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       beam_lib - An Interface To the BEAM File Format


       beam_lib  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.

       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:

         * abstract_code ("Abst")

         * attributes ("Attr")

         * compile_info ("CInf")

         * exports ("ExpT")

         * labeled_exports ("ExpT")

         * imports ("ImpT")

         * indexed_imports ("ImpT")

         * locals ("LocT")

         * labeled_locals ("LocT")

         * atoms ("Atom")


       The  option  debug_info  can be given to the compiler (see compile(3erl)) in order to have
       debug information in the form of abstract code (see The Abstract  Format  in  ERTS  User's
       Guide)  stored  in  the  abstract_code  chunk. Tools such as Debugger and Xref require the
       debug information to be included.

       Source code  can  be  reconstructed  from  the  debug  information.  Use  encrypted  debug
       information (see below) to prevent this.

       The  debug  information  can  also be removed from BEAM files using strip/1, strip_files/1
       and/or strip_release/1.

   Reconstructing source code
       Here is an example of how to reconstruct source code from the debug information in a  BEAM
       file Beam:

             {ok,{_,[{abstract_code,{_,AC}}]}} = beam_lib:chunks(Beam,[abstract_code]).
             io:fwrite("~s~n", [erl_prettypr:format(erl_syntax:form_list(AC))]).

   Encrypted debug information
       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.

       To use encrypted debug information, a key must be provided to the compiler  and  beam_lib.
       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

       The  default type -- and currently the only type -- of crypto algorithm is des3_cbc, three
       rounds of DES. The key string will be scrambled using erlang:md5/1 to generate the  actual
       keys used for des3_cbc.

       As  far  as  we know by the time of writing, it is infeasible to break des3_cbc encryption
       without any knowledge of the key. Therefore, as long as  the  key  is  kept  safe  and  is
       unguessable, the encrypted debug information should be safe from intruders.

       There are two ways to provide the key:

         * Use  the  compiler  option  {debug_info,Key},  see  compile(3erl),  and  the  function
           crypto_key_fun/1 to register a fun which returns the key whenever  beam_lib  needs  to
           decrypt the debug information.

           If  no  such fun is registered, beam_lib will instead search for a .erlang.crypt file,
           see below.

         * Store the key in a text file named .erlang.crypt.

           In this case, the compiler option encrypt_debug_info can be used, see compile(3erl).

       beam_lib searches for .erlang.crypt in the current directory and then the  home  directory
       for  the  current  user. If the file is found and contains a key, beam_lib will implicitly
       create a crypto key fun and register it.

       The .erlang.crypt file should contain a single list of tuples:

             {debug_info, Mode, Module, Key}

       Mode is the type of crypto algorithm; currently, the only allowed value thus is  des3_cbc.
       Module  is  either  an atom, in which case Key will only be used for the module Module, or
       [], in which case Key will be used for all modules. Key is the non-empty key string.

       The Key in the first tuple where both Mode and Module matches will be used.

       Here is an example of an .erlang.crypt file that returns the same key for all modules:

       [{debug_info, des3_cbc, [], "%>7}|pc/DM6Cga*68$Mw]L#&_Gejr]G^"}].

       And here is a slightly more complicated example of an .erlang.crypt which provides one key
       for the module t, and another key for all other modules:

       [{debug_info, des3_cbc, t, "My KEY"},
        {debug_info, des3_cbc, [], "%>7}|pc/DM6Cga*68$Mw]L#&_Gejr]G^"}].

       Do not use any of the keys in these examples. Use your own keys.


       beam() = module() | file:filename() | binary()

              Each  of the functions described below accept either the module name, the filename,
              or a binary containing the beam module.

       chunkdata() = {chunkid(), dataB()}
                   | {abstract_code, abst_code()}
                   | {attributes, [attrib_entry()]}
                   | {compile_info, [compinfo_entry()]}
                   | {exports, [{atom(), arity()}]}
                   | {labeled_exports, [labeled_entry()]}
                   | {imports, [mfa()]}
                   | {indexed_imports,
                        Function :: atom(),
                   | {locals, [{atom(), arity()}]}
                   | {labeled_locals, [labeled_entry()]}
                   | {atoms, [{integer(), atom()}]}

              The list of attributes  is  sorted  on  Attribute  (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.

       chunkid() = nonempty_string()

              "Abst" | "Attr" | "CInf" | "ExpT" | "ImpT" | "LocT" | "Atom"

       dataB() = binary()

       abst_code() = {AbstVersion :: atom(), forms()}
                   | no_abstract_code

              It is not checked that the forms  conform  to  the  abstract  format  indicated  by
              AbstVersion. no_abstract_code means that the "Abst" chunk is present, but empty.

