path: root/lib/kernel/doc/src/code.xml

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<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2018</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
    
    </legalnotice>

    <title>code</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module since="">code</module>
  <modulesummary>Erlang code server.</modulesummary>
  <description>
    <p>This module contains the interface to the Erlang
      <em>code server</em>, which deals with the loading of compiled
      code into a running Erlang runtime system.</p>
    <p>The runtime system can be started in <em>interactive</em> or
      <em>embedded</em> mode. Which one is decided by the command-line
      flag <c>-mode</c>:</p>
    <pre>
% <input>erl -mode interactive</input></pre>
    <p>The modes are as follows:</p>
    <list type="bulleted">
      <item>
        <p>In interactive mode, which is default, only some code is loaded
          during system startup, basically the modules needed by the runtime
          system. Other code is dynamically loaded when first
          referenced. When a call to a function in a certain module is
          made, and the module is not loaded, the code server searches
          for and tries to load the module.</p>
      </item>
      <item>
        <p>In embedded mode, modules are not auto loaded. Trying to use
        a module that has not been loaded results in an error. This mode is
        recommended when the boot script loads all modules, as it is
        typically done in OTP releases. (Code can still be loaded later
        by explicitly ordering the code server to do so).</p>
      </item>
    </list>
    <p>To prevent accidentally reloading of modules affecting the Erlang
      runtime system, directories <c>kernel</c>, <c>stdlib</c>,
      and <c>compiler</c> are considered <em>sticky</em>. This
      means that the system issues a warning and rejects the request if
      a user tries to reload a module residing in any of them.
      The feature can be disabled by using command-line flag
      <c>-nostick</c>.</p>
  </description>

  <section>
    <title>Code Path</title>
    <p>In interactive mode, the code server maintains a search path,
      usually called the <em>code path</em>, consisting of a list of
      directories, which it searches sequentially when trying to load a
      module.</p>
    <p>Initially, the code path consists of the current working
      directory and all Erlang object code directories under library
      directory <c>$OTPROOT/lib</c>, where <c>$OTPROOT</c> is
      the installation directory of Erlang/OTP, <c>code:root_dir()</c>.
      Directories can be named <c>Name[-Vsn]</c> and the code server,
      by default, chooses the directory with the highest version number
      among those having the same <c>Name</c>. Suffix <c>-Vsn</c>
      is optional. If an <c>ebin</c> directory exists under
      <c>Name[-Vsn]</c>, this directory is added to the code path.</p>
    <p>Environment variable <c>ERL_LIBS</c> (defined in the operating
      system) can be used to define more library directories to
      be handled in the same way as the standard OTP library
      directory described above, except that directories without
      an <c>ebin</c> directory are ignored.</p>
    <p>All application directories found in the additional directories
      appears before the standard OTP applications, except for the
      Kernel and STDLIB applications, which are placed before
      any additional applications. In other words, modules found in any
      of the additional library directories override modules with
      the same name in OTP, except for modules in Kernel and
      STDLIB.</p>
    <p>Environment variable <c>ERL_LIBS</c> (if defined) is to contain
      a colon-separated (for Unix-like systems) or semicolon-separated
      (for Windows) list of additional libraries.</p>
      <p><em>Example:</em></p>
    <p>On a Unix-like system, <c>ERL_LIBS</c> can be set to the following</p>
      <code>
/usr/local/jungerl:/home/some_user/my_erlang_lib</code>
    <p>On Windows, use semi-colon as separator.</p>
  </section>

  <section>
    <title>Loading of Code From Archive Files</title>

    <warning><p>The support for loading code from archive files is
     experimental. The purpose of releasing it before it is ready
     is to obtain early feedback. The file format, semantics,
     interfaces, and so on, can be changed in a future release. The function
     <seealso marker="#lib_dir/2"><c>lib_dir/2</c></seealso>
     and  flag <c>-code_path_choice</c> are also
     experimental.</p></warning>

    <p>The Erlang archives are <c>ZIP</c>
     files with extension <c>.ez</c>. Erlang archives can also be
     enclosed in <c>escript</c> files whose file extension is arbitrary.</p>

    <p>Erlang archive files can contain entire Erlang applications or
     parts of applications. The structure in an archive file is the
     same as the directory structure for an application. If you, for
     example, create an archive of <c>mnesia-4.4.7</c>, the
     archive file must be named <c>mnesia-4.4.7.ez</c> and it must
     contain a top directory named <c>mnesia-4.4.7</c>. If the
     version part of the name is omitted, it must also be omitted in
     the archive. That is, a <c>mnesia.ez</c> archive must contain a
     <c>mnesia</c> top directory.</p>

    <p>An archive file for an application can, for example, be
     created like this:</p>

<pre>
zip:create("mnesia-4.4.7.ez",
	["mnesia-4.4.7"],
	[{cwd, code:lib_dir()},
	 {compress, all},
	 {uncompress,[".beam",".app"]}]).</pre>

    <p>Any file in the archive can be compressed, but to
     speed up the access of frequently read files, it can be a good
     idea to store <c>beam</c> and <c>app</c> files uncompressed in
     the archive.</p>

