1beam_lib(3) Erlang Module Definition beam_lib(3)
2
6 beam_lib - An interface to the BEAM file format.
7
9 This module provides an interface to files created by the BEAM Compiler
10 ("BEAM files"). The format used, a variant of "EA IFF 1985" Standard
11 for Interchange Format Files, divides data into chunks.
12 Chunk data can be returned as binaries or as compound terms. Compound
14 terms are returned when chunks are referenced by names (atoms) rather
15 than identifiers (strings). The recognized names and the corresponding
16 identifiers are as follows:
17 * atoms ("Atom")
19 * attributes ("Attr")
21 * compile_info ("CInf")
23 * debug_info ("Dbgi")
25 * exports ("ExpT")
27 * imports ("ImpT")
29 * indexed_imports ("ImpT")
31 * labeled_exports ("ExpT")
33 * labeled_locals ("LocT")
35 * locals ("LocT")
37
39 Option debug_info can be specified to the Compiler (see compile(3)) to
40 have debug information, such as Erlang Abstract Format, stored in the
41 debug_info chunk. Tools such as Debugger and Xref require the debug
42 information to be included.
43 Warning:
45 Source code can be reconstructed from the debug information. To prevent
46 this, use encrypted debug information (see below).
47 The debug information can also be removed from BEAM files using
50 strip/1, strip_files/1, and/or strip_release/1.
51
53 The following example shows how to reconstruct Erlang source code from
54 the debug information in a BEAM file Beam:
55 {ok,{_,[{abstract_code,{_,AC}}]}} = beam_lib:chunks(Beam,[abstract_code]).
57 io:fwrite("~s~n", [erl_prettypr:format(erl_syntax:form_list(AC))]).
58
60 The debug information can be encrypted to keep the source code secret,
61 but still be able to use tools such as Debugger or Xref.
62 To use encrypted debug information, a key must be provided to the com‐
64 piler and beam_lib. The key is specified as a string. It is recommended
65 that the string contains at least 32 characters and that both upper and
66 lower case letters as well as digits and special characters are used.
67 The default type (and currently the only type) of crypto algorithm is
69 des3_cbc, three rounds of DES. The key string is scrambled using
70 erlang:md5/1 to generate the keys used for des3_cbc.
71 Note:
73 As far as we know by the time of writing, it is infeasible to break
74 des3_cbc encryption without any knowledge of the key. Therefore, as
75 long as the key is kept safe and is unguessable, the encrypted debug
76 information should be safe from intruders.
77 The key can be provided in the following two ways:
80 * Use Compiler option {debug_info_key,Key}, see compile(3) and func‐
82 tion crypto_key_fun/1 to register a fun that returns the key when‐
83 ever beam_lib must decrypt the debug information.
84 If no such fun is registered, beam_lib instead searches for an
86 .erlang.crypt file, see the next section.
87 * Store the key in a text file named .erlang.crypt.
89 In this case, Compiler option encrypt_debug_info can be used, see
91 compile(3).
92
94 beam_lib searches for .erlang.crypt in the current directory and then
95 the home directory for the current user. If the file is found and con‐
96 tains a key, beam_lib implicitly creates a crypto key fun and registers
97 it.
98 File .erlang.crypt is to contain a single list of tuples:
100 {debug_info, Mode, Module, Key}
102 Mode is the type of crypto algorithm; currently, the only allowed value
104 is des3_cbc. Module is either an atom, in which case Key is only used
105 for the module Module, or [], in which case Key is used for all mod‐
106 ules. Key is the non-empty key string.
107 Key in the first tuple where both Mode and Module match is used.
109 The following is an example of an .erlang.crypt file that returns the
111 same key for all modules:
112 [{debug_info, des3_cbc, [], "%>7}|pc/DM6Cga*68$Mw]L#&_Gejr]G^"}].
114 The following is a slightly more complicated example of an
116 .erlang.crypt providing one key for module t and another key for all
117 other modules:
118 [{debug_info, des3_cbc, t, "My KEY"},
120 {debug_info, des3_cbc, [], "%>7}|pc/DM6Cga*68$Mw]L#&_Gejr]G^"}].
121 Note:
123 Do not use any of the keys in these examples. Use your own keys.
124
127 beam() = file:filename() | binary()
128 Each of the functions described below accept either the filename
130 (as a string) or a binary containing the BEAM module.
