beam_lib
An interface to the BEAM file format.
This module 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 recognized names and the corresponding identifiers are as follows:
abstract_code ("Abst")
atoms ("Atom")
attributes ("Attr")
compile_info ("CInf")
exports ("ExpT")
imports ("ImpT")
indexed_imports ("ImpT")
labeled_exports ("ExpT")
labeled_locals ("LocT")
locals ("LocT")
Debug Information/Abstract Code
Option debug_info
can be specified to the Compiler (see
compile(3)
)
to have debug information in the form of abstract code (see section
The Abstract Format in the
ERTS User's Guide) stored in the abstract_code
chunk.
Tools such as Debugger and Xref require the debug information to
be included.
Warning!
Source code can be reconstructed from the debug information. To prevent this, use encrypted debug information (see below).
The debug information can also be removed from BEAM files
using strip/1
,
strip_files/1
, and/or
strip_release/1
.
Reconstruct Source Code
The following example shows 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 to keep the source code secret, but still be able to use tools such as Debugger or Xref.
To use encrypted debug information, a key must be provided to
the compiler and beam_lib
. The key is specified as a string.
It is recommended that the string contains at least 32 characters and
that both upper and lower case letters as well as digits and
special characters are used.
The default type (and currently the only type) of crypto
algorithm is des3_cbc
, three rounds of DES. The key string
is scrambled using
erlang:md5/1
to generate the keys used for des3_cbc
.
Note!
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.
The key can be provided in the following two ways:
-
Use Compiler option
{debug_info,Key}
, seecompile(3)
and functioncrypto_key_fun/1
to register a fun that returns the key wheneverbeam_lib
must decrypt the debug information.If no such fun is registered,
beam_lib
instead searches for an.erlang.crypt
file, see the next section. -
Store the key in a text file named
.erlang.crypt
.In this case, Compiler option
encrypt_debug_info
can be used, seecompile(3)
.
.erlang.crypt
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
implicitly creates a crypto key fun and registers it.
File .erlang.crypt
is to contain a single list of tuples:
{debug_info, Mode, Module, Key}
Mode
is the type of crypto algorithm; currently, the only
allowed value is des3_cbc
. Module
is either an
atom, in which case Key
is only used for the module
Module
, or []
, in which case Key
is
used for all modules. Key
is the non-empty key string.
Key
in the first tuple where both Mode
and
Module
match is used.
The following is an example of an .erlang.crypt
file that returns
the same key for all modules:
[{debug_info, des3_cbc, [], "%>7}|pc/DM6Cga*68$Mw]L#&_Gejr]G^"}].
The following is a slightly more complicated example of an
.erlang.crypt
providing one key for module
t
and another key for all other modules:
[{debug_info, des3_cbc, t, "My KEY"}, {debug_info, des3_cbc, [], "%>7}|pc/DM6Cga*68$Mw]L#&_Gejr]G^"}].
Note!
Do not use any of the keys in these examples. Use your own keys.
Types
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,
[{index(), module(), Function :: atom(), arity()}]} |
{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
. no_abstract_code
means that chunk "Abst"
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, file:filename(), abstract_code} |
info_rsn()
info_rsn() =
{chunk_too_big,
file:filename(),
chunkid(),
ChunkSize :: integer() >= 0,
FileSize :: integer() >= 0} |
{invalid_beam_file,
file:filename(),
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()}
Functions
all_chunks(File :: beam()) ->
{ok, beam_lib, [{chunkid(), dataB()}]}
Reads chunk data for all chunks.
build_module(Chunks) -> {ok, Binary}
Builds a BEAM module (as a binary) from a list of chunks.
chunks(Beam, ChunkRefs) ->
{ok, {module(), [chunkdata()]}} |
{error, beam_lib, chnk_rsn()}
Beam = beam()
ChunkRefs = [chunkref()]
Reads chunk data for selected chunks references. 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 references. 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
, an error
tuple is returned.
