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 in‐
42 formation 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 er‐
70 lang: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 .er‐
86 lang.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, then the
95 user's home directory and then filename:basedir(user_config, "erlang").
96 If the file is found and contains a key, beam_lib implicitly creates a
97 crypto key fun and registers 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$Mw]L#&_Gejr]G^"}].
114 The following is a slightly more complicated example of an .er‐
116 lang.crypt providing one key for module t and another key for all other
117 modules:
118 [{debug_info, des3_cbc, t, "My KEY"},
120 {debug_info, des3_cbc, [], "%>7}|pc/DM6Cga*68$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 attrib_en‐
148 try()) and each attribute name occurs once in the list. The at‐
149 tribute values occur in the same order as in the file. The lists
150 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, Backend:de‐
164 bug_info(Format, Module, Data, Opts) must be invoked. Format is
165 an atom, such as erlang_v1 for the Erlang Abstract Format or
166 core_v1 for Core Erlang. Module is the module represented by the
167 beam file and Data is the value stored in the debug info chunk.
168 Opts is any list of values supported by the Backend. Backend:de‐
169 bug_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 | debug_info | attributes | compile_info |
203 exports | labeled_exports | imports | indexed_imports |
204 locals | labeled_locals | atoms
205 chnk_rsn() =
207 {unknown_chunk, file:filename(), atom()} |
208 {key_missing_or_invalid,
209 file:filename(),
210 abstract_code | debug_info} |
211 {missing_backend, file:filename(), module()} |
212 info_rsn()
213 info_rsn() =
215 {chunk_too_big,
216 file:filename(),
217 chunkid(),
218 ChunkSize :: integer() >= 0,
219 FileSize :: integer() >= 0} |
220 {invalid_beam_file,
221 file:filename(),
222 Position :: integer() >= 0} |
223 {invalid_chunk, file:filename(), chunkid()} |
224 {missing_chunk, file:filename(), chunkid()} |
225 {not_a_beam_file, file:filename()} |
226 {file_error, file:filename(), file:posix()}
227
229 all_chunks(File :: beam()) ->
230 {ok, beam_lib, [{chunkid(), dataB()}]} |
231 {error, beam_lib, info_rsn()}
232 Reads chunk data for all chunks.
234 build_module(Chunks) -> {ok, Binary}
236 Types:
238 Chunks = [{chunkid(), dataB()}]
240 Binary = binary()
241 Builds a BEAM module (as a binary) from a list of chunks.
243 chunks(Beam, ChunkRefs) ->
245 {ok, {module(), [chunkdata()]}} |
246 {error, beam_lib, chnk_rsn()}
247 Types:
249 Beam = beam()
251 ChunkRefs = [chunkref()]
252 Reads chunk data for selected chunks references. The order of
254 the returned list of chunk data is determined by the order of
255 the list of chunks references.
256 chunks(Beam, ChunkRefs, Options) ->
258 {ok, {module(), [ChunkResult]}} |
259 {error, beam_lib, chnk_rsn()}
260 Types:
262 Beam = beam()
264 ChunkRefs = [chunkref()]
265 Options = [allow_missing_chunks]
266 ChunkResult =
267 chunkdata() | {ChunkRef :: chunkref(), missing_chunk}
268 Reads chunk data for selected chunks references. The order of
270 the returned list of chunk data is determined by the order of
271 the list of chunks references.
272 By default, if any requested chunk is missing in Beam, an error
274 tuple is returned. However, if option allow_missing_chunks is
275 specified, a result is returned even if chunks are missing. In
276 the result list, any missing chunks are represented as
277 {ChunkRef,missing_chunk}. Notice however that if chunk "Atom" is
278 missing, that is considered a fatal error and the return value
279 is an error tuple.
280 clear_crypto_key_fun() -> undefined | {ok, Result}
282 Types:
284 Result = undefined | term()
286 Unregisters the crypto key fun and terminates the process hold‐
288 ing it, started by crypto_key_fun/1.
289 Returns either {ok, undefined} if no crypto key fun is regis‐
291 tered, or {ok, Term}, where Term is the return value from Cryp‐
292 toKeyFun(clear), see crypto_key_fun/1.
