"><a href="code.html#del_path-1">del_path/1</a></li>
<li title="replace_path-2"><a href="code.html#replace_path-2">replace_path/2</a></li>
<li title="load_file-1"><a href="code.html#load_file-1">load_file/1</a></li>
<li title="load_abs-1"><a href="code.html#load_abs-1">load_abs/1</a></li>
<li title="ensure_loaded-1"><a href="code.html#ensure_loaded-1">ensure_loaded/1</a></li>
<li title="load_binary-3"><a href="code.html#load_binary-3">load_binary/3</a></li>
<li title="delete-1"><a href="code.html#delete-1">delete/1</a></li>
<li title="purge-1"><a href="code.html#purge-1">purge/1</a></li>
<li title="soft_purge-1"><a href="code.html#soft_purge-1">soft_purge/1</a></li>
<li title="is_loaded-1"><a href="code.html#is_loaded-1">is_loaded/1</a></li>
<li title="all_loaded-0"><a href="code.html#all_loaded-0">all_loaded/0</a></li>
<li title="which-1"><a href="code.html#which-1">which/1</a></li>
<li title="get_object_code-1"><a href="code.html#get_object_code-1">get_object_code/1</a></li>
<li title="root_dir-0"><a href="code.html#root_dir-0">root_dir/0</a></li>
<li title="lib_dir-0"><a href="code.html#lib_dir-0">lib_dir/0</a></li>
<li title="lib_dir-1"><a href="code.html#lib_dir-1">lib_dir/1</a></li>
<li title="lib_dir-2"><a href="code.html#lib_dir-2">lib_dir/2</a></li>
<li title="compiler_dir-0"><a href="code.html#compiler_dir-0">compiler_dir/0</a></li>
<li title="priv_dir-1"><a href="code.html#priv_dir-1">priv_dir/1</a></li>
<li title="objfile_extension-0"><a href="code.html#objfile_extension-0">objfile_extension/0</a></li>
<li title="stick_dir-1"><a href="code.html#stick_dir-1">stick_dir/1</a></li>
<li title="unstick_dir-1"><a href="code.html#unstick_dir-1">unstick_dir/1</a></li>
<li title="is_sticky-1"><a href="code.html#is_sticky-1">is_sticky/1</a></li>
<li title="rehash-0"><a href="code.html#rehash-0">rehash/0</a></li>
<li title="where_is_file-1"><a href="code.html#where_is_file-1">where_is_file/1</a></li>
<li title="clash-0"><a href="code.html#clash-0">clash/0</a></li>
<li title="is_module_native-1"><a href="code.html#is_module_native-1">is_module_native/1</a></li>
</ul>
</li>
<li id="no" title="disk_log " expanded="false">disk_log<ul>
<li><a href="disk_log.html">
Top of manual page
</a></li>
<li title="accessible_logs-0"><a href="disk_log.html#accessible_logs-0">accessible_logs/0</a></li>
<li title="alog-2"><a href="disk_log.html#alog-2">alog/2</a></li>
<li title="balog-2"><a href="disk_log.html#balog-2">balog/2</a></li>
<li title="alog_terms-2"><a href="disk_log.html#alog_terms-2">alog_terms/2</a></li>
<li title="balog_terms-2"><a href="disk_log.html#balog_terms-2">balog_terms/2</a></li>
<li title="block-1"><a href="disk_log.html#block-1">block/1</a></li>
<li title="block-2"><a href="disk_log.html#block-2">block/2</a></li>
<li title="change_header-2"><a href="disk_log.html#change_header-2">change_header/2</a></li>
<li title="change_notify-3"><a href="disk_log.html#change_notify-3">change_notify/3</a></li>
<li title="change_size-2"><a href="disk_log.html#change_size-2">change_size/2</a></li>
<li title="chunk-2"><a href="disk_log.html#chunk-2">chunk/2</a></li>
<li title="chunk-3"><a href="disk_log.html#chunk-3">chunk/3</a></li>
<li title="bchunk-2"><a href="disk_log.html#bchunk-2">bchunk/2</a></li>
<li title="bchunk-3"><a href="disk_log.html#bchunk-3">bchunk/3</a></li>
<li title="chunk_info-1"><a href="disk_log.html#chunk_info-1">chunk_info/1</a></li>
<li title="chunk_step-3"><a href="disk_log.html#chunk_step-3">chunk_step/3</a></li>
<li title="close-1"><a href="disk_log.html#close-1">close/1</a></li>
<li title="format_error-1"><a href="disk_log.html#format_error-1">format_error/1</a></li>
<li title="inc_wrap_file-1"><a href="disk_log.html#inc_wrap_file-1">inc_wrap_file/1</a></li>
<li title="info-1"><a href="disk_log.html#info-1">info/1</a></li>
<li title="lclose-1"><a href="disk_log.html#lclose-1">lclose/1</a></li>
<li title="lclose-2"><a href="disk_log.html#lclose-2">lclose/2</a></li>
<li title="log-2"><a href="disk_log.html#log-2">log/2</a></li>
<li title="blog-2"><a href="disk_log.html#blog-2">blog/2</a></li>
<li title="log_terms-2"><a href="disk_log.html#log_terms-2">log_terms/2</a></li>
<li title="blog_terms-2"><a href="disk_log.html#blog_terms-2">blog_terms/2</a></li>
<li title="open-1"><a href="disk_log.html#open-1">open/1</a></li>
<li title="pid2name-1"><a href="disk_log.html#pid2name-1">pid2name/1</a></li>
<li title="reopen-2"><a href="disk_log.html#reopen-2">reopen/2</a></li>
<li title="reopen-3"><a href="disk_log.html#reopen-3">reopen/3</a></li>
<li title="breopen-3"><a href="disk_log.html#breopen-3">breopen/3</a></li>
<li title="sync-1"><a href="disk_log.html#sync-1">sync/1</a></li>
<li title="truncate-1"><a href="disk_log.html#truncate-1">truncate/1</a></li>
<li title="truncate-2"><a href="disk_log.html#truncate-2">truncate/2</a></li>
<li title="btruncate-2"><a href="disk_log.html#btruncate-2">btruncate/2</a></li>
<li title="unblock-1"><a href="disk_log.html#unblock-1">unblock/1</a></li>
</ul>
</li>
<li id="no" title="erl_boot_server " expanded="false">erl_boot_server<ul>
<li><a href="erl_boot_server.html">
Top of manual page
</a></li>
