beam_lib(3erl) Erlang Module Definition beam_lib(3erl)
NAME
beam_lib - An interface to the BEAM file format.
DESCRIPTION
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:
* atoms ("Atom")
* attributes ("Attr")
* compile_info ("CInf")
* debug_info ("Dbgi")
* 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(3erl))
to have debug information, such as Erlang Abstract Format, stored in
the debug_info 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 Erlang 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 com-
piler 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 er-
lang: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,Key}, see compile(3erl) and
function crypto_key_fun/1 to register a fun that returns the key
whenever beam_lib must decrypt the debug information.
If no such fun is registered, beam_lib instead searches for an .er-
lang.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, see
compile(3erl).
.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 con-
tains 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 mod-
ules. 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$Mw]L#&_Gejr]G^"}].
The following is a slightly more complicated example of an .er-
lang.crypt providing one key for module t and another key for all other
modules:
[{debug_info, des3_cbc, t, "My KEY"},
{debug_info, des3_cbc, [], "%>7}|pc/DM6Cga*68$Mw]L#&_Gejr]G^"}].
Note:
Do not use any of the keys in these examples. Use your own keys.
DATA TYPES
beam() = file:filename() | binary()
Each of the functions described below accept either the filename
(as a string) or a binary containing the BEAM module.
chunkdata() =
{chunkid(), dataB()} |
{abstract_code, abst_code()} |
{debug_info, debug_info()} |
{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_en-
try()) and each attribute name occurs once in the list. The at-
tribute values occur in the same order as in the file. The lists
of functions are also sorted.
chunkid() = nonempty_string()
"Attr" | "CInf" | "Dbgi" | "ExpT" | "ImpT" | "LocT" | "AtU8"
dataB() = binary()
debug_info() =
{DbgiVersion :: atom(), Backend :: module(), Data :: term()} |
no_debug_info
The format stored in the debug_info chunk. To retrieve particu-
lar code representation from the backend, Backend:de-
bug_info(Format, Module, Data, Opts) must be invoked. Format is
an atom, such as erlang_v1 for the Erlang Abstract Format or
core_v1 for Core Erlang. Module is the module represented by the
beam file and Data is the value stored in the debug info chunk.
Opts is any list of values supported by the Backend. Backend:de-
bug_info/4 must return {ok, Code} or {error, Term}.
Developers must always invoke the debug_info/4 function and
never rely on the Data stored in the debug_info chunk, as it is
opaque and may change at any moment. no_debug_info means that
chunk "Dbgi" is present, but empty.
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 chunk
"Abst" is present, but empty.
For modules compiled with OTP 20 onwards, the abst_code chunk is
automatically computed from the debug_info chunk.
forms() = [erl_parse:abstract_form() | erl_parse:form_info()]
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 | debug_info | 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 | debug_info} |
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()}
EXPORTS
all_chunks(File :: beam()) ->
{ok, beam_lib, [{chunkid(), dataB()}]} |
{error, beam_lib, info_rsn()}
Reads chunk data for all chunks.
build_module(Chunks) -> {ok, Binary}
Types:
Chunks = [{chunkid(), dataB()}]
Binary = binary()
Builds a BEAM module (as a binary) from a list of chunks.
chunks(Beam, ChunkRefs) ->
{ok, {module(), [chunkdata()]}} |
{error, beam_lib, chnk_rsn()}
Types:
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()}
Types:
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 Beam, 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
{ChunkRef,missing_chunk}. 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}
Types:
Result = undefined | term()
Unregisters the crypto key fun and terminates the process hold-
ing it, started by crypto_key_fun/1.
Returns either {ok, undefined} if no crypto key fun is regis-
tered, or {ok, Term}, where Term is the return value from Cryp-
toKeyFun(clear), see crypto_key_fun/1.
cmp(Beam1, Beam2) -> ok | {error, beam_lib, cmp_rsn()}
Types:
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 con-
taining the compilation information that is returned by Mod-
ule: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}
Types:
Dir1 = Dir2 = atom() | file:filename()
Only1 = Only2 = [file:filename()]
Different =
[{Filename1 :: file:filename(), Filename2 :: file:file-
name()}]
Reason = {not_a_directory, term()} | info_rsn()
Compares the BEAM files in two directories. Only files with ex-
tension ".beam" are compared. BEAM files that exist only in di-
rectory Dir1 (Dir2) are returned in Only1 (Only2). BEAM files
that exist in both directories but are considered different by
cmp/2 are returned as pairs {Filename1, Filename2}, where File-
name1 (Filename2) exists in directory Dir1 (Dir2).
crypto_key_fun(CryptoKeyFun) -> ok | {error, Reason}
Types:
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
debug_info 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 initializa-
tions. If {ok, NewCryptoKeyFun} is returned, NewCryptoKeyFun is
registered instead of CryptoKeyFun. If {error, Term} is re-
turned, 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}
Types:
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 dif-
ferent are presented on standard output.
format_error(Reason) -> io_lib:chars()
Types:
Reason = term()
For a specified error returned by any function in this module,
this function returns a descriptive string of the error in Eng-
lish. For file errors, function file:format_error(Posix) is to
be called.
info(Beam) -> [InfoPair] | {error, beam_lib, info_rsn()}
Types:
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()}
Types:
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()}
Types:
Beam1 = Beam2 = beam()
Removes all chunks from a BEAM file except those needed by the
loader. In particular, the debug information (chunk debug_info
and abstract_code) is removed.
strip(Beam1, AdditionalChunks) ->
{ok, {module(), Beam2}} | {error, beam_lib, info_rsn()}
Types:
Beam1 = beam()
AdditionalChunks = [chunkid()]
Beam2 = beam()
Removes all chunks from a BEAM file except those needed by the
loader or passed in. In particular, the debug information (chunk
debug_info and abstract_code) is removed.
strip_files(Files) ->
{ok, [{module(), Beam}]} |
{error, beam_lib, info_rsn()}
Types:
Files = [beam()]
Beam = beam()
Removes all chunks except those needed by the loader from BEAM
files. In particular, the debug information (chunk debug_info
and abstract_code) is removed. The returned list contains one
element for each specified filename, in the same order as in
Files.
strip_files(Files, AdditionalChunks) ->
{ok, [{module(), Beam}]} |
{error, beam_lib, info_rsn()}
Types:
Files = [beam()]
AdditionalChunks = [chunkid()]
Beam = beam()
Removes all chunks except those needed by the loader or passed
in from BEAM files. In particular, the debug information (chunk
debug_info and abstract_code) is removed. The returned list con-
tains one element for each specified filename, in the same order
as in Files.
strip_release(Dir) ->
{ok, [{module(), file:filename()}]} |
{error, beam_lib, Reason}
Types:
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. Dir is to be the installation root di-
rectory. For example, the current OTP release can be stripped
with the call beam_lib:strip_release(code:root_dir()).
strip_release(Dir, AdditionalChunks) ->
{ok, [{module(), file:filename()}]} |
{error, beam_lib, Reason}
Types:
Dir = atom() | file:filename()
AdditionalChunks = [chunkid()]
Reason = {not_a_directory, term()} | info_rsn()
Removes all chunks except those needed by the loader or passed
in from the BEAM files of a release. Dir is to be the installa-
tion 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()}
Types:
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]}}
Ericsson AB stdlib 3.13 beam_lib(3erl)