path: root/lib/stdlib/doc/src/beam_lib.xml

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<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>2000</year><year>2013</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>beam_lib</title>
    <prepared>Hans Bolinder</prepared>
    <docno></docno>
    <date>1999-10-30</date>
    <rev>PA1</rev>
  </header>
  <module>beam_lib</module>
  <modulesummary>An Interface To the BEAM File Format</modulesummary>
  <description>
    <p><c>beam_lib</c> provides an interface to files created by
      the BEAM compiler ("BEAM files"). The format used, a variant of
      "EA IFF 1985" Standard for Interchange Format Files, divides data
      into chunks.</p>
    <p>Chunk data can be returned as binaries or as compound terms.
      Compound terms are returned when chunks are referenced by names
      (atoms) rather than identifiers (strings).  The names recognized
      and the corresponding identifiers are:</p>
    <list type="bulleted">
      <item><c>abstract_code ("Abst")</c></item>
      <item><c>attributes ("Attr")</c></item>
      <item><c>compile_info ("CInf")</c></item>
      <item><c>exports ("ExpT")</c></item>
      <item><c>labeled_exports ("ExpT")</c></item>
      <item><c>imports ("ImpT")</c></item>
      <item><c>indexed_imports ("ImpT")</c></item>
      <item><c>locals ("LocT")</c></item>
      <item><c>labeled_locals ("LocT")</c></item>
      <item><c>atoms ("Atom")</c></item>
    </list>
  </description>

  <section>
    <marker id="debug_info"></marker>
    <title>Debug Information/Abstract Code</title>
    <p>The option <c>debug_info</c> can be given to the compiler (see
      <seealso marker="compiler:compile#debug_info">compile(3)</seealso>)
      in order to have debug information in the form of abstract code
      (see <seealso marker="erts:absform">The Abstract Format</seealso>
      in ERTS User's Guide) stored in the <c>abstract_code</c> chunk.
      Tools such as Debugger and Xref require the debug information to
      be included.</p>
    <warning>
      <p>Source code can be reconstructed from the debug information.
        Use encrypted debug information (see below) to prevent this.</p>
    </warning>
    <p>The debug information can also be removed from BEAM files
      using <seealso marker="#strip/1">strip/1</seealso>,
      <seealso marker="#strip_files/1">strip_files/1</seealso> and/or
      <seealso marker="#strip_release/1">strip_release/1</seealso>.</p>
    <section>
      <title>Reconstructing source code</title>
      <p>Here is an example of how to reconstruct source code from
        the debug information in a BEAM file <c>Beam</c>:</p>
      <code type="none">
      {ok,{_,[{abstract_code,{_,AC}}]}} = beam_lib:chunks(Beam,[abstract_code]).
      io:fwrite("~s~n", [erl_prettypr:format(erl_syntax:form_list(AC))]).</code>
    </section>
    <section>
    <title>Encrypted debug information</title>
      <p>The debug information can be encrypted in order to keep
        the source code secret, but still being able to use tools such as
        Xref or Debugger. </p>
      <p>To use encrypted debug information, a key must be provided to
        the compiler and <c>beam_lib</c>. The key is given as a string and
        it is recommended that it contains at least 32 characters and
        that both upper and lower case letters as well as digits and
        special characters are used.</p>
      <p>The default type -- and currently the only type -- of crypto
        algorithm is <c>des3_cbc</c>, three rounds of DES. The key string
        will be scrambled using <c>erlang:md5/1</c> to generate
        the actual keys used for <c>des3_cbc</c>.</p>
      <note>
        <p>As far as we know by the time of writing, it is
          infeasible to break <c>des3_cbc</c> encryption without any
          knowledge of the key. Therefore, as long as the key is kept
          safe and is unguessable, the encrypted debug information
          <em>should</em> be safe from intruders.</p>
      </note>
      <p>There are two ways to provide the key:</p>
      <list type="ordered">
        <item>
          <p>Use the compiler option <c>{debug_info,Key}</c>, see
            <seealso marker="compiler:compile#debug_info_key">compile(3)</seealso>,
            and the function
            <seealso marker="#crypto_key_fun/1">crypto_key_fun/1</seealso>
            to register a fun which returns the key whenever
            <c>beam_lib</c> needs to decrypt the debug information.</p>
          <p>If no such fun is registered, <c>beam_lib</c> will instead