       forms() = [erl_parse:abstract_form()]

       compinfo_entry() = {InfoKey :: atom(), term()}

       attrib_entry() =
           {Attribute :: atom(), [AttributeValue :: term()]}

       labeled_entry() = {Function :: atom(), arity(), label()}

       index() = integer() >= 0

       label() = integer()

       chunkref() = chunkname() | chunkid()

       chunkname() = abstract_code
                   | attributes
                   | compile_info
                   | exports
                   | labeled_exports
                   | imports
                   | indexed_imports
                   | locals
                   | labeled_locals
                   | atoms

       chnk_rsn() = {unknown_chunk, file:filename(), atom()}
                  | {key_missing_or_invalid,
                  | info_rsn()

       info_rsn() = {chunk_too_big,
                     ChunkSize :: integer() >= 0,
                     FileSize :: integer() >= 0}
                  | {invalid_beam_file,
                     Position :: integer() >= 0}
                  | {invalid_chunk, file:filename(), chunkid()}
                  | {missing_chunk, file:filename(), chunkid()}
                  | {not_a_beam_file, file:filename()}
                  | {file_error, file:filename(), file:posix()}


       chunks(Beam, ChunkRefs) ->
                 {ok, {module(), [chunkdata()]}} |
                 {error, beam_lib, chnk_rsn()}


                 Beam = beam()
                 ChunkRefs = [chunkref()]

              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.

       chunks(Beam, ChunkRefs, Options) ->
                 {ok, {module(), [ChunkResult]}} |
                 {error, beam_lib, chnk_rsn()}


                 Beam = beam()
                 ChunkRefs = [chunkref()]
                 Options = [allow_missing_chunks]
                 ChunkResult = chunkdata()
                             | {ChunkRef :: chunkref(), missing_chunk}

              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.

              By  default, if any requested chunk is missing in Beam, an error tuple is returned.
              However, if the option allow_missing_chunks  has  been  given,  a  result  will  be
              returned even if chunks are missing. In the result list, any missing chunks will be
              represented as {ChunkRef,missing_chunk}. Note, however, that if the "Atom" chunk if
              missing,  that  is  considered  a fatal error and the return value will be an error

       version(Beam) ->
                  {ok, {module(), [Version :: term()]}} |
                  {error, beam_lib, chnk_rsn()}


                 Beam = beam()

              Returns the module version(s).  A  version  is  defined  by  the  module  attribute
              -vsn(Vsn). If this attribute is not specified, the version defaults to the checksum
              of the module. Note that if the version Vsn is not a list, it  is  made  into  one,
              that  is  {ok,{Module,[Vsn]}}  is  returned.  If  there  are  several  -vsn  module
              attributes, the result is the concatenated list of versions. Examples:

              1> beam_lib:version(a). % -vsn(1).
              2> beam_lib:version(b). % -vsn([1]).
              3> beam_lib:version(c). % -vsn([1]). -vsn(2).
              4> beam_lib:version(d). % no -vsn attribute

       md5(Beam) -> {ok, {module(), MD5}} | {error, beam_lib, chnk_rsn()}


                 Beam = beam()
                 MD5 = binary()

              Calculates an MD5 redundancy check for the code of the module (compilation date and
              other attributes are not included).

       info(Beam) -> [InfoPair] | {error, beam_lib, info_rsn()}


                 Beam = beam()
                 InfoPair = {file, Filename :: file:filename()}
                          | {binary, Binary :: binary()}
                          | {module, Module :: module()}
                          | {chunks,
                             [{ChunkId :: chunkid(),
                               Pos :: integer() >= 0,
                               Size :: integer() >= 0}]}

              Returns  a  list  containing  some  information  about a BEAM file as tuples {Item,

                {file, Filename} | {binary, Binary}:
                  The name (string) of the BEAM file, or the binary from  which  the  information
                  was extracted.

                {module, Module}:
                  The name (atom) of the module.

                {chunks, [{ChunkId, Pos, Size}]}:
                  For  each chunk, the identifier (string) and the position and size of the chunk
                  data, in bytes.

       cmp(Beam1, Beam2) -> ok | {error, beam_lib, cmp_rsn()}


                 Beam1 = Beam2 = beam()
                 cmp_rsn() = {modules_different, module(), module()}
                           | {chunks_different, chunkid()}
                           | different_chunks
                           | info_rsn()

              Compares the contents of two BEAM files. If the module names are the same, and  all
              chunks   except  for  the  "CInf"  chunk  (the  chunk  containing  the  compilation
              information  which  is  returned  by  Module:module_info(compile))  have  the  same
              contents in both files, ok is returned. Otherwise an error message is returned.

