2 The raw data contained within a BFD is maintained through the
3 section abstraction. A single BFD may have any number of
4 sections. It keeps hold of them by pointing to the first;
5 each one points to the next in the list.
7 Sections are supported in BFD in @code{section.c}.
13 * section prototypes::
16 @node Section Input, Section Output, Sections, Sections
17 @subsection Section input
18 When a BFD is opened for reading, the section structures are
19 created and attached to the BFD.
21 Each section has a name which describes the section in the
22 outside world---for example, @code{a.out} would contain at least
23 three sections, called @code{.text}, @code{.data} and @code{.bss}.
25 Names need not be unique; for example a COFF file may have several
26 sections named @code{.data}.
28 Sometimes a BFD will contain more than the ``natural'' number of
29 sections. A back end may attach other sections containing
30 constructor data, or an application may add a section (using
31 @code{bfd_make_section}) to the sections attached to an already open
32 BFD. For example, the linker creates an extra section
33 @code{COMMON} for each input file's BFD to hold information about
36 The raw data is not necessarily read in when
37 the section descriptor is created. Some targets may leave the
38 data in place until a @code{bfd_get_section_contents} call is
39 made. Other back ends may read in all the data at once. For
40 example, an S-record file has to be read once to determine the
41 size of the data. An IEEE-695 file doesn't contain raw data in
42 sections, but data and relocation expressions intermixed, so
43 the data area has to be parsed to get out the data and
46 @node Section Output, typedef asection, Section Input, Sections
47 @subsection Section output
48 To write a new object style BFD, the various sections to be
49 written have to be created. They are attached to the BFD in
50 the same way as input sections; data is written to the
51 sections using @code{bfd_set_section_contents}.
53 Any program that creates or combines sections (e.g., the assembler
54 and linker) must use the @code{asection} fields @code{output_section} and
55 @code{output_offset} to indicate the file sections to which each
56 section must be written. (If the section is being created from
57 scratch, @code{output_section} should probably point to the section
58 itself and @code{output_offset} should probably be zero.)
60 The data to be written comes from input sections attached
61 (via @code{output_section} pointers) to
62 the output sections. The output section structure can be
63 considered a filter for the input section: the output section
64 determines the vma of the output data and the name, but the
65 input section determines the offset into the output section of
66 the data to be written.
68 E.g., to create a section "O", starting at 0x100, 0x123 long,
69 containing two subsections, "A" at offset 0x0 (i.e., at vma
70 0x100) and "B" at offset 0x20 (i.e., at vma 0x120) the @code{asection}
71 structures would look like:
77 output_section -----------> section name "O"
79 section name "B" | size 0x123
82 output_section --------|
85 @subsection Link orders
86 The data within a section is stored in a @dfn{link_order}.
87 These are much like the fixups in @code{gas}. The link_order
88 abstraction allows a section to grow and shrink within itself.
90 A link_order knows how big it is, and which is the next
91 link_order and where the raw data for it is; it also points to
92 a list of relocations which apply to it.
94 The link_order is used by the linker to perform relaxing on
95 final code. The compiler creates code which is as big as
96 necessary to make it work without relaxing, and the user can
97 select whether to relax. Sometimes relaxing takes a lot of
98 time. The linker runs around the relocations to see if any
99 are attached to data which can be shrunk, if so it does it on
100 a link_order by link_order basis.
103 @node typedef asection, section prototypes, Section Output, Sections
104 @subsection typedef asection
105 Here is the section structure:
110 /* This structure is used for a comdat section, as in PE. A comdat
111 section is associated with a particular symbol. When the linker
112 sees a comdat section, it keeps only one of the sections with a
113 given name and associated with a given symbol. */
115 struct bfd_comdat_info
117 /* The name of the symbol associated with a comdat section. */
120 /* The local symbol table index of the symbol associated with a
121 comdat section. This is only meaningful to the object file format
122 specific code; it is not an index into the list returned by
123 bfd_canonicalize_symtab. */
129 /* The name of the section; the name isn't a copy, the pointer is
130 the same as that passed to bfd_make_section. */
133 /* A unique sequence number. */
136 /* Which section in the bfd; 0..n-1 as sections are created in a bfd. */
139 /* The next section in the list belonging to the BFD, or NULL. */
142 /* The field flags contains attributes of the section. Some
143 flags are read in from the object file, and some are
144 synthesized from other information. */
147 #define SEC_NO_FLAGS 0x000
149 /* Tells the OS to allocate space for this section when loading.
150 This is clear for a section containing debug information only. */
151 #define SEC_ALLOC 0x001
153 /* Tells the OS to load the section from the file when loading.