            search for a <c>.erlang.crypt</c> file, see below.</p>
        </item>
        <item>
          <p>Store the key in a text file named <c>.erlang.crypt</c>.</p>
          <p>In this case, the compiler option <c>encrypt_debug_info</c>
            can be used, see
            <seealso marker="compiler:compile#encrypt_debug_info">compile(3)</seealso>.</p>
        </item>
      </list>
    </section>
    <section>
      <title>.erlang.crypt</title>
      <p><c>beam_lib</c> searches for <c>.erlang.crypt</c> in the current
        directory and then the home directory for the current user. If
        the file is found and contains a key, <c>beam_lib</c> will
        implicitly create a crypto key fun and register it.</p>
      <p>The <c>.erlang.crypt</c> file should contain a single list of
        tuples:</p>
      <code type="none">
      {debug_info, Mode, Module, Key}</code>
      <p><c>Mode</c> is the type of crypto algorithm; currently, the only
        allowed value thus is <c>des3_cbc</c>. <c>Module</c> is either an
        atom, in which case <c>Key</c> will only be used for the module
        <c>Module</c>, or <c>[]</c>, in which case <c>Key</c> will be
        used for all modules. <c>Key</c> is the non-empty key string.</p>
      <p>The <c>Key</c> in the first tuple where both <c>Mode</c> and
        <c>Module</c> matches will be used.</p>
      <p>Here is an example of an <c>.erlang.crypt</c> file that returns
        the same key for all modules:</p>
      <code type="none"><![CDATA[
[{debug_info, des3_cbc, [], "%>7}|pc/DM6Cga*68$Mw]L#&_Gejr]G^"}].]]></code>
      <p>And here is a slightly more complicated example of an
        <c>.erlang.crypt</c> which provides one key for the module
        <c>t</c>, and another key for all other modules:</p>
      <code type="none"><![CDATA[
[{debug_info, des3_cbc, t, "My KEY"},
 {debug_info, des3_cbc, [], "%>7}|pc/DM6Cga*68$Mw]L#&_Gejr]G^"}].]]></code>
      <note>
        <p>Do not use any of the keys in these examples. Use your own
          keys.</p>
      </note>
    </section>
  </section>

  <datatypes>
    <datatype>
      <name name="beam"/>
      <desc>
        <p>Each of the functions described below accept either the
          module name, the filename, or a binary containing the beam
          module.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="chunkdata"/>
      <desc>
        <p>The list of attributes is sorted on <c>Attribute</c>
          (in attrib_entry()), and each
          attribute name occurs once in the list. The attribute values
          occur in the same order as in the file. The lists of functions
          are also sorted.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="chunkid"/>
      <desc>
        <p>"Abst" | "Attr" | "CInf" | "ExpT" | "ImpT" | "LocT" | "Atom"</p>
      </desc>
    </datatype>
    <datatype>
      <name name="dataB"/>
    </datatype>
    <datatype>
      <name name="abst_code"/>
      <desc>
        <p>It is not checked that the forms conform to the abstract format
          indicated by <c><anno>AbstVersion</anno></c>. <c>no_abstract_code</c> means
          that the <c>"Abst"</c> chunk is present, but empty.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="forms"/>
    </datatype>
    <datatype>
      <name name="compinfo_entry"/>
    </datatype>
    <datatype>
      <name name="attrib_entry"/>
    </datatype>
    <datatype>
      <name name="labeled_entry"/>
    </datatype>
    <datatype>
      <name name="index"/>
    </datatype>
    <datatype>
      <name name="label"/>
    </datatype>
    <datatype>
      <name name="chunkref"/>
    </datatype>
    <datatype>
      <name name="chunkname"/>
    </datatype>
    <datatype>
      <name name="chnk_rsn"/>
    </datatype>
    <datatype>
      <name name="info_rsn"/>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="chunks" arity="2"/>
      <fsummary>Read selected chunks from a BEAM file or binary</fsummary>
      <desc>
        <p>Reads chunk data for selected chunks refs. The order of
          the returned list of chunk data is determined by the order
          of the list of chunks references.</p>
      </desc>
    </func>
    <func>
      <name name="chunks" arity="3"/>
      <fsummary>Read selected chunks from a BEAM file or binary</fsummary>
      <desc>
        <p>Reads chunk data for selected chunks refs. The order of
          the returned list of chunk data is determined by the order
          of the list of chunks references.</p>
        <p>By default, if any requested chunk is missing in <c><anno>Beam</anno></c>,
          an <c>error</c> tuple is returned.