    <p>Normally the top directory of an application is located
     in library directory <c>$OTPROOT/lib</c> or in a directory
     referred to by environment variable <c>ERL_LIBS</c>. At
     startup, when the initial code path is computed, the code server
     also looks for archive files in these directories and
     possibly adds <c>ebin</c> directories in archives to the code path. The
     code path then contains paths to directories that look like
     <c>$OTPROOT/lib/mnesia.ez/mnesia/ebin</c> or
     <c>$OTPROOT/lib/mnesia-4.4.7.ez/mnesia-4.4.7/ebin</c>.</p>

    <p>The code server uses module <c>erl_prim_loader</c> in ERTS
     (possibly through <c>erl_boot_server</c>) to read code files from
     archives. However, the functions in <c>erl_prim_loader</c> can also be
     used by other applications to read files from archives. For
     example, the call
     <c>erl_prim_loader:list_dir( "/otp/root/lib/mnesia-4.4.7.ez/mnesia-4.4.7/examples/bench)"</c>
     would list the contents of a directory inside an archive.
     See <seealso marker="erts:erl_prim_loader"><c>erl_prim_loader(3)</c></seealso>.</p>

    <p>An application archive file and a regular application directory
     can coexist. This can be useful when it is needed to have
     parts of the application as regular files. A typical case is the
     <c>priv</c> directory, which must reside as a regular directory
     to link in drivers dynamically and start port programs.
     For other applications that do not need this, directory
     <c>priv</c> can reside in the archive and the files
     under the directory <c>priv</c> can be read through
     <c>erl_prim_loader</c>.</p>

    <p>When a directory is added to the code path and
     when the entire code path is (re)set, the code server
     decides which subdirectories in an application that are to be
     read from the archive and which that are to be read as regular
     files. If directories are added or removed afterwards, the file
     access can fail if the code path is not updated (possibly to the
     same path as before, to trigger the directory resolution
     update).</p>

     <p>For each directory on the second level in the application archive
     (<c>ebin</c>, <c>priv</c>, <c>src</c>, and so on), the code server first
     chooses the regular directory if it exists and second from the
     archive. Function <c>code:lib_dir/2</c> returns the path to the
     subdirectory. For example, <c>code:lib_dir(megaco,ebin)</c> can return
     <c>/otp/root/lib/megaco-3.9.1.1.ez/megaco-3.9.1.1/ebin</c> while
     <c>code:lib_dir(megaco,priv)</c> can return
     <c>/otp/root/lib/megaco-3.9.1.1/priv</c>.</p>

    <p>When an <c>escript</c> file contains an archive, there are
     no restrictions on the name of the <c>escript</c> and no restrictions
     on how many applications that can be stored in the embedded
     archive. Single Beam files can also reside on the top
     level in the archive. At startup, the top directory in the
     embedded archive and all (second level) <c>ebin</c>
     directories in the embedded archive are added to the code path.
     See <seealso marker="erts:escript"><c>erts:escript(1)</c></seealso>.</p>

    <p>When the choice of directories in the code path is
     <c>strict</c>, the directory that ends up in the code path is
     exactly the stated one. This means that if, for example, the
     directory <c>$OTPROOT/lib/mnesia-4.4.7/ebin</c> is explicitly
     added to the code path, the code server does not load files from
     <c>$OTPROOT/lib/mnesia-4.4.7.ez/mnesia-4.4.7/ebin</c>.</p>

    <p>This behavior can be controlled through command-line flag
     <c>-code_path_choice Choice</c>. If the flag is set to <c>relaxed</c>,
     the code server instead chooses a suitable directory
     depending on the actual file structure. If a regular
     application <c>ebin</c> directory exists, it is chosen. Otherwise,
     the directory <c>ebin</c> in the archive is chosen if it
     exists. If neither of them exists, the original directory is
     chosen.</p>

    <p>Command-line flag <c>-code_path_choice Choice</c> also
     affects how module <c>init</c> interprets the <c>boot script</c>.
     The interpretation of the explicit code paths in the <c>boot
     script</c> can be <c>strict</c> or <c>relaxed</c>. It is
     particularly useful to set the flag to <c>relaxed</c> when
     elaborating with code loading from archives without editing the
     <c>boot script</c>. The default is <c>relaxed</c>. See <seealso
     marker="erts:init"><c>erts:init(3)</c></seealso>.</p></section>

  <section>

    <title>Current and Old Code</title>
    <p>The code for a module can exist in two variants in a system:
      <em>current code</em> and <em>old code</em>. When a module is
      loaded into the system for the first time, the module code
      becomes 'current' and the global <em>export table</em> is updated
      with references to all functions exported from the module.</p>
    <p>If then a new instance of the module is loaded (for example, because of
      error correction), the code of the previous
      instance becomes 'old', and all export entries referring to
      the previous instance are removed. After that, the new instance is
      loaded as for the first time, and becomes 'current'.</p>
    <p>Both old and current code for a module are valid, and can even be
      evaluated concurrently. The difference is that exported functions
      in old code are unavailable. Hence, a global call cannot be made
      to an exported function in old code, but old code can
      still be evaluated because of processes lingering in it.</p>
    <p>If a third instance of the module is loaded, the code server
      removes (purges) the old code and any processes lingering in it
      are terminated. Then the third instance becomes 'current' and
      the previously current code becomes 'old'.</p>
    <p>For more information about old and current code, and how to
      make a process switch from old to current code, see section
      Compilation and Code Loading in the
      <seealso marker="doc/reference_manual:code_loading">Erlang Reference Manual</seealso>.</p>
  </section>