293 cmp(Beam1, Beam2) -> ok | {error, beam_lib, cmp_rsn()}
295 Types:
297 Beam1 = Beam2 = beam()
299 cmp_rsn() =
300 {modules_different, module(), module()} |
301 {chunks_different, chunkid()} |
302 different_chunks |
303 info_rsn()
304 Compares the contents of two BEAM files. If the module names are
306 the same, and all chunks except for chunk "CInf" (the chunk con‐
307 taining the compilation information that is returned by Mod‐
308 ule:module_info(compile)) have the same contents in both files,
309 ok is returned. Otherwise an error message is returned.
310 cmp_dirs(Dir1, Dir2) ->
312 {Only1, Only2, Different} | {error, beam_lib, Reason}
313 Types:
315 Dir1 = Dir2 = atom() | file:filename()
317 Only1 = Only2 = [file:filename()]
318 Different =
319 [{Filename1 :: file:filename(), Filename2 :: file:file‐
320 name()}]
321 Reason = {not_a_directory, term()} | info_rsn()
322 Compares the BEAM files in two directories. Only files with ex‐
324 tension ".beam" are compared. BEAM files that exist only in di‐
325 rectory Dir1 (Dir2) are returned in Only1 (Only2). BEAM files
326 that exist in both directories but are considered different by
327 cmp/2 are returned as pairs {Filename1, Filename2}, where File‐
328 name1 (Filename2) exists in directory Dir1 (Dir2).
329 crypto_key_fun(CryptoKeyFun) -> ok | {error, Reason}
331 Types:
333 CryptoKeyFun = crypto_fun()
335 Reason = badfun | exists | term()
336 crypto_fun() = fun((crypto_fun_arg()) -> term())
337 crypto_fun_arg() =
338 init | clear | {debug_info, mode(), module(), file:filename()}
339 mode() = des3_cbc
340 Registers an unary fun that is called if beam_lib must read an
342 debug_info chunk that has been encrypted. The fun is held in a
343 process that is started by the function.
344 If a fun is already registered when attempting to register a
346 fun, {error, exists} is returned.
347 The fun must handle the following arguments:
349 CryptoKeyFun(init) -> ok | {ok, NewCryptoKeyFun} | {error, Term}
351 Called when the fun is registered, in the process that holds the
353 fun. Here the crypto key fun can do any necessary initializa‐
354 tions. If {ok, NewCryptoKeyFun} is returned, NewCryptoKeyFun is
355 registered instead of CryptoKeyFun. If {error, Term} is re‐
356 turned, the registration is aborted and crypto_key_fun/1 also
357 returns {error, Term}.
358 CryptoKeyFun({debug_info, Mode, Module, Filename}) -> Key
360 Called when the key is needed for module Module in the file
362 named Filename. Mode is the type of crypto algorithm; currently,
363 the only possible value is des3_cbc. The call is to fail (raise
364 an exception) if no key is available.
365 CryptoKeyFun(clear) -> term()
367 Called before the fun is unregistered. Here any cleaning up can
369 be done. The return value is not important, but is passed back
370 to the caller of clear_crypto_key_fun/0 as part of its return
371 value.
372 diff_dirs(Dir1, Dir2) -> ok | {error, beam_lib, Reason}
374 Types:
376 Dir1 = Dir2 = atom() | file:filename()
378 Reason = {not_a_directory, term()} | info_rsn()
379 Compares the BEAM files in two directories as cmp_dirs/2, but
381 the names of files that exist in only one directory or are dif‐
382 ferent are presented on standard output.
383 format_error(Reason) -> io_lib:chars()
385 Types:
387 Reason = term()
389 For a specified error returned by any function in this module,
391 this function returns a descriptive string of the error in Eng‐
392 lish. For file errors, function file:format_error(Posix) is to
393 be called.
394 info(Beam) -> [InfoPair] | {error, beam_lib, info_rsn()}
396 Types:
398 Beam = beam()
400 InfoPair =
401 {file, Filename :: file:filename()} |
402 {binary, Binary :: binary()} |
403 {module, Module :: module()} |
404 {chunks,
405 [{ChunkId :: chunkid(),
406 Pos :: integer() >= 0,
407 Size :: integer() >= 0}]}
408 Returns a list containing some information about a BEAM file as
410 tuples {Item, Info}:
411 {file, Filename} | {binary, Binary}:
413 The name (string) of the BEAM file, or the binary from which
414 the information was extracted.