131 chunkdata() =
133 {chunkid(), dataB()} |
134 {abstract_code, abst_code()} |
135 {debug_info, debug_info()} |
136 {attributes, [attrib_entry()]} |
137 {compile_info, [compinfo_entry()]} |
138 {exports, [{atom(), arity()}]} |
139 {labeled_exports, [labeled_entry()]} |
140 {imports, [mfa()]} |
141 {indexed_imports,
142 [{index(), module(), Function :: atom(), arity()}]} |
143 {locals, [{atom(), arity()}]} |
144 {labeled_locals, [labeled_entry()]} |
145 {atoms, [{integer(), atom()}]}
146 The list of attributes is sorted on Attribute (in
148 attrib_entry()) and each attribute name occurs once in the list.
149 The attribute values occur in the same order as in the file. The
150 lists of functions are also sorted.
151 chunkid() = nonempty_string()
153 "Attr" | "CInf" | "Dbgi" | "ExpT" | "ImpT" | "LocT" | "AtU8"
155 dataB() = binary()
157 debug_info() =
159 {DbgiVersion :: atom(), Backend :: module(), Data :: term()} |
160 no_debug_info
161 The format stored in the debug_info chunk. To retrieve particu‐
163 lar code representation from the backend, Back‐
164 end:debug_info(Format, Module, Data, Opts) must be invoked. For‐
165 mat is an atom, such as erlang_v1 for the Erlang Abstract Format
166 or core_v1 for Core Erlang. Module is the module represented by
167 the beam file and Data is the value stored in the debug info
168 chunk. Opts is any list of values supported by the Backend.
169 Backend:debug_info/4 must return {ok, Code} or {error, Term}.
170 Developers must always invoke the debug_info/4 function and
172 never rely on the Data stored in the debug_info chunk, as it is
173 opaque and may change at any moment. no_debug_info means that
174 chunk "Dbgi" is present, but empty.
175 abst_code() =
177 {AbstVersion :: atom(), forms()} | no_abstract_code
178 It is not checked that the forms conform to the abstract format
180 indicated by AbstVersion. no_abstract_code means that chunk
181 "Abst" is present, but empty.
182 For modules compiled with OTP 20 onwards, the abst_code chunk is
184 automatically computed from the debug_info chunk.
185 forms() = [erl_parse:abstract_form() | erl_parse:form_info()]
187 compinfo_entry() = {InfoKey :: atom(), term()}
189 attrib_entry() =
191 {Attribute :: atom(), [AttributeValue :: term()]}
192 labeled_entry() = {Function :: atom(), arity(), label()}
194 index() = integer() >= 0
196 label() = integer()
198 chunkref() = chunkname() | chunkid()
200 chunkname() =
202 abstract_code |
203 debug_info |
204 attributes |
205 compile_info |
206 exports |
207 labeled_exports |
208 imports |
209 indexed_imports |
210 locals |
211 labeled_locals |
212 atoms
213 chnk_rsn() =
215 {unknown_chunk, file:filename(), atom()} |
216 {key_missing_or_invalid,
217 file:filename(),
218 abstract_code | debug_info} |
219 info_rsn()
220 info_rsn() =
222 {chunk_too_big,
223 file:filename(),
224 chunkid(),
225 ChunkSize :: integer() >= 0,
226 FileSize :: integer() >= 0} |
227 {invalid_beam_file,
228 file:filename(),
229 Position :: integer() >= 0} |
230 {invalid_chunk, file:filename(), chunkid()} |
231 {missing_chunk, file:filename(), chunkid()} |
232 {not_a_beam_file, file:filename()} |
233 {file_error, file:filename(), file:posix()}
234
236 all_chunks(File :: beam()) ->
237 {ok, beam_lib, [{chunkid(), dataB()}]} |
238 {error, beam_lib, info_rsn()}
239 Reads chunk data for all chunks.
241 build_module(Chunks) -> {ok, Binary}
243 Types:
245 Chunks = [{chunkid(), dataB()}]
247 Binary = binary()
248 Builds a BEAM module (as a binary) from a list of chunks.
250 chunks(Beam, ChunkRefs) ->
252 {ok, {module(), [chunkdata()]}} |
253 {error, beam_lib, chnk_rsn()}
254 Types:
256 Beam = beam()
258 ChunkRefs = [chunkref()]
259 Reads chunk data for selected chunks references. The order of
261 the returned list of chunk data is determined by the order of
262 the list of chunks references.