<li title="start-1"><a href="erl_boot_server.html#start-1">start/1</a></li>
<li title="start_link-1"><a href="erl_boot_server.html#start_link-1">start_link/1</a></li>
<li title="add_slave-1"><a href="erl_boot_server.html#add_slave-1">add_slave/1</a></li>
<li title="delete_slave-1"><a href="erl_boot_server.html#delete_slave-1">delete_slave/1</a></li>
<li title="which_slaves-0"><a href="erl_boot_server.html#which_slaves-0">which_slaves/0</a></li>
</ul>
</li>
<li id="no" title="erl_ddll " expanded="false">erl_ddll<ul>
<li><a href="erl_ddll.html">
Top of manual page
</a></li>
<li title="demonitor-1"><a href="erl_ddll.html#demonitor-1">demonitor/1</a></li>
<li title="info-0"><a href="erl_ddll.html#info-0">info/0</a></li>
<li title="info-1"><a href="erl_ddll.html#info-1">info/1</a></li>
<li title="info-2"><a href="erl_ddll.html#info-2">info/2</a></li>
<li title="load-2"><a href="erl_ddll.html#load-2">load/2</a></li>
<li title="load_driver-2"><a href="erl_ddll.html#load_driver-2">load_driver/2</a></li>
<li title="monitor-2"><a href="erl_ddll.html#monitor-2">monitor/2</a></li>
<li title="reload-2"><a href="erl_ddll.html#reload-2">reload/2</a></li>
<li title="reload_driver-2"><a href="erl_ddll.html#reload_driver-2">reload_driver/2</a></li>
<li title="try_load-3"><a href="erl_ddll.html#try_load-3">try_load/3</a></li>
<li title="try_unload-2"><a href="erl_ddll.html#try_unload-2">try_unload/2</a></li>
<li title="unload-1"><a href="erl_ddll.html#unload-1">unload/1</a></li>
<li title="unload_driver-1"><a href="erl_ddll.html#unload_driver-1">unload_driver/1</a></li>
<li title="loaded_drivers-0"><a href="erl_ddll.html#loaded_drivers-0">loaded_drivers/0</a></li>
<li title="format_error-1"><a href="erl_ddll.html#format_error-1">format_error/1</a></li>
</ul>
</li>
<li title="erl_prim_loader"><a href="erl_prim_loader.html">erl_prim_loader</a></li>
<li title="erlang"><a href="erlang.html">erlang</a></li>
<li id="no" title="error_handler " expanded="false">error_handler<ul>
<li><a href="error_handler.html">
Top of manual page
</a></li>
<li title="undefined_function-3"><a href="error_handler.html#undefined_function-3">undefined_function/3</a></li>
<li title="undefined_lambda-3"><a href="error_handler.html#undefined_lambda-3">undefined_lambda/3</a></li>
</ul>
</li>
<li id="no" title="error_logger " expanded="false">error_logger<ul>
<li><a href="error_logger.html">
Top of manual page
</a></li>
<li title="error_msg-1"><a href="error_logger.html#error_msg-1">error_msg/1</a></li>
<li title="error_msg-2"><a href="error_logger.html#error_msg-2">error_msg/2</a></li>
<li title="format-2"><a href="error_logger.html#format-2">format/2</a></li>
<li title="error_report-1"><a href="error_logger.html#error_report-1">error_report/1</a></li>
<li title="error_report-2"><a href="error_logger.html#error_report-2">error_report/2</a></li>
<li title="warning_map-0"><a href="error_logger.html#warning_map-0">warning_map/0</a></li>
<li title="warning_msg-1"><a href="error_logger.html#warning_msg-1">warning_msg/1</a></li>
<li title="warning_msg-2"><a href="error_logger.html#warning_msg-2">warning_msg/2</a></li>
<li title="warning_report-1"><a href="error_logger.html#warning_report-1">warning_report/1</a></li>
<li title="warning_report-2"><a href="error_logger.html#warning_report-2">warning_report/2</a></li>
<li title="info_msg-1"><a href="error_logger.html#info_msg-1">info_msg/1</a></li>
<li title="info_msg-2"><a href="error_logger.html#info_msg-2">info_msg/2</a></li>
<li title="info_report-1"><a href="error_logger.html#info_report-1">info_report/1</a></li>
<li title="info_report-2"><a href="error_logger.html#info_report-2">info_report/2</a></li>
<li title="add_report_handler-1"><a href="error_logger.html#add_report_handler-1">add_report_handler/1</a></li>
<li title="add_report_handler-2"><a href="error_logger.html#add_report_handler-2">add_report_handler/2</a></li>
<li title="delete_report_handler-1"><a href="error_logger.html#delete_report_handler-1">delete_report_handler/1</a></li>
<li title="tty-1"><a href="error_logger.html#tty-1">tty/1</a></li>
<li title="logfile-1"><a href="error_logger.html#logfile-1">logfile/1</a></li>
</ul>
</li>
<li id="no" title="file " expanded="false">file<ul>
<li><a href="file.html">
Top of manual page
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<li title="advise-4"><a href="file.html#advise-4">advise/4</a></li>
<li title="change_group-2"><a href="file.html#change_group-2">change_group/2</a></li>
<li title="change_mode-2"><a href="file.html#change_mode-2">change_mode/2</a></li>
<li title="change_owner-2"><a href="file.html#change_owner-2">change_owner/2</a></li>
<li title="change_owner-3"><a href="file.html#change_owner-3">change_owner/3</a></li>
<li title="change_time-2"><a href="file.html#change_time-2">change_time/2</a></li>
<li title="change_time-3"><a href="file.html#change_time-3">change_time/3</a></li>
<li title="close-1"><a href="file.html#close-1">close/1</a></li>
<li title="consult-1"><a href="file.html#consult-1">consult/1</a></li>
<li title="copy-2"><a href="file.html#copy-2">copy/2</a></li>
<li title="copy-3"><a href="file.html#copy-3">copy/3</a></li>
