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26 .Dd October 21, 2007
27 .Os
28 .Dt ELF 3
29 .Sh NAME
30 .Nm elf
31 .Nd API for manipulating ELF objects
32 .Sh LIBRARY
33 .Lb libelf
34 .Sh SYNOPSIS
35 .In libelf.h
36 .Sh DESCRIPTION
37 The
38 .Lb libelf
39 provides functions that allow an application to read and manipulate
40 ELF object files, and to read
41 .Xr ar 1
42 archives.
43 The library allows the manipulation of ELF objects in a byte ordering
44 and word-size independent way, allowing an application to read and
45 create ELF objects for 32 and 64 bit architectures and for little-
46 and big-endian machines.
47 The library is capable of processing ELF objects that use extended
48 section numbering.
49 .Pp
50 This manual page serves to provide an overview of the functionality in
51 the ELF library.
52 Further information may found in the manual pages for individual
53 .Xr ELF 3
54 functions that comprise the library.
55 .Ss ELF Concepts
56 As described in
57 .Xr elf 5 ,
58 ELF files contain several data structures that are laid out in a
59 specific way.
60 ELF files begin with an
61 .Dq Executable Header ,
62 and may contain an optional
63 .Dq Program Header Table ,
64 and optional data in the form of ELF
65 .Dq sections .
66 A
67 .Dq Section Header Table
68 describes the content of the data in these sections.
69 .Pp
70 ELF objects have an associated
71 .Dq "ELF class"
72 which denotes the natural machine word size for the architecture
73 the object is associated with.
74 Objects for 32 bit architectures have an ELF class of
75 .Dv ELFCLASS32 .
76 Objects for 64 bit architectures have an ELF class of
77 .Dv ELFCLASS64 .
78 .Pp
79 ELF objects also have an associated
80 .Dq endianness
81 which denotes the endianness of the machine architecture associated
82 with the object.
83 This may be
84 .Dv ELFDATA2LSB
85 for little-endian architectures and
86 .Dv ELFDATA2MSB
87 for big-endian architectures.
88 .Pp
89 ELF objects are also associated with an API version number.
90 This version number determines the layout of the individual components
91 of an ELF file and the semantics associated with these.
92 .Ss Data Representation And Translation
93 The
94 .Xr ELF 3
95 library distinguishes between
96 .Dq native
97 representations of ELF data structures and their
98 .Dq file
99 representations.
100 .Pp
101 An application would work with ELF data in its
102 .Dq native
103 representation, i.e., using the native byteorder and alignment mandated
104 by the processor the application is running on.
105 The
106 .Dq file
107 representation of the same data could use a different byte ordering
108 and follow different constraints on object alignment than these native
109 constraints.
110 .Pp
111 Accordingly, the
112 .Xr ELF 3
113 library offers translation facilities
114 .Xr ( elf32_xlatetof 3 ,
115 .Xr elf32_xlatetom 3 ,
116 .Xr elf64_xlatetof 3
117 and
118 .Xr elf64_xlatetom 3 )
119 to and from these
120 representations and also provides higher-level APIs that retrieve and store
121 data from the ELF object in a transparent manner.
122 .Ss Library Working Version
123 Conceptually, there are three version numbers associated with an
124 application using the ELF library to manipulate ELF objects:
125 .Bl -bullet -compact -offset indent
126 .It
127 The ELF version that the application was compiled against.
128 This version determines the ABI expected by the application.
129 .It
130 The ELF version of the ELF object being manipulated by the
131 application through the ELF library.
132 .It
133 The ELF version (or set of versions) supported by the ELF library itself.
134 .El
135 .Pp
136 In order to facilitate working with ELF objects of differing versions,
137 the ELF library requires the application to call the
138 .Fn elf_version
139 function before invoking many of its operations, in order to inform
140 the library of the application's desired working version.
141 .Pp
142 In the current implementation, all three versions have to be
143 .Dv EV_CURRENT .