       cmp_dirs(Dir1, Dir2) ->
                   {Only1, Only2, Different} | {error, beam_lib, Reason}


                 Dir1 = Dir2 = atom() | file:filename()
                 Only1 = Only2 = [file:filename()]
                 Different =
                     [{Filename1 :: file:filename(), Filename2 :: file:filename()}]
                 Reason = {not_a_directory, term()} | info_rsn()

              The cmp_dirs/2 function compares the BEAM files in two directories. Only files with
              extension ".beam" are compared. BEAM files that exist in directory Dir1 (Dir2) only
              are  returned  in  Only1 (Only2). BEAM files that exist on both directories but are
              considered different by cmp/2 are returned as pairs  {Filename1,  Filename2}  where
              Filename1 (Filename2) exists in directory Dir1 (Dir2).

       diff_dirs(Dir1, Dir2) -> ok | {error, beam_lib, Reason}


                 Dir1 = Dir2 = atom() | file:filename()
                 Reason = {not_a_directory, term()} | info_rsn()

              The  diff_dirs/2  function  compares  the  BEAM  files  in  two directories the way
              cmp_dirs/2 does, but names of files  that  exist  in  only  one  directory  or  are
              different are presented on standard output.

       strip(Beam1) ->
                {ok, {module(), Beam2}} | {error, beam_lib, info_rsn()}


                 Beam1 = Beam2 = beam()

              The strip/1 function removes all chunks from a BEAM file except those needed by the
              loader. In particular, the debug information (abstract_code chunk) is removed.

       strip_files(Files) ->
                      {ok, [{module(), Beam}]} |
                      {error, beam_lib, info_rsn()}


                 Files = [beam()]
                 Beam = beam()

              The strip_files/1 function removes all chunks except those  needed  by  the  loader
              from  BEAM  files.  In  particular,  the debug information (abstract_code chunk) is
              removed. The returned list contains one element for each given file  name,  in  the
              same order as in Files.

       strip_release(Dir) ->
                        {ok, [{module(), file:filename()}]} |
                        {error, beam_lib, Reason}


                 Dir = atom() | file:filename()
                 Reason = {not_a_directory, term()} | info_rsn()

              The  strip_release/1  function removes all chunks except those needed by the loader
              from the BEAM files of a release. Dir should be the  installation  root  directory.
              For   example,   the   current   OTP   release   can  be  stripped  with  the  call

       format_error(Reason) -> io_lib:chars()


                 Reason = term()

              Given the error returned by any function in this module, the function  format_error
              returns a descriptive string of the error in English. For file errors, the function
              file:format_error(Posix) should be called.

       crypto_key_fun(CryptoKeyFun) -> ok | {error, Reason}


                 CryptoKeyFun = crypto_fun()
                 Reason = badfun | exists | term()
                 crypto_fun() = fun((crypto_fun_arg()) -> term())
                 crypto_fun_arg() = init
                                  | clear
                                  | {debug_info,
                 mode() = des3_cbc

              The crypto_key_fun/1 function registers a unary fun that will be called if beam_lib
              needs  to read an abstract_code chunk that has been encrypted. The fun is held in a
              process that is started by the function.

              If there already is a fun registered when attempting to  register  a  fun,  {error,
              exists} is returned.

              The fun must handle the following arguments:

                        CryptoKeyFun(init) -> ok | {ok, NewCryptoKeyFun} | {error, Term}

              Called  when  the  fun  is  registered, in the process that holds the fun. Here the
              crypto key fun can do any necessary initializations. If  {ok,  NewCryptoKeyFun}  is
              returned  then  NewCryptoKeyFun  will  be  registered  instead  of CryptoKeyFun. If
              {error, Term} is returned, the registration is aborted and crypto_key_fun/1 returns
              {error, Term} as well.

                        CryptoKeyFun({debug_info, Mode, Module, Filename}) -> Key

              Called  when  the  key  is needed for the module Module in the file named Filename.
              Mode is the type of crypto algorithm; currently, the only possible  value  thus  is
              des3_cbc. The call should fail (raise an exception) if there is no key available.

                        CryptoKeyFun(clear) -> term()

              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  clear_crypto_key_fun/0
              as part of its return value.

       clear_crypto_key_fun() -> undefined | {ok, Result}


                 Result = undefined | term()

              Unregisters  the  crypto  key fun and terminates the process holding it, started by

              The clear_crypto_key_fun/1 either returns {ok, undefined} if there  was  no  crypto
              key   fun  registered,  or  {ok,  Term},  where  Term  is  the  return  value  from
              CryptoKeyFun(clear), see crypto_key_fun/1.