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")
Debug Information/Abstract Code
The option debug_info
can be given to the compiler (see
compile(3))
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.
Warning!
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 used.
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
.
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.
There are two ways to provide the key:
-
Use the compiler option
{debug_info,Key}
, see compile(3), and the function crypto_key_fun/1 to register a fun which returns the key wheneverbeam_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(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
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^"}].
Note!
Do not use any of the keys in these examples. Use your own keys.
DATA TYPES
beam() -> Module | Filename | binary() Module = atom() Filename = string() | atom()
Each of the functions described below accept either the module name, the filename, or a binary containing the beam module.
chunkdata() = {ChunkId, DataB} | {ChunkName, DataT} ChunkId = chunkid() DataB = binary() {ChunkName, DataT} = {abstract_code, AbstractCode} | {attributes, [{Attribute, [AttributeValue]}]} | {compile_info, [{InfoKey, [InfoValue]}]} | {exports, [{Function, Arity}]} | {labeled_exports, [{Function, Arity, Label}]} | {imports, [{Module, Function, Arity}]} | {indexed_imports, [{Index, Module, Function, Arity}]} | {locals, [{Function, Arity}]}]} | {labeled_locals, [{Function, Arity, Label}]}]} | {atoms, [{integer(), atom()}]} AbstractCode = {AbstVersion, Forms} | no_abstract_code AbstVersion = atom() Attribute = atom() AttributeValue = term() Module = Function = atom() Arity = int() Label = int()
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.
The list of attributes is sorted on Attribute
, 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() = "Abst" | "Attr" | "CInf" | "ExpT" | "ImpT" | "LocT" | "Atom" chunkname() = abstract_code | attributes | compile_info | exports | labeled_exports | imports | indexed_imports | locals | labeled_locals | atoms chunkref() = chunkname() | chunkid()
Functions
chunks(Beam, [ChunkRef]) -> {ok, {Module, [ChunkData]}} | {error, beam_lib, Reason}
Beam = beam()
ChunkRef = chunkref()
Module = atom()
ChunkData = chunkdata()
Reason = {unknown_chunk, Filename, atom()}
| {key_missing_or_invalid, Filename, abstract_code}
| Reason1 -- see info/1
Filename = string()
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, [ChunkRef], [Option]) -> {ok, {Module, [ChunkResult]}} | {error, beam_lib, Reason}
Beam = beam()
ChunkRef = chunkref()
Module = atom()
Option = allow_missing_chunks
ChunkResult = {chunkref(), ChunkContents} | {chunkref(), missing_chunk}
Reason = {missing_chunk, Filename, atom()}
| {key_missing_or_invalid, Filename, abstract_code}
| Reason1 -- see info/1
Filename = string()
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
tuple.
version(Beam) -> {ok, {Module, [Version]}} | {error, beam_lib, Reason}
Beam = beam()
Module = atom()
Version = term()
Reason -- see chunks/2
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). {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]}}
md5(Beam) -> {ok, {Module, MD5}} | {error, beam_lib, Reason}
Beam = beam()
Module = atom()
MD5 = binary()
Reason -- see chunks/2
Calculates an MD5 redundancy check for the code of the module (compilation date and other attributes are not included).
info(Beam) -> [{Item, Info}] | {error, beam_lib, Reason1}
Beam = beam()
Item, Info -- see below
Reason1 = {chunk_too_big, Filename, ChunkId, ChunkSize, FileSize}
| {invalid_beam_file, Filename, Pos}
| {invalid_chunk, Filename, ChunkId}
| {missing_chunk, Filename, ChunkId}
| {not_a_beam_file, Filename}
| {file_error, Filename, Posix}
Filename = string()
ChunkId = chunkid()
ChunkSize = FileSize = int()
Pos = int()
Posix = posix() -- see file(3)
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.
cmp(Beam1, Beam2) -> ok | {error, beam_lib, Reason}
Beam1 = Beam2 = beam()
Reason = {modules_different, Module1, Module2}
| {chunks_different, ChunkId}
| Reason1 -- see info/1
Module1 = Module2 = atom()
ChunkId = chunkid()
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, Reason1}
Dir1 = Dir2 = string() | atom()
Different = [{Filename1, Filename2}]
Only1 = Only2 = [Filename]
Filename = Filename1 = Filename2 = string()
Reason1 = {not_a_directory, term()} | -- see info/1
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, Reason1}
Dir1 = Dir2 = string() | atom()
Reason1 = {not_a_directory, term()} | -- see info/1
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, Reason1}
Beam1 = Beam2 = beam()
Module = atom()
Reason1 -- see info/1
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, Beam2}]} | {error, beam_lib, Reason1}
Files = [Beam1]
Beam1 = beam()
Module = atom()
Beam2 = beam()
Reason1 -- see info/1
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, Filename]}} | {error, beam_lib, Reason1}
Dir = string() | atom()
Module = atom()
Filename = string()
Reason1 = {not_a_directory, term()} | -- see info/1
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
beam_lib:strip_release(code:root_dir())
.
format_error(Reason) -> Chars
Reason -- see other functions
Chars = [char() | Chars]
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 = fun() -- see below
Reason = badfun | exists | term()
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() -> {ok, Result}
Result = undefined | term()
Unregisters the crypto key fun and terminates the process
holding it, started by crypto_key_fun/1
.
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
.