  <section>
    <title>Argument Types and Invalid Arguments</title>

    <p>Module and application names are atoms, while file and directory
    names are strings. For backward compatibility reasons, some functions accept
    both strings and atoms, but a future release will probably only allow
    the arguments that are documented.</p>

    <p>Functions in this module generally fail with an
    exception if they are passed an incorrect type (for example, an integer or a tuple
    where an atom is expected). An error tuple is returned if the argument type
    is correct, but there are some other errors (for example, a non-existing directory
    is specified to <c>set_path/1</c>).</p>
  </section>

  <section>
    <marker id="error_reasons"></marker>
    <title>Error Reasons for Code-Loading Functions</title>

    <p>Functions that load code (such as <c>load_file/1</c>) will
    return <c>{error,Reason}</c> if the load operation fails.
    Here follows a description of the common reasons.</p>

    <taglist>
      <tag><c>badfile</c></tag>
      <item>
	<p>The object code has an incorrect format or the module
	name in the object code is not the expected module name.</p>
      </item>

      <tag><c>nofile</c></tag>
      <item>
	<p>No file with object code was found.</p>
      </item>

      <tag><c>not_purged</c></tag>
      <item>
	<p>The object code could not be loaded because an old version
	of the code already existed.</p>
      </item>

      <tag><c>on_load_failure</c></tag>
      <item>
	<p>The module has an
	<seealso marker="doc/reference_manual:code_loading#on_load">-on_load function</seealso>
	that failed when it was called.</p>
      </item>

      <tag><c>sticky_directory</c></tag>
      <item>
	<p>The object code resides in a sticky directory.</p>
      </item>