415 {module, Module}:
417 The name (atom) of the module.
418 {chunks, [{ChunkId, Pos, Size}]}:
420 For each chunk, the identifier (string) and the position and
421 size of the chunk data, in bytes.
422 md5(Beam) -> {ok, {module(), MD5}} | {error, beam_lib, chnk_rsn()}
424 Types:
426 Beam = beam()
428 MD5 = binary()
429 Calculates an MD5 redundancy check for the code of the module
431 (compilation date and other attributes are not included).
432 strip(Beam1) ->
434 {ok, {module(), Beam2}} | {error, beam_lib, info_rsn()}
435 Types:
437 Beam1 = Beam2 = beam()
439 Removes all chunks from a BEAM file except those used by the
441 loader. In particular, the debug information (chunk debug_info
442 and abstract_code) is removed.
443 strip(Beam1, AdditionalChunks) ->
445 {ok, {module(), Beam2}} | {error, beam_lib, info_rsn()}
446 Types:
448 Beam1 = beam()
450 AdditionalChunks = [chunkid()]
451 Beam2 = beam()
452 Removes all chunks from a BEAM file except those used by the
454 loader or mentioned in AdditionalChunks. In particular, the de‐
455 bug information (chunk debug_info and abstract_code) is removed.
456 strip_files(Files) ->
458 {ok, [{module(), Beam}]} |
459 {error, beam_lib, info_rsn()}
460 Types:
462 Files = [beam()]
464 Beam = beam()
465 Removes all chunks except those used by the loader from BEAM
467 files. In particular, the debug information (chunk debug_info
468 and abstract_code) is removed. The returned list contains one
469 element for each specified filename, in the same order as in
470 Files.
471 strip_files(Files, AdditionalChunks) ->
473 {ok, [{module(), Beam}]} |
474 {error, beam_lib, info_rsn()}
475 Types:
477 Files = [beam()]
479 AdditionalChunks = [chunkid()]
480 Beam = beam()
481 Removes all chunks except those used by the loader or mentioned
483 in AdditionalChunks. In particular, the debug information (chunk
484 debug_info and abstract_code) is removed. The returned list con‐
485 tains one element for each specified filename, in the same order
486 as in Files.
487 strip_release(Dir) ->
489 {ok, [{module(), file:filename()}]} |
490 {error, beam_lib, Reason}
491 Types:
493 Dir = atom() | file:filename()
495 Reason = {not_a_directory, term()} | info_rsn()
496 Removes all chunks except those used by the loader from the BEAM
498 files of a release. Dir is to be the installation root direc‐
499 tory. For example, the current OTP release can be stripped with
500 the call beam_lib:strip_release(code:root_dir()).
501 strip_release(Dir, AdditionalChunks) ->
503 {ok, [{module(), file:filename()}]} |
504 {error, beam_lib, Reason}
505 Types:
507 Dir = atom() | file:filename()
509 AdditionalChunks = [chunkid()]
510 Reason = {not_a_directory, term()} | info_rsn()
511 Removes all chunks except those used by the loader or mentioned
513 in AdditionalChunks. Dir is to be the installation root direc‐
514 tory. For example, the current OTP release can be stripped with
515 the call beam_lib:strip_release(code:root_dir()).
516 version(Beam) ->
518 {ok, {module(), [Version :: term()]}} |
519 {error, beam_lib, chnk_rsn()}
520 Types:
522 Beam = beam()
524 Returns the module version or versions. A version is defined by
526 module attribute -vsn(Vsn). If this attribute is not specified,
527 the version defaults to the checksum of the module. Notice that
528 if version Vsn is not a list, it is made into one, that is
529 {ok,{Module,[Vsn]}} is returned. If there are many -vsn module
530 attributes, the result is the concatenated list of versions.
531 Examples:
533 1> beam_lib:version(a). % -vsn(1).
535 {ok,{a,[1]}}
536 2> beam_lib:version(b). % -vsn([1]).
537 {ok,{b,[1]}}
538 3> beam_lib:version(c). % -vsn([1]). -vsn(2).
539 {ok,{c,[1,2]}}
540 4> beam_lib:version(d). % no -vsn attribute
541 {ok,{d,[275613208176997377698094100858909383631]}}
542Ericsson AB stdlib 4.3.1.3 beam_lib(3)