<li title="del_dir-1"><a href="file.html#del_dir-1">del_dir/1</a></li>
<li title="delete-1"><a href="file.html#delete-1">delete/1</a></li>
<li title="eval-1"><a href="file.html#eval-1">eval/1</a></li>
<li title="eval-2"><a href="file.html#eval-2">eval/2</a></li>
<li title="file_info-1"><a href="file.html#file_info-1">file_info/1</a></li>
<li title="format_error-1"><a href="file.html#format_error-1">format_error/1</a></li>
<li title="get_cwd-0"><a href="file.html#get_cwd-0">get_cwd/0</a></li>
<li title="get_cwd-1"><a href="file.html#get_cwd-1">get_cwd/1</a></li>
<li title="list_dir-1"><a href="file.html#list_dir-1">list_dir/1</a></li>
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<li title="make_link-2"><a href="file.html#make_link-2">make_link/2</a></li>
<li title="make_symlink-2"><a href="file.html#make_symlink-2">make_symlink/2</a></li>
<li title="native_name_encoding-0"><a href="file.html#native_name_encoding-0">native_name_encoding/0</a></li>
<li title="open-2"><a href="file.html#open-2">open/2</a></li>
<li title="path_consult-2"><a href="file.html#path_consult-2">path_consult/2</a></li>
<li title="path_eval-2"><a href="file.html#path_eval-2">path_eval/2</a></li>
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<li title="position-2"><a href="file.html#position-2">position/2</a></li>
<li title="pread-2"><a href="file.html#pread-2">pread/2</a></li>
<li title="pread-3"><a href="file.html#pread-3">pread/3</a></li>
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<li title="read-2"><a href="file.html#read-2">read/2</a></li>
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<div id="content">
<div class="innertube">
<!-- refpage --><center><h1>application</h1></center>
<h3>MODULE</h3>
<div class="REFBODY">application</div>
<h3>MODULE SUMMARY</h3>
<div class="REFBODY">Generic OTP application functions</div>
<h3>DESCRIPTION</h3>
<div class="REFBODY"><p>
<p>In OTP, <strong>application</strong> denotes a component implementing
some specific functionality, that can be started and stopped as a
unit, and which can be re-used in other systems as well. This
module interfaces the <strong>application controller</strong>, a process
started at every Erlang runtime system, and contains functions
for controlling applications (for example starting and stopping
applications), and functions to access information about
applications (for example configuration parameters).</p>
<p>An application is defined by an <strong>application specification</strong>. The specification is normally located in an
<strong>application resource file</strong> called <span class="code">Application.app</span>,
where <span class="code">Application</span> is the name of the application. Refer to
<span class="bold_code"><a href="app.html">app(4)</a></span> for more information about
the application specification.</p>
<p>This module can also be viewed as a behaviour for an application
implemented according to the OTP design principles as a
supervision tree. The definition of how to start and stop
the tree should be located in an <strong>application callback module</strong> exporting a pre-defined set of functions.</p>
<p>Refer to <span class="bold_code"><a href="javascript:erlhref('../../../../doc/../','doc/design_principles','des_princ.html');">OTP Design Principles</a></span> for more information about
applications and behaviours.</p>
</p></div>
<h3>DATA TYPES</h3>
<p>
<span class="bold_code"><a name="type-start_type">start_type()</a> = normal<br> | {takeover, Node :: node()}<br> | {failover, Node :: node()}</span><br></p>
<p>
<span class="bold_code"><a name="type-restart_type">restart_type()</a> = permanent | transient | temporary</span><br></p>
<p><span class="bold_code"><a name="type-tuple_of">tuple_of(T)</a></span></p>
<div class="REFBODY"><p><p>A tuple where the elements are of type <span class="code">T</span>.</p></p></div>
<h3>EXPORTS</h3>
<p><a name="get_all_env-0"></a><span class="bold_code">get_all_env() -> Env</span><br><a name="get_all_env-1"></a><span class="bold_code">get_all_env(Application) -> Env</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Application = atom()</span></div>
<div class="REFTYPES"><span class="bold_code">Env = [{Par :: atom(), Val :: term()}]</span></div>
</div></p>
<div class="REFBODY"><p>
<p>Returns the configuration parameters and their values for
<span class="code">Application</span>. If the argument is omitted, it defaults to
the application of the calling process.</p>
<p>If the specified application is not loaded, or if the process
executing the call does not belong to any application,
the function returns <span class="code">[]</span>.</p>
</p></div>
<p><a name="get_all_key-0"></a><span class="bold_code">get_all_key() -> [] | {ok, Keys}</span><br><a name="get_all_key-1"></a><span class="bold_code">get_all_key(Application) -> undefined | Keys</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Application = atom()</span></div>
<div class="REFTYPES"><span class="bold_code">Keys = {ok, [{Key :: atom(), Val :: term()}, ...]}</span></div>
</div></p>
<div class="REFBODY"><p>
<p>Returns the application specification keys and their values
for <span class="code">Application</span>. If the argument is omitted, it