          However, if the option <c>allow_missing_chunks</c> has been given,
          a result will be returned even if chunks are missing.
          In the result list, any missing chunks will be represented as
          <c>{<anno>ChunkRef</anno>,missing_chunk}</c>.
          Note, however, that if the <c>"Atom"</c> chunk if missing, that is
          considered a fatal error and the return value will be an <c>error</c>
          tuple.</p>
      </desc>
    </func>
    <func>
      <name name="version" arity="1"/>
      <fsummary>Read the BEAM file's module version</fsummary>
      <desc>
        <p>Returns the module version(s). A version is defined by
          the module attribute <c>-vsn(Vsn)</c>. If this attribute is
          not specified, the version defaults to the checksum of
          the module. Note that if the version <c>Vsn</c> is not a list,
          it is made into one, that is <c>{ok,{Module,[Vsn]}}</c> is
          returned. If there are several <c>-vsn</c> module attributes,
          the result is the concatenated list of versions. Examples:</p>
        <pre>
1> <input>beam_lib:version(a).</input> % -vsn(1).
{ok,{a,[1]}}
2> <input>beam_lib:version(b).</input> % -vsn([1]).
{ok,{b,[1]}}
3> <input>beam_lib:version(c).</input> % -vsn([1]). -vsn(2).
{ok,{c,[1,2]}}
4> <input>beam_lib:version(d).</input> % no -vsn attribute
{ok,{d,[275613208176997377698094100858909383631]}}</pre>
      </desc>
    </func>
    <func>
      <name name="md5" arity="1"/>
      <fsummary>Read the BEAM file's module version</fsummary>
      <desc>
        <p>Calculates an MD5 redundancy check for the code of the module
          (compilation date and other attributes are not included).</p>
      </desc>
    </func>
    <func>
      <name name="info" arity="1"/>
      <fsummary>Information about a BEAM file</fsummary>
      <desc>
        <p>Returns a list containing some information about a BEAM file
          as tuples <c>{Item, Info}</c>:</p>
        <taglist>
          <tag><c>{file, <anno>Filename</anno>} | {binary, <anno>Binary</anno>}</c></tag>
          <item>
            <p>The name (string) of the BEAM file, or the binary from
              which the information was extracted.</p>
          </item>
          <tag><c>{module, <anno>Module</anno>}</c></tag>
          <item>
            <p>The name (atom) of the module.</p>
          </item>
          <tag><c>{chunks, [{<anno>ChunkId</anno>, <anno>Pos</anno>, <anno>Size</anno>}]}</c></tag>
          <item>
            <p>For each chunk, the identifier (string) and the position
              and size of the chunk data, in bytes.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="cmp" arity="2"/>
      <fsummary>Compare two BEAM files</fsummary>
      <type name="cmp_rsn"/>
      <desc>
        <p>Compares the contents of two BEAM files. If the module names
          are the same, and all chunks except for the <c>"CInf"</c> chunk
	  (the chunk containing the compilation information which is
	  returned by <c>Module:module_info(compile)</c>)
	  have the same contents in both files,
          <c>ok</c> is returned. Otherwise an error message is returned.</p>
      </desc>
    </func>
    <func>
      <name name="cmp_dirs" arity="2"/>
      <fsummary>Compare the BEAM files in two directories</fsummary>
      <desc>
        <p>The <c>cmp_dirs/2</c> function compares the BEAM files in
          two directories. Only files with extension <c>".beam"</c> are
          compared. BEAM files that exist in directory <c><anno>Dir1</anno></c>
          (<c><anno>Dir2</anno></c>) only are returned in <c><anno>Only1</anno></c>
          (<c><anno>Only2</anno></c>). BEAM files that exist on both directories but
          are considered different by <c>cmp/2</c> are returned as
          pairs {<c><anno>Filename1</anno></c>, <c><anno>Filename2</anno></c>} where
          <c><anno>Filename1</anno></c> (<c><anno>Filename2</anno></c>) exists in directory
          <c><anno>Dir1</anno></c> (<c><anno>Dir2</anno></c>).</p>
      </desc>
    </func>
    <func>
      <name name="diff_dirs" arity="2"/>
      <fsummary>Compare the BEAM files in two directories</fsummary>
      <desc>
        <p>The <c>diff_dirs/2</c> function compares the BEAM files in
          two directories the way <c>cmp_dirs/2</c> does, but names of
          files that exist in only one directory or are different are
          presented on standard output.</p>
      </desc>
    </func>
    <func>
      <name name="strip" arity="1"/>
      <fsummary>Removes chunks not needed by the loader from a BEAM file</fsummary>
      <desc>
        <p>The <c>strip/1</c> function removes all chunks from a BEAM
          file except those needed by the loader. In particular,
          the debug information (<c>abstract_code</c> chunk) is removed.</p>
      </desc>
    </func>
    <func>
      <name name="strip_files" arity="1"/>
      <fsummary>Removes chunks not needed by the loader from BEAM files</fsummary>
      <desc>
        <p>The <c>strip_files/1</c> function removes all chunks except
          those needed by the loader from BEAM files. In particular,
          the debug information (<c>abstract_code</c> chunk) is removed.