    </taglist>
    </section>
  <datatypes>
    <datatype>
      <name name="load_ret"/>
    </datatype>
    <datatype>
      <name name="load_error_rsn"/>
    </datatype>
    <datatype>
      <name name="prepared_code"/>
      <desc><p>An opaque term holding prepared code.</p></desc>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="set_path" arity="1" since=""/>
      <fsummary>Set the code server search path.</fsummary>
      <desc>
        <p>Sets the code path to the list of directories <c><anno>Path</anno></c>.</p>
        <p>Returns:</p>
	<taglist>
	<tag><c>true</c></tag>
	<item><p>If successful</p></item>
        <tag><c>{error, bad_directory}</c></tag>
	<item><p>If any <c><anno>Dir</anno></c> is not a directory name</p></item>
	</taglist>
      </desc>
    </func>
    <func>
      <name name="get_path" arity="0" since=""/>
      <fsummary>Return the code server search path.</fsummary>
      <desc>
        <p>Returns the code path.</p>
      </desc>
    </func>
    <func>
      <name name="add_path" arity="1" since=""/>
      <name name="add_pathz" arity="1" since=""/>
      <fsummary>Add a directory to the end of the code path.</fsummary>
      <type name="add_path_ret"/>
      <desc>
        <p>Adds <c><anno>Dir</anno></c> to the code path. The directory is added as
          the last directory in the new path. If <c><anno>Dir</anno></c> already
          exists in the path, it is not added.</p>
        <p>Returns <c>true</c> if successful, or
          <c>{error, bad_directory}</c> if <c><anno>Dir</anno></c> is not the name
          of a directory.</p>
      </desc>
    </func>
    <func>
      <name name="add_patha" arity="1" since=""/>
      <fsummary>Add a directory to the beginning of the code path.</fsummary>
      <type name="add_path_ret"/>
      <desc>
        <p>Adds <c><anno>Dir</anno></c> to the beginning of the code path. If
          <c><anno>Dir</anno></c> exists, it is removed from the old
          position in the code path.</p>
        <p>Returns <c>true</c> if successful, or
          <c>{error, bad_directory}</c> if <c><anno>Dir</anno></c> is not the name
          of a directory.</p>
      </desc>
    </func>
    <func>
      <name name="add_paths" arity="1" since=""/>
      <name name="add_pathsz" arity="1" since=""/>
      <fsummary>Add directories to the end of the code path.</fsummary>
      <desc>
        <p>Adds the directories in <c><anno>Dirs</anno></c> to the end of the code
          path. If a <c><anno>Dir</anno></c> exists, it is not added.</p>
	<p>Always returns <c>ok</c>, regardless of the validity
          of each individual <c><anno>Dir</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="add_pathsa" arity="1" since=""/>
      <fsummary>Add directories to the beginning of the code path.</fsummary>
      <desc>
	<p>Traverses <c><anno>Dirs</anno></c> and adds
	  each <c><anno>Dir</anno></c> to the beginning of the code
	  path. This means that the order of <c><anno>Dirs</anno></c>
	  is reversed in the resulting code path. For example, if you
	  add <c>[Dir1,Dir2]</c>, the resulting path will
	  be <c>[Dir2,Dir1|OldCodePath]</c>.</p>
	<p>If a <c><anno>Dir</anno></c> already exists in the code
	  path, it is removed from the old position.</p>
	<p>Always returns <c>ok</c>, regardless of the validity of each
          individual <c><anno>Dir</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="del_path" arity="1" since=""/>
      <fsummary>Delete a directory from the code path.</fsummary>
      <desc>
        <p>Deletes a directory from the code path. The argument can be
          an atom <c><anno>Name</anno></c>, in which case the directory with
          the name <c>.../<anno>Name</anno>[-Vsn][/ebin]</c> is deleted from the code
          path. Also, the complete directory name <c><anno>Dir</anno></c> can be
	  specified as argument.</p>
        <p>Returns:</p>
	<taglist>
	<tag><c>true</c></tag>
	<item><p>If successful</p></item>
	<tag><c>false</c></tag>
	<item><p>If the directory is not found</p></item>
	<tag><c>{error, bad_name}</c></tag>
	<item><p>If the argument is invalid</p></item>
	</taglist>
      </desc>
    </func>
    <func>
      <name name="replace_path" arity="2" since=""/>
      <fsummary>Replace a directory with another in the code path.</fsummary>
      <desc>
        <p>Replaces an old occurrence of a directory
          named <c>.../<anno>Name</anno>[-Vsn][/ebin]</c> in the code path, with
          <c><anno>Dir</anno></c>. If <c><anno>Name</anno></c> does not exist, it adds
	  the new directory <c><anno>Dir</anno></c> last in the code path. The new
	  directory must also be named <c>.../<anno>Name</anno>[-Vsn][/ebin]</c>.
	  This function is to be used if a new version of the directory (library) is
          added to a running system.</p>
        <p>Returns:</p>
	<taglist>
	  <tag><c>true</c></tag>
	  <item><p>If successful</p></item>
          <tag><c>{error, bad_name}</c></tag>
	  <item><p>If <c><anno>Name</anno></c> is not found</p></item>
          <tag><c>{error, bad_directory}</c></tag>
	  <item><p>If <c><anno>Dir</anno></c> does not exist</p></item>
          <tag><c>{error, {badarg, [<anno>Name</anno>, <anno>Dir</anno>]}}</c></tag>
	  <item><p>If <c><anno>Name</anno></c> or <c><anno>Dir</anno></c> is invalid</p></item>
	</taglist>
      </desc>
    </func>
    <func>
      <name name="load_file" arity="1" since=""/>
      <fsummary>Load a module.</fsummary>
      <type name="load_ret"/>
      <desc>
        <p>Tries to load the Erlang module <c><anno>Module</anno></c>, using
          the code path. It looks for the object code file with an
          extension corresponding to the Erlang machine used, for
          example, <c><anno>Module</anno>.beam</c>. The loading fails if the module
          name found in the object code differs from the name
          <c><anno>Module</anno></c>.
          <seealso marker="#load_binary/3"><c>load_binary/3</c></seealso> must
          be used to load object code with a module name that is
          different from the file name.</p>
        <p>Returns <c>{module, <anno>Module</anno>}</c> if successful, or
	<c>{error, Reason}</c> if loading fails.
	See <seealso marker="#error_reasons">Error Reasons for Code-Loading Functions</seealso> for a description of the possible error reasons.</p>
      </desc>
    </func>
    <func>
      <name name="load_abs" arity="1" since=""/>
      <fsummary>Load a module, residing in a specified file.</fsummary>
      <type name="load_ret"/>
      <type name="loaded_filename"/>
      <type name="loaded_ret_atoms"/>
      <desc>
        <p>Same as <c>load_file(<anno>Module</anno>)</c>, but
          <c><anno>Filename</anno></c> is an absolute or
          relative filename. The code path is not searched. It returns
          a value in the same way as
          <seealso marker="#load_file/1"><c>load_file/1</c></seealso>. Notice
          that <c><anno>Filename</anno></c> must not contain the extension (for
          example, <c>.beam</c>) because <c>load_abs/1</c> adds the correct
          extension.</p>