defaults to the application of the calling process.</p>
<p>If the specified application is not loaded, the function
returns <span class="code">undefined</span>. If the process executing the call
does not belong to any application, the function returns
<span class="code">[]</span>.</p>
</p></div>
<p><a name="get_application-0"></a><span class="bold_code">get_application() -> undefined | {ok, Application}</span><br><a name="get_application-1"></a><span class="bold_code">get_application(PidOrModule) -> undefined | {ok, Application}</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">PidOrModule = (Pid :: pid()) | (Module :: module())</span></div>
<div class="REFTYPES"><span class="bold_code">Application = atom()</span></div>
</div></p>
<div class="REFBODY"><p>
<p>Returns the name of the application to which the process
<span class="code">Pid</span> or the module <span class="code">Module</span> belongs. Providing no
argument is the same as calling
<span class="code">get_application(self())</span>.</p>
<p>If the specified process does not belong to any application,
or if the specified process or module does not exist,
the function returns <span class="code">undefined</span>.</p>
</p></div>
<p><a name="get_env-1"></a><span class="bold_code">get_env(Par) -> undefined | {ok, Val}</span><br><a name="get_env-2"></a><span class="bold_code">get_env(Application, Par) -> undefined | {ok, Val}</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Application = Par = atom()</span></div>
<div class="REFTYPES"><span class="bold_code"></span></div>
<div class="REFTYPES"><span class="bold_code">Val = term()</span></div>
</div></p>
<div class="REFBODY"><p>
<p>Returns the value of the configuration parameter <span class="code">Par</span>
for <span class="code">Application</span>. If the application argument is
omitted, it defaults to the application of the calling
process.</p>
<p>If the specified application is not loaded, or
the configuration parameter does not exist, or if the process
executing the call does not belong to any application,
the function returns <span class="code">undefined</span>.</p>
</p></div>
<p><a name="get_key-1"></a><span class="bold_code">get_key(Key) -> undefined | {ok, Val}</span><br><a name="get_key-2"></a><span class="bold_code">get_key(Application, Key) -> undefined | {ok, Val}</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Application = Key = atom()</span></div>
<div class="REFTYPES"><span class="bold_code"></span></div>
<div class="REFTYPES"><span class="bold_code">Val = term()</span></div>
</div></p>
<div class="REFBODY"><p>
<p>Returns the value of the application specification key
<span class="code">Key</span> for <span class="code">Application</span>. If the application
argument is omitted, it defaults to the application of
the calling process.</p>
<p>If the specified application is not loaded, or
the specification key does not exist, or if the process
executing the call does not belong to any application,
the function returns <span class="code">undefined</span>.</p>
</p></div>
<p><a name="load-1"></a><span class="bold_code">load(AppDescr) -> ok | {error, Reason}</span><br><a name="load-2"></a><span class="bold_code">load(AppDescr, Distributed) -> ok | {error, Reason}</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">AppDescr = Application | (AppSpec :: application_spec())</span></div>
<div class="REFTYPES"><span class="bold_code">Application = atom()</span></div>
<div class="REFTYPES"><span class="bold_code">Distributed = {Application, Nodes}<br> | {Application, Time, Nodes}<br> | default</span></div>
<div class="REFTYPES"><span class="bold_code">Nodes = [node() | <span class="bold_code"><a href="#type-tuple_of">tuple_of</a></span>(node())]</span></div>
<div class="REFTYPES"><span class="bold_code">Time = integer() >= 1</span></div>
<div class="REFTYPES"><span class="bold_code">Reason = term()</span></div>
<div class="REFTYPES">
<span class="bold_code">application_spec() = <br> {application,<br> Application :: atom(),<br> AppSpecKeys :: [application_opt()]}</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">application_opt() = {description, Description :: string()}<br> | {vsn, Vsn :: string()}<br> | {id, Id :: string()}<br> | {modules,<br> [(Module :: module()) |<br> {Module :: module(), Version :: term()}]}<br> | {registered, Names :: [Name :: atom()]}<br> | {applications, [Application :: atom()]}<br> | {included_applications,<br> [Application :: atom()]}<br> | {env, [{Par :: atom(), Val :: term()}]}<br> | {start_phases,<br> [{Phase :: atom(), PhaseArgs :: term()}] |<br> undefined}<br> | {maxT, MaxT :: timeout()}<br> | {maxP, MaxP :: integer() >= 1 | infinity}<br> | {mod,<br> Start ::<br> {Module :: module(),<br> StartArgs :: term()}}</span><br>
</div>
</div></p>
<div class="REFBODY"><p>
<p>Loads the application specification for an application into
the application controller. It will also load the application
specifications for any included applications. Note that
the function does not load the actual Erlang object code.</p>
<p>The application can be given by its name <span class="code">Application</span>.