263 chunks(Beam, ChunkRefs, Options) ->
265 {ok, {module(), [ChunkResult]}} |
266 {error, beam_lib, chnk_rsn()}
267 Types:
269 Beam = beam()
271 ChunkRefs = [chunkref()]
272 Options = [allow_missing_chunks]
273 ChunkResult =
274 chunkdata() | {ChunkRef :: chunkref(), missing_chunk}
275 Reads chunk data for selected chunks references. The order of
277 the returned list of chunk data is determined by the order of
278 the list of chunks references.
279 By default, if any requested chunk is missing in Beam, an error
281 tuple is returned. However, if option allow_missing_chunks is
282 specified, a result is returned even if chunks are missing. In
283 the result list, any missing chunks are represented as
284 {ChunkRef,missing_chunk}. Notice however that if chunk "Atom" is
285 missing, that is considered a fatal error and the return value
286 is an error tuple.
287 clear_crypto_key_fun() -> undefined | {ok, Result}
289 Types:
291 Result = undefined | term()
293 Unregisters the crypto key fun and terminates the process hold‐
295 ing it, started by crypto_key_fun/1.
296 Returns either {ok, undefined} if no crypto key fun is regis‐
298 tered, or {ok, Term}, where Term is the return value from Cryp‐
299 toKeyFun(clear), see crypto_key_fun/1.
300 cmp(Beam1, Beam2) -> ok | {error, beam_lib, cmp_rsn()}
302 Types:
304 Beam1 = Beam2 = beam()
306 cmp_rsn() =
307 {modules_different, module(), module()} |
308 {chunks_different, chunkid()} |
309 different_chunks |
310 info_rsn()
311 Compares the contents of two BEAM files. If the module names are
313 the same, and all chunks except for chunk "CInf" (the chunk con‐
314 taining the compilation information that is returned by Mod‐
315 ule:module_info(compile)) have the same contents in both files,
316 ok is returned. Otherwise an error message is returned.
317 cmp_dirs(Dir1, Dir2) ->
319 {Only1, Only2, Different} | {error, beam_lib, Reason}
320 Types:
322 Dir1 = Dir2 = atom() | file:filename()
324 Only1 = Only2 = [file:filename()]
325 Different =
326 [{Filename1 :: file:filename(), Filename2 :: file:file‐
327 name()}]
328 Reason = {not_a_directory, term()} | info_rsn()
329 Compares the BEAM files in two directories. Only files with
331 extension ".beam" are compared. BEAM files that exist only in
332 directory Dir1 (Dir2) are returned in Only1 (Only2). BEAM files
333 that exist in both directories but are considered different by
334 cmp/2 are returned as pairs {Filename1, Filename2}, where File‐
335 name1 (Filename2) exists in directory Dir1 (Dir2).
336 crypto_key_fun(CryptoKeyFun) -> ok | {error, Reason}
338 Types:
340 CryptoKeyFun = crypto_fun()
342 Reason = badfun | exists | term()
343 crypto_fun() = fun((crypto_fun_arg()) -> term())
344 crypto_fun_arg() =
345 init | clear | {debug_info, mode(), module(), file:filename()}
346 mode() = des3_cbc
347 Registers an unary fun that is called if beam_lib must read an
349 debug_info chunk that has been encrypted. The fun is held in a
350 process that is started by the function.
351 If a fun is already registered when attempting to register a
353 fun, {error, exists} is returned.
354 The fun must handle the following arguments:
356 CryptoKeyFun(init) -> ok | {ok, NewCryptoKeyFun} | {error, Term}
358 Called when the fun is registered, in the process that holds the
360 fun. Here the crypto key fun can do any necessary initializa‐
361 tions. If {ok, NewCryptoKeyFun} is returned, NewCryptoKeyFun is
362 registered instead of CryptoKeyFun. If {error, Term} is
363 returned, the registration is aborted and crypto_key_fun/1 also
364 returns {error, Term}.
365 CryptoKeyFun({debug_info, Mode, Module, Filename}) -> Key
367 Called when the key is needed for module Module in the file
369 named Filename. Mode is the type of crypto algorithm; currently,
370 the only possible value is des3_cbc. The call is to fail (raise
371 an exception) if no key is available.
