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beam_libHans Bolinder1999-10-30PA1beam_libAn 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:
Option debug_info can be specified to the Compiler (see
compile(3))
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.
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 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.
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(3)
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 .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, see
compile(3).
.erlang.crypt
beam_lib searches for .erlang.crypt in the current
directory, then the
user's home directory and then
filename:basedir(user_config, "erlang").
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:
7}|pc/DM6Cga*68$Mw]L#&_Gejr]G^"}].]]>
The following is a slightly more complicated example of an
.erlang.crypt providing one key for module
t and another key for all other modules:
7}|pc/DM6Cga*68$Mw]L#&_Gejr]G^"}].]]>
Do not use any of the keys in these examples. Use your own keys.
Each of the functions described below accept either the
filename (as a string) or a binary containing the BEAM
module.
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.
The format stored in the debug_info chunk.
To retrieve particular code representation from the backend,
Backend:debug_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:debug_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.
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.
Read all chunks from a BEAM file or binary
Reads chunk data for all chunks.
Create a BEAM module from a list of chunks.
Builds a BEAM module (as a binary) from a list of chunks.
Read selected chunks from a BEAM file or binary.
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.
Read selected chunks from a BEAM file or binary.
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.
Unregister the current crypto key fun.
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.
Compare two BEAM files.
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.
Compare the BEAM files in two directories.
Compares the BEAM files in
two directories. Only files with extension ".beam" are
compared. BEAM files that exist only in directory
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 Filename1 (Filename2)
exists in directory Dir1
(Dir2).
Register a fun that provides a crypto key.
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
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}.
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.
Compare the BEAM files in two directories.
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.
Return an English description of a BEAM read error reply.
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.
Information about a BEAM file.
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.
Read the module version of the BEAM file.
Calculates an MD5 redundancy check for the code of the module
(compilation date and other attributes are not included).
Remove chunks not used by the loader from a BEAM file.
Removes all chunks from a BEAM
file except those used by the loader. In particular,
the debug information (chunk debug_info and abstract_code)
is removed.
Remove chunks not used by the loader from a BEAM file.
Removes all chunks from a BEAM file except those used by
the loader or mentioned in AdditionalChunks. In
particular, the debug information (chunk debug_info and
abstract_code) is removed.
Removes chunks not used by the loader from BEAM files.
Removes all chunks except
those used 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.
Removes chunks not used by the loader from BEAM files.
Removes all chunks except those used by the loader or
mentioned in AdditionalChunks. 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.
Remove chunks not used by the loader from all BEAM files of
a release.
Removes all chunks
except those used by the loader from the BEAM files of a
release. Dir 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()).
Remove chunks not needed by the loader from all BEAM files of
a release.
Removes all chunks except those used by the loader or
mentioned in AdditionalChunks.
Dir 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()).
Read the module version of the BEAM file.
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.