In this case the application controller will search the code
path for the application resource file <span class="code">Application.app</span>
and load the specification it contains.</p>
<p>The application specification can also be given directly as a
tuple <span class="code">AppSpec</span>. This tuple should have the format and
contents as described in <span class="code">app(4)</span>.</p>
<p>If <span class="code">Distributed == {Application,[Time,]Nodes}</span>,
the application will be distributed. The argument overrides
the value for the application in the Kernel configuration
parameter <span class="code">distributed</span>. <span class="code">Application</span> must be
the name of the application (same as in the first argument).
If a node crashes and <span class="code">Time</span> has been specified, then
the application controller will wait for <span class="code">Time</span>
milliseconds before attempting to restart the application on
another node. If <span class="code">Time</span> is not specified, it will
default to 0 and the application will be restarted
immediately.</p>
<p><span class="code">Nodes</span> is a list of node names where the application
may run, in priority from left to right. Node names can be
grouped using tuples to indicate that they have the same
priority. Example:</p>
<div class="example"><pre>
Nodes = [cp1@cave, {cp2@cave, cp3@cave}]</pre></div>
<p>This means that the application should preferably be started
at <span class="code">cp1@cave</span>. If <span class="code">cp1@cave</span> is down,
the application should be started at either <span class="code">cp2@cave</span>
or <span class="code">cp3@cave</span>.</p>
<p>If <span class="code">Distributed == default</span>, the value for
the application in the Kernel configuration parameter
<span class="code">distributed</span> will be used.</p>
</p></div>
<p><a name="loaded_applications-0"></a><span class="bold_code">loaded_applications() -> [{Application, Description, Vsn}]</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Application = atom()</span></div>
<div class="REFTYPES"><span class="bold_code">Description = Vsn = string()</span></div>
<div class="REFTYPES"><span class="bold_code"></span></div>
</div></p>
<div class="REFBODY"><p>
<p>Returns a list with information about the applications which
have been loaded using <span class="code">load/1,2</span>, also included
applications. <span class="code">Application</span> is the application name.
<span class="code">Description</span> and <span class="code">Vsn</span> are the values of its
<span class="code">description</span> and <span class="code">vsn</span> application specification
keys, respectively.</p>
</p></div>
<p><a name="permit-2"></a><span class="bold_code">permit(Application, Permission) -> ok | {error, Reason}</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Application = atom()</span></div>
<div class="REFTYPES"><span class="bold_code">Permission = boolean()</span></div>
<div class="REFTYPES"><span class="bold_code">Reason = term()</span></div>
</div></p>
<div class="REFBODY"><p>
<p>Changes the permission for <span class="code">Application</span> to run at
the current node. The application must have been loaded using
<span class="code">load/1,2</span> for the function to have effect.</p>
<p>If the permission of a loaded, but not started, application
is set to <span class="code">false</span>, <span class="code">start</span> will return <span class="code">ok</span> but
the application will not be started until the permission is
set to <span class="code">true</span>.</p>
<p>If the permission of a running application is set to
<span class="code">false</span>, the application will be stopped. If
the permission later is set to <span class="code">true</span>, it will be
restarted.</p>
<p>If the application is distributed, setting the permission to
<span class="code">false</span> means that the application will be started at, or
moved to, another node according to how its distribution is
configured (see <span class="code">load/2</span> above).</p>
<p>The function does not return until the application is
started, stopped or successfully moved to another node.