      </desc>
    </func>
    <func>
      <name name="ensure_loaded" arity="1" since=""/>
      <fsummary>Ensure that a module is loaded.</fsummary>
      <desc>
        <p>Tries to load a module in the same way as
          <seealso marker="#load_file/1"><c>load_file/1</c></seealso>,
	  unless the module is already loaded.
          However, in embedded mode it does not load a module that is not
          already loaded, but returns <c>{error, embedded}</c> instead.
	  See <seealso marker="#error_reasons">Error Reasons for Code-Loading Functions</seealso> for a description of other possible error reasons.</p>
      </desc>
    </func>
    <func>
      <name name="load_binary" arity="3" since=""/>
      <fsummary>Load object code for a module.</fsummary>
      <type name="loaded_filename"/>
      <type name="loaded_ret_atoms"/>
      <desc>
        <p>This function can be used to load object code on remote
          Erlang nodes. Argument <c><anno>Binary</anno></c> must contain
	  object code for <c><anno>Module</anno></c>.
          <c><anno>Filename</anno></c> is only used by the code server to keep a
          record of from which file the object code for <c><anno>Module</anno></c>
          comes. Thus, <c><anno>Filename</anno></c> is not opened and read by
          the code server.</p>
        <p>Returns <c>{module, <anno>Module</anno>}</c> if successful, or
	<c>{error, Reason}</c> if loading fails.
	See <seealso marker="#error_reasons">Error Reasons for Code-Loading Functions</seealso> for a description of the possible error reasons.</p>
      </desc>
    </func>
    <func>
      <name name="atomic_load" arity="1" since="OTP 19.0"/>
      <fsummary>Load a list of modules atomically</fsummary>
      <desc>
        <p>Tries to load all of the modules in the list
        <c><anno>Modules</anno></c> atomically.  That means that
        either all modules are loaded at the same time, or
        none of the modules are loaded if there is a problem with any
	of the modules.</p>
	<p>Loading can fail for one the following reasons:</p>
	<taglist>
	  <tag><c>badfile</c></tag>
	  <item>
	    <p>The object code has an incorrect format or the module
	    name in the object code is not the expected module name.</p>
	  </item>
	  <tag><c>nofile</c></tag>
	  <item>
	    <p>No file with object code exists.</p>
	  </item>
	  <tag><c>on_load_not_allowed</c></tag>
	  <item>
	    <p>A module contains an
	    <seealso marker="doc/reference_manual:code_loading#on_load">-on_load function</seealso>.</p>
	  </item>
	  <tag><c>duplicated</c></tag>
	  <item>
	    <p>A module is included more than once in
	    <c><anno>Modules</anno></c>.</p>
	  </item>
	  <tag><c>not_purged</c></tag>
	  <item>
	    <p>The object code can not be loaded because an old version
	    of the code already exists.</p>
	  </item>
	  <tag><c>sticky_directory</c></tag>
	  <item>
	    <p>The object code resides in a sticky directory.</p>
	  </item>
	  <tag><c>pending_on_load</c></tag>
	  <item>
	    <p>A previously loaded module contains an
	    <c>-on_load</c> function that never finished.</p>
	  </item>
	</taglist>
	<p>If it is important to minimize the time that an application
	is inactive while changing code, use
	<seealso marker="#prepare_loading/1">prepare_loading/1</seealso>
	and
	<seealso marker="#finish_loading/1">finish_loading/1</seealso>
	instead of <c>atomic_load/1</c>. Here is an example:</p>
<pre>
{ok,Prepared} = code:prepare_loading(Modules),
%% Put the application into an inactive state or do any
%% other preparation needed before changing the code.
ok = code:finish_loading(Prepared),
%% Resume the application.</pre>
      </desc>
    </func>
    <func>
      <name name="prepare_loading" arity="1" since="OTP 19.0"/>
      <fsummary>Prepare a list of modules atomically</fsummary>
      <desc>
        <p>Prepares to load the modules in the list
        <c><anno>Modules</anno></c>.
	Finish the loading by calling
	<seealso marker="#finish_loading/1">finish_loading(Prepared)</seealso>.</p>
	<p>This function can fail with one of the following error reasons:</p>
	<taglist>
	  <tag><c>badfile</c></tag>
	  <item>
	    <p>The object code has an incorrect format or the module
	    name in the object code is not the expected module name.</p>
	  </item>
	  <tag><c>nofile</c></tag>
	  <item>
	    <p>No file with object code exists.</p>
	  </item>
	  <tag><c>on_load_not_allowed</c></tag>
	  <item>
	    <p>A module contains an
	    <seealso marker="doc/reference_manual:code_loading#on_load">-on_load function</seealso>.</p>
	  </item>
	  <tag><c>duplicated</c></tag>
	  <item>
	    <p>A module is included more than once in
	    <c><anno>Modules</anno></c>.</p>
	  </item>
	</taglist>
      </desc>
    </func>
    <func>
      <name name="finish_loading" arity="1" since="OTP 19.0"/>
      <fsummary>Finish loading a list of prepared modules atomically</fsummary>
      <desc>
        <p>Tries to load code for all modules that have been previously
	prepared by
	<seealso marker="#prepare_loading/1">prepare_loading/1</seealso>.
	The loading occurs atomically, meaning that
        either all modules are loaded at the same time, or
        none of the modules are loaded.</p>
	<p>This function can fail with one of the following error reasons:</p>
	<taglist>
	  <tag><c>not_purged</c></tag>
	  <item>
	    <p>The object code can not be loaded because an old version
	    of the code already exists.</p>
	  </item>
	  <tag><c>sticky_directory</c></tag>
	  <item>
	    <p>The object code resides in a sticky directory.</p>
	  </item>
	  <tag><c>pending_on_load</c></tag>
	  <item>
	    <p>A previously loaded module contains an
	    <c>-on_load</c> function that never finished.</p>
	  </item>
	</taglist>
      </desc>
    </func>
    <func>
      <name name="ensure_modules_loaded" arity="1" since="OTP 19.0"/>
      <fsummary>Ensure that a list of modules is loaded</fsummary>
      <desc>
        <p>Tries to load any modules not already loaded in the list
	<c><anno>Modules</anno></c> in the same way as
          <seealso marker="#load_file/1">load_file/1</seealso>.</p>
	  <p>Returns <c>ok</c> if successful, or
	  <c>{error,[{Module,Reason}]}</c> if loading of some modules fails.
	  See <seealso marker="#error_reasons">Error Reasons for Code-Loading Functions</seealso> for a description of other possible error reasons.</p>
      </desc>
    </func>
    <func>
      <name name="delete" arity="1" since=""/>
      <fsummary>Remove current code for a module.</fsummary>
      <desc>
        <p>Removes the current code for <c><anno>Module</anno></c>, that is,
          the current code for <c><anno>Module</anno></c> is made old. This means
          that processes can continue to execute the code in the module,
          but no external function calls can be made to it.</p>
        <p>Returns <c>true</c> if successful, or <c>false</c> if there
          is old code for <c><anno>Module</anno></c> that must be purged first, or
          if <c><anno>Module</anno></c> is not a (loaded) module.</p>
      </desc>
    </func>