144 .Ss Namespace use
145 The ELF library uses the following prefixes:
146 .Bl -tag -width "ELF_F_*"
147 .It Dv elf_
148 Used for class-independent functions.
149 .It Dv elf32_
150 Used for functions working with 32 bit ELF objects.
151 .It Dv elf64_
152 Used for functions working with 64 bit ELF objects.
153 .It Dv Elf_
154 Used for class-independent data types.
155 .It Dv ELF_C_
156 Used for command values used in a few functions.
157 These symbols are defined as members of the
158 .Vt Dv Elf_Cmd
159 enumeration.
160 .It Dv ELF_E_
161 Used for error numbers.
162 .It Dv ELF_F_
163 Used for flags.
164 .It Dv ELF_K_
165 These constants define the kind of file associated with an ELF
166 descriptor.
167 See
168 .Xr elf_kind 3 .
169 The symbols are defined by the
170 .Vt Elf_Kind
171 enumeration.
172 .It Dv ELF_T_
173 These values are defined by the
174 .Vt Elf_Type
175 enumeration, and denote the types of ELF data structures
176 that can be present in an ELF object.
177 .El
178 .Pp
179 In addition, the library uses symbols with prefixes
180 .Dv _ELF
181 and
182 .Dv _libelf
183 for its internal use.
184 .Ss Descriptors
185 Applications communicate with the library using descriptors.
186 These are:
187 .Bl -tag -width ".Vt Elf_Data"
188 .It Vt Elf
189 An
190 .Vt Elf
191 descriptor represents an ELF object or an
192 .Xr ar 1
193 archive.
194 It is allocated using one of the
195 .Fn elf_begin
196 or
197 .Fn elf_memory
198 functions.
199 An
200 .Vt Elf
201 descriptor can be used to read and write data to an ELF file.
202 An
203 .Vt Elf
204 descriptor can be associated with zero or more
205 .Vt Elf_Scn
206 section descriptors.
207 .Pp
208 Given an ELF descriptor, the application may retrieve the ELF
209 object's class-dependent
210 .Dq "Executable Header"
211 structures using the
212 .Fn elf32_getehdr
213 or
214 .Fn elf64_getehdr
215 functions.
216 A new Ehdr structure may be allocated using the
217 .Fn elf64_newehdr
218 or
219 .Fn elf64_newehdr
220 functions.
221 .Pp
222 The
223 .Dq "Program Header Table"
224 associated with an ELF descriptor may be allocated using the
225 .Fn elf32_getphdr
226 or
227 .Fn elf64_getphdr
228 functions.
229 A new program header table may be allocated or an existing table
230 resized using the
231 .Fn elf32_newphdr
232 or
233 .Fn elf64_newphdr
234 functions.
235 .Pp
236 The
237 .Vt Elf
238 structure is opaque and has no members visible to the
239 application.
240 .\" TODO describe the Elf_Arhdr and Elf_Arsym structures.
241 .It Vt Elf_Data
242 An
243 .Vt Elf_Data
244 data structure describes an individual chunk of a ELF file as
245 represented in memory.
246 It has the following application visible members:
247 .Bl -tag -width ".Vt unsigned int d_version" -compact
248 .It Vt "uint64_t d_align"
249 The in-file alignment of the data buffer within its containing ELF section.
250 This value must be a power of two.
251 .It Vt "uint64_t d_off"
252 The offset with the containing section where this descriptors data
253 would be placed.
254 This field will be computed by the library unless the application
255 requests full control of the ELF object's layout.
256 .It Vt "uint64_t d_size"
257 The number of bytes of data in this descriptor.
258 .It Vt "void *d_buf"
259 A pointer to data in memory.
260 .It Vt "Elf_Type d_type"
261 The ELF type (see below) of the data in this descriptor.
262 .It Vt "unsigned int d_version"
263 The operating version for the data in this buffer.
264 .El
265 .Pp
266 .Vt Elf_Data
267 descriptors are usually associated with
268 .Vt Elf_Scn
269 descriptors.
270 Existing data descriptors associated with an ELF section may be
271 structures are retrieved using the
272 .Fn elf_getdata
273 function.
274 The
275 .Fn elf_newdata
276 function may be used to attach new data descriptors to an ELF section.
277 .It Vt Elf_Scn
278 .Vt Elf_Scn
279 descriptors represent a section in an ELF object.
280 .Pp
281 They are retrieved using the
282 .Fn elf_getscn
283 function.