However, in some cases where permission is set to <span class="code">true</span>
the function may return <span class="code">ok</span> even though the application
itself has not started. This is true when an application
cannot start because it has dependencies to other
applications which have not yet been started. When they have
been started, <span class="code">Application</span> will be started as well.</p>
<p>By default, all applications are loaded with permission
<span class="code">true</span> on all nodes. The permission is configurable by
using the Kernel configuration parameter <span class="code">permissions</span>.</p>
</p></div>
<p><a name="set_env-3"></a><span class="bold_code">set_env(Application, Par, Val) -> ok</span><br><a name="set_env-4"></a><span class="bold_code">set_env(Application, Par, Val, Timeout) -> ok</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Application = Par = atom()</span></div>
<div class="REFTYPES"><span class="bold_code"></span></div>
<div class="REFTYPES"><span class="bold_code">Val = term()</span></div>
<div class="REFTYPES"><span class="bold_code">Timeout = timeout()</span></div>
</div></p>
<div class="REFBODY"><p>
<p>Sets the value of the configuration parameter <span class="code">Par</span> for
<span class="code">Application</span>.</p>
<p><span class="code">set_env/3</span> uses the standard <span class="code">gen_server</span> timeout
value (5000 ms). A <span class="code">Timeout</span> argument can be provided
if another timeout value is useful, for example, in situations
where the application controller is heavily loaded.</p>
<div class="warning">
<div class="label">Warning</div>
<div class="content"><p>
<p>Use this function only if you know what you are doing,
that is, on your own applications. It is very application
and configuration parameter dependent when and how often
the value is read by the application, and careless use
of this function may put the application in a
weird, inconsistent, and malfunctioning state. </p>
</p></div>
</div>
</p></div>
<p><a name="start-1"></a><span class="bold_code">start(Application) -> ok | {error, Reason}</span><br><a name="start-2"></a><span class="bold_code">start(Application, Type) -> ok | {error, Reason}</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Application = atom()</span></div>
<div class="REFTYPES"><span class="bold_code">Type = <span class="bold_code"><a href="#type-restart_type">restart_type()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">Reason = term()</span></div>
</div></p>
<div class="REFBODY"><p>
<p>Starts <span class="code">Application</span>. If it is not loaded,
the application controller will first load it using
<span class="code">load/1</span>. It will make sure any included applications
are loaded, but will not start them. That is assumed to be
taken care of in the code for <span class="code">Application</span>.</p>
<p>The application controller checks the value of
the application specification key <span class="code">applications</span>, to
ensure that all applications that should be started before
this application are running. If not,
<span class="code">{error,{not_started,App}}</span> is returned, where <span class="code">App</span>
is the name of the missing application.</p>
<p>The application controller then creates an <strong>application master</strong> for the application. The application master is
the group leader of all the processes in the application.
The application master starts the application by calling
the application callback function <span class="code">Module:start/2</span> as
defined by the application specification key <span class="code">mod</span>.</p>
<p>The <span class="code">Type</span> argument specifies the type of
the application. If omitted, it defaults to <span class="code">temporary</span>.</p>
<ul>
<li>If a permanent application terminates, all other
applications and the entire Erlang node are also terminated.</li>
<li>If a transient application terminates with <span class="code">Reason == normal</span>, this is reported but no other applications are
terminated. If a transient application terminates
abnormally, all other applications and the entire Erlang
node are also terminated.</li>
<li>If a temporary application terminates, this is reported
but no other applications are terminated.</li>
</ul>
<p>Note that it is always possible to stop an application
explicitly by calling <span class="code">stop/1</span>. Regardless of the type of
the application, no other applications will be affected.</p>
<p>Note also that the transient type is of little practical use,
since when a supervision tree terminates, the reason is set to
<span class="code">shutdown</span>, not <span class="code">normal</span>.</p>
</p></div>
<p><a name="start_type-0"></a><span class="bold_code">start_type() -> StartType | undefined | local</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">StartType = <span class="bold_code"><a href="#type-start_type">start_type()</a></span></span></div>
</div></p>
<div class="REFBODY"><p>
<p>This function is intended to be called by a process belonging
to an application, when the application is being started, to
determine the start type which is either <span class="code">StartType</span> or
<span class="code">local</span>.</p>
<p>See <span class="bold_code"><a href="#start_type"><span class="code">Module:start/2</span></a></span> for a description of
<span class="code">StartType</span>.</p>
<p><span class="code">local</span> is returned if only parts of the application is
being restarted (by a supervisor), or if the function is
called outside a startup.</p>
<p>If the process executing the call does not belong to any
application, the function returns <span class="code">undefined</span>.</p>
</p></div>
<p><a name="stop-1"></a><span class="bold_code">stop(Application) -> ok | {error, Reason}</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Application = atom()</span></div>
<div class="REFTYPES"><span class="bold_code">Reason = term()</span></div>
</div></p>
<div class="REFBODY"><p>
<p>Stops <span class="code">Application</span>. The application master calls
<span class="code">Module:prep_stop/1</span>, if such a function is defined, and
then tells the top supervisor of the application to shutdown
(see <span class="code">supervisor(3)</span>). This means that the entire
supervision tree, including included applications, is
terminated in reversed start order. After the shutdown,
the application master calls <span class="code">Module:stop/1</span>.