    <func>
      <name name="purge" arity="1" since=""/>
      <fsummary>Remove old code for a module.</fsummary>
      <desc>
        <p>Purges the code for <c><anno>Module</anno></c>, that is, removes code
          marked as old. If some processes still linger in the old code,
          these processes are killed before the code is removed.</p>
	<note><p>As of ERTS version 9.0, a process is only considered
	to be lingering in the code if it has direct references to the code.
	For more information see documentation of
	<seealso marker="erts:erlang#check_process_code/3"><c>erlang:check_process_code/3</c></seealso>,
	which is used in order to determine this.</p></note>
        <p>Returns <c>true</c> if successful and any process is needed to
          be killed, otherwise <c>false</c>.</p>
      </desc>
    </func>
    <func>
      <name name="soft_purge" arity="1" since=""/>
      <fsummary>Remove old code for a module, unless no process uses it.</fsummary>
      <desc>
        <p>Purges the code for <c><anno>Module</anno></c>, that is, removes code
          marked as old, but only if no processes linger in it.</p>
	<note><p>As of ERTS version 9.0, a process is only considered
	to be lingering in the code if it has direct references to the code.
	For more information see documentation of
	<seealso marker="erts:erlang#check_process_code/3"><c>erlang:check_process_code/3</c></seealso>,
	which is used in order to determine this.</p></note>
        <p>Returns <c>false</c> if the module cannot be purged because
          of processes lingering in old code, otherwise <c>true</c>.</p>
      </desc>
    </func>
    <func>
      <name name="is_loaded" arity="1" since=""/>
      <fsummary>Check if a module is loaded.</fsummary>
      <type name="loaded_filename"/>
      <type name="loaded_ret_atoms"/>
      <type_desc name="loaded_filename"><c><anno>Filename</anno></c> is an absolute
      filename.</type_desc>
      <desc>
        <p>Checks if <c><anno>Module</anno></c> is loaded. If it is,
          <c>{file, <anno>Loaded</anno>}</c> is returned, otherwise <c>false</c>.</p>
        <p>Normally, <c><anno>Loaded</anno></c> is the absolute filename
          <c>Filename</c> from which the code is obtained. If the module
          is preloaded (see
          <seealso marker="sasl:script"><c>script(4)</c></seealso>),
          <c>Loaded==preloaded</c>. If the module is Cover-compiled (see
          <seealso marker="tools:cover"><c>cover(3)</c></seealso>),
          <c>Loaded==cover_compiled</c>.</p>
      </desc>
    </func>
    <func>
      <name name="all_loaded" arity="0" since=""/>
      <fsummary>Get all loaded modules.</fsummary>
      <type name="loaded_filename"/>
      <type name="loaded_ret_atoms"/>
      <type_desc name="loaded_filename"><c><anno>Filename</anno></c> is an absolute
      filename.</type_desc>
      <desc>
        <p>Returns a list of tuples <c>{<anno>Module</anno>, <anno>Loaded</anno>}</c> for all
          loaded modules. <c><anno>Loaded</anno></c> is normally the absolute filename,
          as described for
          <seealso marker="#is_loaded/1"><c>is_loaded/1</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="which" arity="1" since=""/>
      <fsummary>The object code file of a module.</fsummary>
      <type name="loaded_ret_atoms"/>
      <desc>
        <p>If the module is not loaded, this function searches the code
          path for the first file containing object code for
          <c><anno>Module</anno></c> and returns the absolute filename.</p>
	  <p>If the module is loaded, it returns the name of the file
          containing the loaded object code.</p>
	  <p>If the module is preloaded, <c>preloaded</c> is returned.</p>
	  <p>If the module is Cover-compiled, <c>cover_compiled</c> is returned.</p>
	  <p>If the module cannot be found, <c>non_existing</c> is returned.</p>
      </desc>
    </func>
    <func>
      <name name="get_object_code" arity="1" since=""/>
      <fsummary>Gets the object code for a module.</fsummary>
      <desc>
        <p>Searches the code path for the object code of module
          <c><anno>Module</anno></c>. Returns <c>{<anno>Module</anno>, <anno>Binary</anno>, <anno>Filename</anno>}</c>
          if successful, otherwise <c>error</c>. <c><anno>Binary</anno></c> is a
          binary data object, which contains the object code for
          the module. This can be useful if code is to be loaded on a
          remote node in a distributed system. For example, loading
          module <c><anno>Module</anno></c> on a node <c>Node</c> is done as
          follows:</p>
        <code type="none">
...
{_Module, Binary, Filename} = code:get_object_code(Module),
rpc:call(Node, code, load_binary, [Module, Filename, Binary]),
...</code>
      </desc>
    </func>
    <func>
      <name name="root_dir" arity="0" since=""/>
      <fsummary>Root directory of Erlang/OTP.</fsummary>
      <desc>
        <p>Returns the root directory of Erlang/OTP, which is
          the directory where it is installed.</p>
	<p><em>Example:</em></p>
        <pre>
> <input>code:root_dir().</input>
"/usr/local/otp"</pre>
      </desc>
    </func>
    <func>
      <name name="lib_dir" arity="0" since=""/>
      <fsummary>Library directory of Erlang/OTP.</fsummary>
      <desc>
        <p>Returns the library directory, <c>$OTPROOT/lib</c>, where
          <c>$OTPROOT</c> is the root directory of Erlang/OTP.</p>
	<p><em>Example:</em></p>
        <pre>
> <input>code:lib_dir().</input>
"/usr/local/otp/lib"</pre>
      </desc>
    </func>
    <func>
      <name name="lib_dir" arity="1" since=""/>
      <fsummary>Library directory for an application.</fsummary>
      <desc>
        <p>Returns the path
          for the "library directory", the top directory, for an
          application <c><anno>Name</anno></c> located under <c>$OTPROOT/lib</c> or
          on a directory referred to with environment variable <c>ERL_LIBS</c>.</p>
        <p>If a regular directory called <c><anno>Name</anno></c> or
          <c><anno>Name</anno>-Vsn</c> exists in the code path with an <c>ebin</c>
          subdirectory, the path to this directory is returned (not
          the <c>ebin</c> directory).</p>
	<p>If the directory refers to a directory in an archive, the
	  archive name is stripped away before the path is returned.
	  For example, if directory
          <c>/usr/local/otp/lib/mnesia-4.2.2.ez/mnesia-4.2.2/ebin</c>
          is in the path, <c>/usr/local/otp/lib/mnesia-4.2.2/ebin</c>
          is returned. This means that the library directory for
          an application is the same, regardless if the
          application resides in an archive or not.</p>
	<p><em>Example:</em></p>
        <pre>
> <input>code:lib_dir(mnesia).</input>
"/usr/local/otp/lib/mnesia-4.2.2"</pre>
        <p>Returns <c>{error, bad_name}</c> if <c><anno>Name</anno></c>
	 is not the name of an application under <c>$OTPROOT/lib</c> or
	 on a directory referred to through environment variable <c>ERL_LIBS</c>.
	 Fails with an exception if <c>Name</c> has the wrong type.</p>