284 An application may iterate through the existing sections of an ELF
285 object using the
286 .Fn elf_nextscn
287 function.
288 New sections may be allocated using the
289 .Fn elf_newscn
290 function.
291 .Pp
292 The
293 .Vt Elf_Scn
294 descriptor is opaque and contains no application modifiable fields.
295 .El
296 .Ss Supported Elf Types
297 The following ELF datatypes are supported by the library.
298 .Pp
299 .Bl -tag -width ".Dv ELF_T_SYMINFO" -compact
300 .It Dv ELF_T_ADDR
301 Machine addresses.
302 .It Dv ELF_T_BYTE
303 Byte data.
304 The library will not attempt to translate byte data.
305 .It Dv ELF_T_CAP
306 Software and hardware capability records.
307 .It Dv ELF_T_DYN
308 Records used in a section of type
309 .Dv SHT_DYNAMIC .
310 .It Dv ELF_T_EHDR
311 ELF executable header.
312 .It Dv ELF_T_HALF
313 16-bit unsigned words.
314 .It Dv ELF_T_LWORD
315 64 bit unsigned words.
316 .It Dv ELF_T_MOVE
317 ELF Move records.
318 .\".It Dv ELF_T_MOVEP
319 .\" As yet unsupported.
320 .It Dv ELF_T_NOTE
321 ELF Note structures.
322 .It Dv ELF_T_OFF
323 File offsets.
324 .It Dv ELF_T_PHDR
325 ELF program header table entries.
326 .It Dv ELF_T_REL
327 ELF relocation entries.
328 .It Dv ELF_T_RELA
329 ELF relocation entries with addends.
330 .It Dv ELF_T_SHDR
331 ELF section header entries.
332 .It Dv ELF_T_SWORD
333 Signed 32-bit words.
334 .It Dv ELF_T_SXWORD
335 Signed 64-bit words.
336 .It Dv ELF_T_SYMINFO
337 ELF symbol information.
338 .It Dv ELF_T_SYM
339 ELF symbol table entries.
340 .It Dv ELF_T_VDEF
341 Symbol version definition records.
342 .It Dv ELF_T_VNEED
343 Symbol version requirement records.
344 .It Dv ELF_T_WORD
345 Unsigned 32-bit words.
346 .It Dv ELF_T_XWORD
347 Unsigned 64-bit words.
348 .El
349 .Pp
350 The symbol
351 .Dv ELF_T_NUM
352 denotes the number of Elf types known to the library.
353 .Pp
354 The following table shows the mapping between ELF section types
355 defined in
356 .Xr elf 5
357 and the types supported by the library.
358 .Bl -column ".Dv SHT_PREINIT_ARRAY" ".Dv ELF_T_SYMINFO"
359 .It Em Section Type Ta Em "Library Type" Ta Em Description
360 .It Dv SHT_DYNAMIC Ta Dv ELF_T_DYN Ta Xo
361 .Sq .dynamic
362 section entries.
363 .Xc
364 .It Dv SHT_DYNSYM Ta Dv ELF_T_SYM Ta Symbols for dynamic linking.
365 .It Dv SHT_FINI_ARRAY Ta Dv ELF_T_ADDR Ta Termination function pointers.
366 .It Dv SHT_GROUP Ta Dv ELF_T_WORD Ta Section group marker.
367 .It Dv SHT_HASH Ta Dv ELF_T_HASH Ta Symbol hashes.
368 .It Dv SHT_INIT_ARRAY Ta Dv ELF_T_ADDR Ta Initialization function pointers.
369 .It Dv SHT_NOBITS Ta Dv ELF_T_BYTE Ta Xo
370 Empty sections.
371 See
372 .Xr elf 5 .
373 .Xc
374 .It Dv SHT_NOTE Ta Dv ELF_T_NOTE Ta ELF note records.
375 .It Dv SHT_PREINIT_ARRAY Ta Dv ELF_T_ADDR Ta Pre-initialization function pointers.
376 .It Dv SHT_PROGBITS Ta Dv ELF_T_BYTE Ta Machine code.
377 .It Dv SHT_REL Ta Dv ELF_T_REL Ta ELF relocation records.