<span class="code">Module</span> is the callback module as defined by
the application specification key <span class="code">mod</span>.</p>
<p>Last, the application master itself terminates. Note that all
processes with the application master as group leader, i.e.
processes spawned from a process belonging to the application,
thus are terminated as well.</p>
<p>When stopped, the application is still loaded.</p>
<p>In order to stop a distributed application, <span class="code">stop/1</span>
has to be called on all nodes where it can execute (that is,
on all nodes where it has been started). The call to
<span class="code">stop/1</span> on the node where the application currently
executes will stop its execution. The application will not be
moved between nodes due to <span class="code">stop/1</span> being called on
the node where the application currently executes before
<span class="code">stop/1</span> is called on the other nodes.</p>
</p></div>
<p><a name="takeover-2"></a><span class="bold_code">takeover(Application, Type) -> ok | {error, Reason}</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Application = atom()</span></div>
<div class="REFTYPES"><span class="bold_code">Type = <span class="bold_code"><a href="#type-restart_type">restart_type()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code">Reason = term()</span></div>
</div></p>
<div class="REFBODY"><p>
<p>Performs a takeover of the distributed application
<span class="code">Application</span>, which executes at another node
<span class="code">Node</span>. At the current node, the application is
restarted by calling
<span class="code">Module:start({takeover,Node},StartArgs)</span>. <span class="code">Module</span>
and <span class="code">StartArgs</span> are retrieved from the loaded application
specification. The application at the other node is not
stopped until the startup is completed, i.e. when
<span class="code">Module:start/2</span> and any calls to
<span class="code">Module:start_phase/3</span> have returned.</p>
<p>Thus two instances of the application will run simultaneously
during the takeover, which makes it possible to transfer data
from the old to the new instance. If this is not acceptable
behavior, parts of the old instance may be shut down when
the new instance is started. Note that the application may
not be stopped entirely however, at least the top supervisor
must remain alive.</p>
<p>See <span class="code">start/1,2</span> for a description of <span class="code">Type</span>.</p>
</p></div>
<p><a name="unload-1"></a><span class="bold_code">unload(Application) -> ok | {error, Reason}</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Application = atom()</span></div>
<div class="REFTYPES"><span class="bold_code">Reason = term()</span></div>
</div></p>
<div class="REFBODY"><p>
<p>Unloads the application specification for <span class="code">Application</span>
from the application controller. It will also unload
the application specifications for any included applications.
Note that the function does not purge the actual Erlang
object code.</p>
</p></div>
<p><a name="unset_env-2"></a><span class="bold_code">unset_env(Application, Par) -> ok</span><br><a name="unset_env-3"></a><span class="bold_code">unset_env(Application, Par, Timeout) -> ok</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Application = Par = atom()</span></div>
<div class="REFTYPES"><span class="bold_code"></span></div>
<div class="REFTYPES"><span class="bold_code">Timeout = timeout()</span></div>
</div></p>
<div class="REFBODY"><p>
<p>Removes the configuration parameter <span class="code">Par</span> and its value
for <span class="code">Application</span>.</p>
<p><span class="code">unset_env/2</span> uses the standard <span class="code">gen_server</span>
timeout value (5000 ms). A <span class="code">Timeout</span> argument can be
provided if another timeout value is useful, for example, in
situations where the application controller is heavily loaded.</p>
<div class="warning">
<div class="label">Warning</div>
<div class="content"><p>
<p>Use this function only if you know what you are doing,
that is, on your own applications. It is very application
and configuration parameter dependent when and how often
the value is read by the application, and careless use
of this function may put the application in a
weird, inconsistent, and malfunctioning state. </p>
</p></div>
</div>
</p></div>
<p><a name="which_applications-0"></a><span class="bold_code">which_applications() -> [{Application, Description, Vsn}]</span><br><a name="which_applications-1"></a><span class="bold_code">which_applications(Timeout) -> [{Application, Description, Vsn}]</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Timeout = timeout()</span></div>
<div class="REFTYPES"><span class="bold_code">Application = atom()</span></div>
<div class="REFTYPES"><span class="bold_code">Description = Vsn = string()</span></div>
<div class="REFTYPES"><span class="bold_code"></span></div>
</div></p>
<div class="REFBODY"><p>
<p>Returns a list with information about the applications which
are currently running. <span class="code">Application</span> is the application
name. <span class="code">Description</span> and <span class="code">Vsn</span> are the values of its
<span class="code">description</span> and <span class="code">vsn</span> application specification
keys, respectively.</p>
<p><span class="code">which_applications/0</span> uses the standard
<span class="code">gen_server</span> timeout value (5000 ms). A <span class="code">Timeout</span>
argument can be provided if another timeout value is useful,
for example, in situations where the application controller
is heavily loaded.</p>
</p></div>
<h3><a name="id100888">CALLBACK MODULE</a></h3>
<div class="REFBODY">
<p>The following functions should be exported from an
<span class="code">application</span> callback module.</p>
</div>
<h3>EXPORTS</h3>
<p><a name="Module:start-2"><span class="bold_code">Module:start(StartType, StartArgs) -> {ok, Pid} | {ok, Pid, State} | {error, Reason}</span></a><br></p>
<div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES">
<span class="bold_code">StartType = normal | {takeover,Node} | {failover,Node}</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code"> Node = node()</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">StartArgs = term()</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Pid = pid()</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">State = term()</span><br>