However, if option allow_missing_chunks
is specified,
a result is returned even if chunks are missing.
In the result list, any missing chunks are represented as
{
.
Notice however that if chunk "Atom"
is missing, that is
considered a fatal error and the return value is an error
tuple.
clear_crypto_key_fun() -> undefined | {ok, Result}
Result = undefined | term()
Unregisters the crypto key fun and terminates the process
holding it, started by
crypto_key_fun/1
.
Returns either {ok, undefined}
if no crypto key fun is
registered, or {ok, Term}
, where Term
is
the return value from CryptoKeyFun(clear)
, see
crypto_key_fun/1
.
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 chunk "CInf"
(the chunk containing the compilation information that 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()
Compares the BEAM files in
two directories. Only files with extension ".beam"
are
compared. BEAM files that exist only in directory
(
) are returned in
(
).
BEAM files that exist in both directories but
are considered different by cmp/2
are returned as
pairs {
,
},
where
(
)
exists in directory
(
).
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(), module(), file:filename()}
mode() = des3_cbc
Registers an unary fun
that is called if beam_lib
must read an
abstract_code
chunk that has been encrypted. The fun
is held in a process that is started by the function.
If a fun is already 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,
NewCryptoKeyFun
is registered instead of
CryptoKeyFun
. If {error, Term}
is returned,
the registration is aborted and crypto_key_fun/1
also returns {error, Term}
.
CryptoKeyFun({debug_info, Mode, Module, Filename}) -> Key
Called when the key is needed for module Module
in the file named Filename
. Mode
is the type of
crypto algorithm; currently, the only possible value is
des3_cbc
. The call is to fail (raise an exception) if
no key is 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.
diff_dirs(Dir1, Dir2) -> ok | {error, beam_lib, Reason}
Dir1 = Dir2 = atom() | file:filename()
Reason = {not_a_directory, term()} | info_rsn()
Compares the BEAM files in two directories as
cmp_dirs/2
, but the
names of files that exist in only one directory or are different are
presented on standard output.
format_error(Reason) -> io_lib:chars()
Reason = term()
For a specified error returned by any function in this module,
this function returns a descriptive string
of the error in English. For file errors, function
file:format_error(Posix)
is to be called.
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, Info}
:
{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.
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).
strip(Beam1) ->
{ok, {module(), Beam2}} | {error, beam_lib, info_rsn()}
Beam1 = Beam2 = beam()
Removes all chunks from a BEAM
file except those needed by the loader. In particular,
the debug information (chunk abstract_code
) is removed.
strip_files(Files) ->
{ok, [{module(), Beam}]} |
{error, beam_lib, info_rsn()}
Removes all chunks except
those needed by the loader from BEAM files. In particular,
the debug information (chunk abstract_code
) is removed.
The returned list contains one element for each specified filename,
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()
Removes all chunks
except those needed by the loader from the BEAM files of a
release.
is to be the installation root
directory. For example, the current OTP release can be
stripped with the call
beam_lib:strip_release(code:root_dir())
.
version(Beam) ->
{ok, {module(), [Version :: term()]}} |
{error, beam_lib, chnk_rsn()}
Beam = beam()
Returns the module version or versions. A version is defined by
module attribute -vsn(Vsn)
. If this attribute is
not specified, the version defaults to the checksum of
the module. Notice that if version Vsn
is not a list,
it is made into one, that is {ok,{Module,[Vsn]}}
is
returned. If there are many -vsn
module attributes,
the result is the concatenated list of versions.
Examples:
1>beam_lib:version(a).
% -vsn(1). {ok,{a,[1]}} 2>beam_lib:version(b).
% -vsn([1]). {ok,{b,[1]}} 3>beam_lib:version(c).
% -vsn([1]). -vsn(2). {ok,{c,[1,2]}} 4>beam_lib:version(d).
% no -vsn attribute {ok,{d,[275613208176997377698094100858909383631]}}