	<warning><p>For backward compatibility, <c><anno>Name</anno></c> is also
	allowed to be a string. That will probably change in a future release.</p></warning>
      </desc>
    </func>
    <func>
      <name name="lib_dir" arity="2" since=""/>
      <fsummary>Subdirectory for an application.</fsummary>
      <desc>
        <p>Returns the path to a subdirectory directly under the top
         directory of an application. Normally the subdirectories
         reside under the top directory for the application, but when
         applications at least partly resides in an archive, the
         situation is different. Some of the subdirectories can reside
         as regular directories while other reside in an archive
         file. It is not checked whether this directory exists.</p>
	 <p><em>Example:</em></p>
        <pre>
> <input>code:lib_dir(megaco, priv).</input>
"/usr/local/otp/lib/megaco-3.9.1.1/priv"</pre>

        <p>Fails with an exception if <c><anno>Name</anno></c> or <c><anno>SubDir</anno></c> has
        the wrong type.</p>
      </desc>
    </func>
    <func>
      <name name="compiler_dir" arity="0" since=""/>
      <fsummary>Library directory for the compiler.</fsummary>
      <desc>
        <p>Returns the compiler library directory. Equivalent to
          <c>code:lib_dir(compiler)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="priv_dir" arity="1" since=""/>
      <fsummary>Priv directory for an application.</fsummary>
      <desc>
        <p>Returns the path to the <c>priv</c> directory in an
         application. Equivalent to <c>code:lib_dir(<anno>Name</anno>, priv)</c>.</p>