372 CryptoKeyFun(clear) -> term()
374 Called before the fun is unregistered. Here any cleaning up can
376 be done. The return value is not important, but is passed back
377 to the caller of clear_crypto_key_fun/0 as part of its return
378 value.
379 diff_dirs(Dir1, Dir2) -> ok | {error, beam_lib, Reason}
381 Types:
383 Dir1 = Dir2 = atom() | file:filename()
385 Reason = {not_a_directory, term()} | info_rsn()
386 Compares the BEAM files in two directories as cmp_dirs/2, but
388 the names of files that exist in only one directory or are dif‐
389 ferent are presented on standard output.
390 format_error(Reason) -> io_lib:chars()
392 Types:
394 Reason = term()
396 For a specified error returned by any function in this module,
398 this function returns a descriptive string of the error in Eng‐
399 lish. For file errors, function file:format_error(Posix) is to
400 be called.
401 info(Beam) -> [InfoPair] | {error, beam_lib, info_rsn()}
403 Types:
405 Beam = beam()
407 InfoPair =
408 {file, Filename :: file:filename()} |
409 {binary, Binary :: binary()} |
410 {module, Module :: module()} |
411 {chunks,
412 [{ChunkId :: chunkid(),
413 Pos :: integer() >= 0,
414 Size :: integer() >= 0}]}
415 Returns a list containing some information about a BEAM file as
417 tuples {Item, Info}:
418 {file, Filename} | {binary, Binary}:
420 The name (string) of the BEAM file, or the binary from which
421 the information was extracted.
422 {module, Module}:
424 The name (atom) of the module.
425 {chunks, [{ChunkId, Pos, Size}]}:
427 For each chunk, the identifier (string) and the position and
428 size of the chunk data, in bytes.
429 md5(Beam) -> {ok, {module(), MD5}} | {error, beam_lib, chnk_rsn()}
431 Types:
433 Beam = beam()
435 MD5 = binary()
436 Calculates an MD5 redundancy check for the code of the module
438 (compilation date and other attributes are not included).
439 strip(Beam1) ->
441 {ok, {module(), Beam2}} | {error, beam_lib, info_rsn()}
442 Types:
444 Beam1 = Beam2 = beam()
446 Removes all chunks from a BEAM file except those needed by the
448 loader. In particular, the debug information (chunk debug_info
449 and abstract_code) is removed.
450 strip_files(Files) ->
452 {ok, [{module(), Beam}]} |
453 {error, beam_lib, info_rsn()}
454 Types:
456 Files = [beam()]
458 Beam = beam()
459 Removes all chunks except those needed by the loader from BEAM
461 files. In particular, the debug information (chunk debug_info
462 and abstract_code) is removed. The returned list contains one
463 element for each specified filename, in the same order as in
464 Files.
465 strip_release(Dir) ->
467 {ok, [{module(), file:filename()}]} |
468 {error, beam_lib, Reason}
469 Types:
471 Dir = atom() | file:filename()
473 Reason = {not_a_directory, term()} | info_rsn()
474 Removes all chunks except those needed by the loader from the
476 BEAM files of a release. Dir is to be the installation root
477 directory. For example, the current OTP release can be stripped
478 with the call beam_lib:strip_release(code:root_dir()).
479 version(Beam) ->
481 {ok, {module(), [Version :: term()]}} |
482 {error, beam_lib, chnk_rsn()}
483 Types:
485 Beam = beam()
487 Returns the module version or versions. A version is defined by
489 module attribute -vsn(Vsn). If this attribute is not specified,
490 the version defaults to the checksum of the module. Notice that
491 if version Vsn is not a list, it is made into one, that is
492 {ok,{Module,[Vsn]}} is returned. If there are many -vsn module
493 attributes, the result is the concatenated list of versions.
494 Examples:
496 1> beam_lib:version(a). % -vsn(1).
498 {ok,{a,[1]}}
499 2> beam_lib:version(b). % -vsn([1]).
500 {ok,{b,[1]}}
501 3> beam_lib:version(c). % -vsn([1]). -vsn(2).
502 {ok,{c,[1,2]}}
503 4> beam_lib:version(d). % no -vsn attribute
504 {ok,{d,[275613208176997377698094100858909383631]}}
505Ericsson AB stdlib 3.8.2.1 beam_lib(3)