378 .It Dv SHT_RELA Ta Dv ELF_T_RELA Ta Relocation records with addends.
379 .It Dv SHT_STRTAB Ta Dv ELF_T_BYTE Ta String tables.
380 .It Dv SHT_SYMTAB Ta Dv ELF_T_SYM Ta Symbol tables.
381 .It Dv SHT_SYMTAB_SHNDX Ta Dv ELF_T_WORD Ta Used with extended section numbering.
382 .It Dv SHT_GNU_verdef Ta Dv ELF_T_VDEF Ta Symbol version definitions.
383 .It Dv SHT_GNU_verneed Ta Dv ELF_T_VNEED Ta Symbol versioning requirements.
384 .It Dv SHT_GNU_versym Ta Dv ELF_T_HALF Ta Version symbols.
385 .It Dv SHT_SUNW_move Ta Dv ELF_T_MOVE Ta ELF move records.
386 .It Dv SHT_SUNW_syminfo Ta Dv ELF_T_SYMINFO Ta Additional symbol flags.
387 .El
388 .TE
389 .Ss Functional Grouping
390 This section contains a brief overview of the available functionality
391 in the ELF library.
392 Each function listed here is described further in its own manual page.
393 .Bl -tag -width indent
394 .It "Archive Access"
395 .Bl -tag -compact
396 .It Fn elf_getarsym
397 Retrieve the archive symbol table.
398 .It Fn elf_getarhdr
399 Retrieve the archive header for an object.
400 .It Fn elf_getbase
401 Retrieve the offset of a member inside an archive.
402 .It Fn elf_next
403 Iterate through an
404 .Xr ar 1
405 archive.
406 .It Fn elf_rand
407 Random access inside an
408 .Xr ar 1
409 archive.
410 .El
411 .It "Data Structures"
412 .Bl -tag -compact
413 .It Fn elf_getdata
414 Retrieve translated data for an ELF section.
415 .It Fn elf_getscn
416 Retrieve the section descriptor for a named section.
417 .It Fn elf_ndxscn
418 Retrieve the index for a section.
419 .It Fn elf_newdata
420 Add a new
421 .Vt Elf_Data
422 descriptor to an ELF section.
423 .It Fn elf_newscn
424 Add a new section descriptor to an ELF descriptor.
425 .It Fn elf_nextscn
426 Iterate through the sections in an ELF object.
427 .It Fn elf_rawdata
428 Retrieve untranslated data for an ELF sectino.
429 .It Fn elf_rawfile
430 Return a pointer to the untranslated file contents for an ELF object.
431 .It Fn elf32_getehdr , Fn elf64_getehdr
432 Retrieve the Executable Header in an ELF object.
433 .It Fn elf32_getphdr , Fn elf64_getphdr
434 Retrieve the Program Header Table in an ELF object.
435 .It Fn elf32_getshdr , Fn elf64_getshdr
436 Retrieve the ELF section header associated with an
437 .Vt Elf_Scn
438 descriptor.
439 .It Fn elf32_newehdr , Fn elf64_newehdr
440 Allocate an Executable Header in an ELF object.
441 .It Fn elf32_newphdr , Fn elf64_newphdr
442 Allocate or resize the Program Header Table in an ELF object.
443 .El
444 .It "Data Translation"
445 .Bl -tag -compact
446 .It Fn elf32_xlatetof , Fn elf64_xlatetof
447 Translate an ELF data structure from its native representation to its
448 file representation.
449 .It Fn elf32_xlatetom , Fn elf64_xlatetom
450 Translate an ELF data structure from its file representation to a
451 native representation.
452 .El
453 .It "Error Reporting"
454 .Bl -tag -compact
455 .It Fn elf_errno
456 Retrieve the current error.
457 .It Fn elf_errmsg
458 Retrieve a human readable description of the current error.
459 .El
460 .It "Initialization"
461 .Bl -tag -compact
462 .It Fn elf_begin
463 Opens an
464 .Xr ar 1
465 archive or ELF object given a file descriptor.
466 .It Fn elf_end
467 Close an ELF descriptor and release all its resources.
468 .It Fn elf_memory
469 Opens an
470 .Xr ar 1
471 archive or ELF object present in a memory arena.