          The returned list contains one element for each given file
          name, in the same order as in <c>Files</c>.</p>
      </desc>
    </func>
    <func>
      <name name="strip_release" arity="1"/>
      <fsummary>Removes chunks not needed by the loader from all BEAM files of a release</fsummary>
      <desc>
        <p>The <c>strip_release/1</c> function removes all chunks
          except those needed by the loader from the BEAM files of a
          release. <c><anno>Dir</anno></c> should be the installation root
          directory. For example, the current OTP release can be
          stripped with the call
          <c>beam_lib:strip_release(code:root_dir())</c>.</p>
      </desc>
    </func>
    <func>
      <name name="format_error" arity="1"/>
      <fsummary>Return an English description of a BEAM read error reply</fsummary>
      <desc>
        <p>Given the error returned by any function in this module, 
          the function <c>format_error</c> returns a descriptive string
          of the error in English. For file errors, the function 
          <c>file:format_error(Posix)</c> should be called.</p>
      </desc>
    </func>
    <func>
      <name name="crypto_key_fun" arity="1"/>
      <fsummary>Register a fun that provides a crypto key</fsummary>
      <type name="crypto_fun"/>
      <type name="crypto_fun_arg"/>
      <type name="mode"/>
      <desc>
        <p>The <c>crypto_key_fun/1</c> function registers a unary fun
          that will be called if <c>beam_lib</c> needs to read an
          <c>abstract_code</c> chunk that has been encrypted. The fun
          is held in a process that is started by the function.</p>
        <p>If there already is a fun registered when attempting to
          register a fun, <c>{error, exists}</c> is returned.</p>
        <p>The fun must handle the following arguments:</p>
        <code type="none">
          CryptoKeyFun(init) -> ok | {ok, NewCryptoKeyFun} | {error, Term}</code>
        <p>Called when the fun is registered, in the process that holds
          the fun. Here the crypto key fun can do any necessary
          initializations. If <c>{ok, NewCryptoKeyFun}</c> is returned
          then <c>NewCryptoKeyFun</c> will be registered instead of
          <c>CryptoKeyFun</c>. If <c>{error, Term}</c> is returned,
          the registration is aborted and <c>crypto_key_fun/1</c>
          returns <c>{error, Term}</c> as well.</p>
        <code type="none">
          CryptoKeyFun({debug_info, Mode, Module, Filename}) -> Key</code>
        <p>Called when the key is needed for the module <c>Module</c>
          in the file named <c>Filename</c>. <c>Mode</c> is the type of
          crypto algorithm; currently, the only possible value thus is
          <c>des3_cbc</c>. The call should fail (raise an exception) if
          there is no key available.</p>
        <code type="none">
          CryptoKeyFun(clear) -> term()</code>
        <p>Called before the fun is unregistered. Here any cleaning up
          can be done. The return value is not important, but is passed
          back to the caller of <c>clear_crypto_key_fun/0</c> as part
          of its return value.</p>
      </desc>
    </func>
    <func>
      <name name="clear_crypto_key_fun" arity="0"/>
      <fsummary>Unregister the current crypto key fun</fsummary>
      <desc>
        <p>Unregisters the crypto key fun and terminates the process
          holding it, started by <c>crypto_key_fun/1</c>.</p>
        <p>The <c>clear_crypto_key_fun/1</c> either returns
          <c>{ok, undefined}</c> if there was no crypto key fun
          registered, or <c>{ok, Term}</c>, where <c>Term</c> is
          the return value from <c>CryptoKeyFun(clear)</c>, see
          <c>crypto_key_fun/1</c>.</p>
      </desc>
    </func>
  </funcs>
</erlref>