	<warning><p>For backward compatibility, <c><anno>Name</anno></c> is also allowed to
	be a string. That will probably change in a future release.</p></warning>
      </desc>
    </func>
    <func>
      <name name="objfile_extension" arity="0" since=""/>
      <fsummary>Object code file extension.</fsummary>
      <desc>
        <p>Returns the object code file extension corresponding to
          the Erlang machine used, namely <c>.beam</c>.</p>
      </desc>
    </func>
    <func>
      <name name="stick_dir" arity="1" since=""/>
      <fsummary>Mark a directory as sticky.</fsummary>
      <desc>
        <p>Marks <c><anno>Dir</anno></c> as sticky.</p>
        <p>Returns <c>ok</c> if successful, otherwise <c>error</c>.</p>
      </desc>
    </func>
    <func>
      <name name="unstick_dir" arity="1" since=""/>
      <fsummary>Remove a sticky directory mark.</fsummary>
      <desc>
        <p>Unsticks a directory that is marked as
          sticky.</p>
        <p>Returns <c>ok</c> if successful, otherwise <c>error</c>.</p>
      </desc>
    </func>
    <func>
      <name name="is_sticky" arity="1" since=""/>
      <fsummary>Test if a module is sticky.</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>Module</anno></c> is the
	name of a module that has been loaded from a sticky directory
	(in other words: an attempt to reload the module will fail),
	or <c>false</c> if <c><anno>Module</anno></c> is not a loaded module or is
	not sticky.</p>
      </desc>
    </func>
    <func>
      <name name="where_is_file" arity="1" since=""/>
      <fsummary>Full name of a file located in the code path.</fsummary>
      <desc>
        <p>Searches the code path for <c><anno>Filename</anno></c>, a file of
          arbitrary type. If found, the full name is returned.
          <c>non_existing</c> is returned if the file cannot be found.
          The function can be useful, for example, to locate
          application resource files.</p>
      </desc>
    </func>
    <func>
      <name name="clash" arity="0" since=""/>
      <fsummary>Search for modules with identical names.</fsummary>
      <desc>
        <p>Searches all directories in the code path for module names with
          identical names and writes a report to <c>stdout</c>.</p>
      </desc>
    </func>
    <func>
      <name name="module_status" arity="1" since="OTP 20.0"/>
      <fsummary>Return the status of the module in relation to object file on disk.</fsummary>
      <desc>
	<p>Returns:</p>
	<taglist>
	<tag><c>not_loaded</c></tag>
	<item><p>If <c><anno>Module</anno></c> is not currently loaded.</p></item>
        <tag><c>loaded</c></tag>
	<item><p>If <c><anno>Module</anno></c> is loaded and the object file
	exists and contains the same code.</p></item>
	<tag><c>removed</c></tag>
	<item><p>If <c><anno>Module</anno></c> is loaded but no
	corresponding object file can be found in the code path.</p></item>
        <tag><c>modified</c></tag>
	<item><p>If <c><anno>Module</anno></c> is loaded but the object file
	contains code with a different MD5 checksum.</p></item>
	</taglist>
        <p>Preloaded modules are always reported as <c>loaded</c>, without
        inspecting the contents on disk. Cover compiled modules will always
        be reported as <c>modified</c> if an object file exists, or as
        <c>removed</c> otherwise. Modules whose load path is an empty string
        (which is the convention for auto-generated code) will only be
        reported as <c>loaded</c> or <c>not_loaded</c>.</p>
        <p>For modules that have native code loaded (see
        <seealso marker="#is_module_native/1"><c>is_module_native/1</c></seealso>),
        the MD5 sum of the native code in the object file is used for the
        comparison, if it exists; the Beam code in the file is ignored.
        Reversely, for modules that do not currently have native code
        loaded, any native code in the file will be ignored.</p>
        <p>See also <seealso marker="#modified_modules/0"><c>modified_modules/0</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="modified_modules" arity="0" since="OTP 20.0"/>
      <fsummary>Return a list of all modules modified on disk.</fsummary>
      <desc>
        <p>Returns the list of all currently loaded modules for which
        <seealso marker="#module_status/1"><c>module_status/1</c></seealso>
        returns <c>modified</c>. See also <seealso marker="#all_loaded/0"><c>all_loaded/0</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="is_module_native" arity="1" since=""/>
      <fsummary>Test if a module has native code.</fsummary>
      <desc>
	<p>Returns:</p>
	<taglist>
	<tag><c>true</c></tag>
	<item><p>If <c><anno>Module</anno></c> is the
	name of a loaded module that has native code loaded</p></item>
        <tag><c>false</c></tag>
	<item><p>If <c><anno>Module</anno></c> is loaded but does not have
	native code</p></item>
	<tag><c>undefined</c></tag>
	<item><p>If <c><anno>Module</anno></c> is not loaded</p></item>
	</taglist>
      </desc>
    </func>

    <func>
      <name name="get_mode" arity="0" since="OTP R16B"/>
      <fsummary>The mode of the code server.</fsummary>
      <desc>
        <p>Returns an atom describing the mode of the code server:
        <c>interactive</c> or <c>embedded</c>.</p>
        <p>This information is useful when an external entity (for example,
        an IDE) provides additional code for a running node. If the code server is
	in interactive mode, it only has to add the path to the code. If the code server
	is in embedded mode, the code must be loaded with
	<seealso marker="#load_binary/3"><c>load_binary/3</c></seealso>.</p>
      </desc>
    </func>
  </funcs>
</erlref>