</div>
</div>
<div class="REFBODY"><p>
<p>This function is called whenever an application is started
using <span class="code">application:start/1,2</span>, and should start
the processes of the application. If the application is
structured according to the OTP design principles as a
supervision tree, this means starting the top supervisor of
the tree.</p>
<p><a name="start_type"></a><span class="code">StartType</span> defines the type of start:</p>
<ul>
<li>
<span class="code">normal</span> if it's a normal startup.</li>
<li>
<span class="code">normal</span> also if the application is distributed and
started at the current node due to a failover from another
node, and the application specification key <span class="code">start_phases == undefined</span>.</li>
<li>
<span class="code">{takeover,Node}</span> if the application is
distributed and started at the current node due to a
takeover from <span class="code">Node</span>, either because
<span class="code">application:takeover/2</span> has been called or because
the current node has higher priority than <span class="code">Node</span>.</li>
<li>
<span class="code">{failover,Node}</span> if the application is
distributed and started at the current node due to a
failover from <span class="code">Node</span>, and the application
specification key <span class="code">start_phases /= undefined</span>.</li>
</ul>
<p><span class="code">StartArgs</span> is the <span class="code">StartArgs</span> argument defined by
the application specification key <span class="code">mod</span>.</p>
<p>The function should return <span class="code">{ok,Pid}</span> or
<span class="code">{ok,Pid,State}</span> where <span class="code">Pid</span> is the pid of the top
supervisor and <span class="code">State</span> is any term. If omitted,
<span class="code">State</span> defaults to <span class="code">[]</span>. If later the application
is stopped, <span class="code">State</span> is passed to
<span class="code">Module:prep_stop/1</span>.</p>
</p></div>
<p><a name="Module:start_phase-3"><span class="bold_code">Module:start_phase(Phase, StartType, PhaseArgs) -> ok | {error, Reason}</span></a><br></p>
<div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES">
<span class="bold_code">Phase = atom()</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">StartType = <span class="bold_code"><a href="#type-start_type">start_type()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">PhaseArgs = term()</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Pid = pid()</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">State = state()</span><br>
</div>
</div>
<div class="REFBODY"><p>
<p>This function is used to start an application with included
applications, when there is a need for synchronization between
processes in the different applications during startup.</p>
<p>The start phases is defined by the application specification
key <span class="code">start_phases == [{Phase,PhaseArgs}]</span>. For included
applications, the set of phases must be a subset of the set of
phases defined for the including application.</p>
<p>The function is called for each start phase (as defined for
the primary application) for the primary application and all
included applications, for which the start phase is defined.</p>
<p>See <span class="code">Module:start/2</span> for a description of
<span class="code">StartType</span>.</p>
</p></div>
<p><a name="Module:prep_stop-1"><span class="bold_code">Module:prep_stop(State) -> NewState</span></a><br></p>
<div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES">
<span class="bold_code">State = NewState = term()</span><br>
</div>
</div>
<div class="REFBODY"><p>
<p>This function is called when an application is about to be
stopped, before shutting down the processes of
the application.</p>
<p><span class="code">State</span> is the state returned from
<span class="code">Module:start/2</span>, or <span class="code">[]</span> if no state was returned.
<span class="code">NewState</span> is any term and will be passed to
<span class="code">Module:stop/1</span>.</p>
<p>The function is optional. If it is not defined, the processes
will be terminated and then <span class="code">Module:stop(State)</span> is
called.</p>
</p></div>
<p><a name="Module:stop-1"><span class="bold_code">Module:stop(State)</span></a><br></p>
<div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES">
<span class="bold_code">State = term()</span><br>
</div>
</div>
<div class="REFBODY"><p>
<p>This function is called whenever an application has stopped.
It is intended to be the opposite of <span class="code">Module:start/2</span>
and should do any necessary cleaning up. The return value is
ignored.</p>
<p><span class="code">State</span> is the return value of
<span class="code">Module:prep_stop/1</span>, if such a function exists.
Otherwise <span class="code">State</span> is taken from the return value of
<span class="code">Module:start/2</span>.</p>
</p></div>
<p><a name="Module:config_change-3"><span class="bold_code">Module:config_change(Changed, New, Removed) -> ok</span></a><br></p>
<div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES">
<span class="bold_code">Changed = [{Par,Val}]</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">New = [{Par,Val}]</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Removed = [Par]</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code"> Par = atom()</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code"> Val = term()</span><br>
</div>
</div>
<div class="REFBODY"><p>
<p>This function is called by an application after a code
replacement, if there are any changes to the configuration
parameters.</p>
<p><span class="code">Changed</span> is a list of parameter-value tuples with all
configuration parameters with changed values, <span class="code">New</span> is
a list of parameter-value tuples with all configuration
parameters that have been added, and <span class="code">Removed</span> is a list
of all parameters that have been removed.</p>
</p></div>
<h3><a name="id101309">SEE ALSO</a></h3>
<div class="REFBODY">
<p><span class="bold_code"><a href="javascript:erlhref('../../../../doc/../','doc/design_principles','des_princ.html');">OTP Design Principles</a></span>,
<span class="bold_code"><a href="kernel_app.html">kernel(6)</a></span>,
<span class="bold_code"><a href="app.html">app(4)</a></span></p>
</div>
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