472 .It Fn elf_version
473 Sets the operating version.
474 .El
475 .It "IO Control"
476 .Bl -tag -width ".Fn elf_setshstrndx" -compact
477 .It Fn elf_cntl
478 Manage the association between and ELF descriptor and its underlying file.
479 .It Fn elf_flagdata
480 Mark an
481 .Vt Elf_Data
482 descriptor as dirty.
483 .It Fn elf_flagehdr
484 Mark the ELF Executable Header in an ELF descriptor as dirty.
485 .It Fn elf_flagphdr
486 Mark the ELF Program Header Table in an ELF descriptor as dirty.
487 .It Fn elf_flagscn
488 Mark an
489 .Vt Elf_Scn
490 descriptor as dirty.
491 .It Fn elf_flagshdr
492 Mark an ELF Section Header as dirty.
493 .It Fn elf_setshstrndx
494 Set the index of the section name string table for the ELF object.
495 .It Fn elf_update
496 Recompute ELF object layout and optionally write the modified object
497 back to the underlying file.
498 .El
499 .It "Queries"
500 .Bl -tag -width ".Fn elf_getshstrndx" -compact
501 .It Fn elf32_checksum , Fn elf64_checkum
502 Compute checksum of an ELF object.
503 .It Fn elf_getident
504 Retrieve the identification bytes for an ELF object.
505 .It Fn elf_getshnum
506 Retrieve the number of sections in an ELF object.
507 .It Fn elf_getshstrndx
508 Retrieve the section index of the section name string table in
509 an ELF object.
510 .It Fn elf_hash
511 Compute the ELF hash value of a string.
512 .It Fn elf_kind
513 Query the kind of object associated with an ELF descriptor.
514 .It Fn elf32_fsize , Fn elf64_fsize
515 Return the size of the file representation of an ELF type.
516 .El
517 .El
518 .Ss Controlling ELF Object Layout
519 In the usual mode of operation, library will compute section
520 offsets and alignments based on the contents of an ELF descriptor's
521 sections without need for further intervention by the
522 application.
523 .Pp
524 However, if the application wishes to take complete charge of the
525 layout of the ELF file, it may set the
526 .Dv ELF_F_LAYOUT
527 flag on an ELF descriptor using
528 .Xr elf_flagelf 3 ,
529 following which the library will use the data offsets and alignments
530 specified by the application when laying out the file.
531 Application control of file layout is described further in the
532 .Xr elf_update 3
533 manual page.
534 .Pp
535 Gaps in between sections will be filled with the fill character
536 set by function
537 .Fn elf_fill .
538 .Ss Error Handling
539 In case an error is encountered, these library functions set an
540 internal error number and signal the presence of the error by
541 returning an special return value.
542 The application can check the
543 current error number by calling
544 .Xr elf_errno 3 .
545 A human readable description of the recorded error is available by
546 calling
547 .Xr elf_errmsg 3 .
548 .Ss Memory Management Rules
549 The library keeps track of all
550 .Vt Elf_Scn
551 and
552 .Vt Elf_Data
553 descriptors associated with an ELF descriptor and recovers them
554 when the descriptor is closed using
555 .Xr elf_end 3 .
556 Thus the application must not call
557 .Xr free 3
558 on data structures allocated by the ELF library.
559 .Pp
560 Conversely the library will not
561 free data that it has not allocated.
562 As an example, an application may call
563 .Xr elf_newdata 3
564 to allocate a new
565 .Vt Elf_Data
566 descriptor and can set the
567 .Va d_off
568 member of the descriptor to point to a region of memory allocated
569 using
570 .Xr malloc 3 .
571 It is the applications responsibility to free this arena, though the
572 library will reclaim the space used by the
573 .Vt Elf_Data
574 descriptor itself.
575 .Sh SEE ALSO
576 .Xr gelf 3 ,
577 .Xr elf 5
578 .Sh HISTORY
579 The original ELF(3) API was developed for Unix System V.
580 The current implementation of the ELF(3) API appeared in
581 .Fx 7.0 .
582 .Sh AUTHORS
583 The ELF library was written by
584 .An "Joseph Koshy"
585 .Aq jkoshy@FreeBSD.org .