154 This is clear for a .bss section. */
155 #define SEC_LOAD 0x002
157 /* The section contains data still to be relocated, so there is
158 some relocation information too. */
159 #define SEC_RELOC 0x004
161 /* ELF reserves 4 processor specific bits and 8 operating system
162 specific bits in sh_flags; at present we can get away with just
163 one in communicating between the assembler and BFD, but this
164 isn't a good long-term solution. */
165 #define SEC_ARCH_BIT_0 0x008
167 /* A signal to the OS that the section contains read only data. */
168 #define SEC_READONLY 0x010
170 /* The section contains code only. */
171 #define SEC_CODE 0x020
173 /* The section contains data only. */
174 #define SEC_DATA 0x040
176 /* The section will reside in ROM. */
177 #define SEC_ROM 0x080
179 /* The section contains constructor information. This section
180 type is used by the linker to create lists of constructors and
181 destructors used by @code{g++}. When a back end sees a symbol
182 which should be used in a constructor list, it creates a new
183 section for the type of name (e.g., @code{__CTOR_LIST__}), attaches
184 the symbol to it, and builds a relocation. To build the lists
185 of constructors, all the linker has to do is catenate all the
186 sections called @code{__CTOR_LIST__} and relocate the data
187 contained within - exactly the operations it would peform on
189 #define SEC_CONSTRUCTOR 0x100
191 /* The section is a constructor, and should be placed at the
192 end of the text, data, or bss section(?). */
193 #define SEC_CONSTRUCTOR_TEXT 0x1100
194 #define SEC_CONSTRUCTOR_DATA 0x2100
195 #define SEC_CONSTRUCTOR_BSS 0x3100
197 /* The section has contents - a data section could be
198 @code{SEC_ALLOC} | @code{SEC_HAS_CONTENTS}; a debug section could be
199 @code{SEC_HAS_CONTENTS} */
200 #define SEC_HAS_CONTENTS 0x200
202 /* An instruction to the linker to not output the section
203 even if it has information which would normally be written. */
204 #define SEC_NEVER_LOAD 0x400
206 /* The section is a COFF shared library section. This flag is
207 only for the linker. If this type of section appears in
208 the input file, the linker must copy it to the output file
209 without changing the vma or size. FIXME: Although this
210 was originally intended to be general, it really is COFF
211 specific (and the flag was renamed to indicate this). It
212 might be cleaner to have some more general mechanism to
213 allow the back end to control what the linker does with
215 #define SEC_COFF_SHARED_LIBRARY 0x800
217 /* The section has GOT references. This flag is only for the
218 linker, and is currently only used by the elf32-hppa back end.
219 It will be set if global offset table references were detected
220 in this section, which indicate to the linker that the section
221 contains PIC code, and must be handled specially when doing a
223 #define SEC_HAS_GOT_REF 0x4000
225 /* The section contains common symbols (symbols may be defined
226 multiple times, the value of a symbol is the amount of
227 space it requires, and the largest symbol value is the one
228 used). Most targets have exactly one of these (which we
229 translate to bfd_com_section_ptr), but ECOFF has two. */
230 #define SEC_IS_COMMON 0x8000
232 /* The section contains only debugging information. For
233 example, this is set for ELF .debug and .stab sections.
234 strip tests this flag to see if a section can be
236 #define SEC_DEBUGGING 0x10000
238 /* The contents of this section are held in memory pointed to
239 by the contents field. This is checked by bfd_get_section_contents,
240 and the data is retrieved from memory if appropriate. */
241 #define SEC_IN_MEMORY 0x20000
243 /* The contents of this section are to be excluded by the
244 linker for executable and shared objects unless those
245 objects are to be further relocated. */
246 #define SEC_EXCLUDE 0x40000
248 /* The contents of this section are to be sorted based on the sum of
249 the symbol and addend values specified by the associated relocation
250 entries. Entries without associated relocation entries will be
251 appended to the end of the section in an unspecified order. */
252 #define SEC_SORT_ENTRIES 0x80000
254 /* When linking, duplicate sections of the same name should be
255 discarded, rather than being combined into a single section as
256 is usually done. This is similar to how common symbols are
257 handled. See SEC_LINK_DUPLICATES below. */
258 #define SEC_LINK_ONCE 0x100000
260 /* If SEC_LINK_ONCE is set, this bitfield describes how the linker
261 should handle duplicate sections. */
262 #define SEC_LINK_DUPLICATES 0x600000
264 /* This value for SEC_LINK_DUPLICATES means that duplicate
265 sections with the same name should simply be discarded. */
266 #define SEC_LINK_DUPLICATES_DISCARD 0x0
268 /* This value for SEC_LINK_DUPLICATES means that the linker
269 should warn if there are any duplicate sections, although
270 it should still only link one copy. */
271 #define SEC_LINK_DUPLICATES_ONE_ONLY 0x200000
273 /* This value for SEC_LINK_DUPLICATES means that the linker
274 should warn if any duplicate sections are a different size. */
275 #define SEC_LINK_DUPLICATES_SAME_SIZE 0x400000
277 /* This value for SEC_LINK_DUPLICATES means that the linker
278 should warn if any duplicate sections contain different
280 #define SEC_LINK_DUPLICATES_SAME_CONTENTS 0x600000
282 /* This section was created by the linker as part of dynamic
283 relocation or other arcane processing. It is skipped when
284 going through the first-pass output, trusting that someone
285 else up the line will take care of it later. */
286 #define SEC_LINKER_CREATED 0x800000
288 /* This section should not be subject to garbage collection. */
289 #define SEC_KEEP 0x1000000
291 /* This section contains "short" data, and should be placed
293 #define SEC_SMALL_DATA 0x2000000
295 /* This section contains data which may be shared with other
296 executables or shared objects. */
297 #define SEC_SHARED 0x4000000
299 /* When a section with this flag is being linked, then if the size of
300 the input section is less than a page, it should not cross a page
301 boundary. If the size of the input section is one page or more, it
302 should be aligned on a page boundary. */
303 #define SEC_BLOCK 0x8000000
305 /* Conditionally link this section; do not link if there are no
306 references found to any symbol in the section. */
307 #define SEC_CLINK 0x10000000
309 /* Attempt to merge identical entities in the section.
310 Entity size is given in the entsize field. */
311 #define SEC_MERGE 0x20000000
313 /* If given with SEC_MERGE, entities to merge are zero terminated
314 strings where entsize specifies character size instead of fixed
316 #define SEC_STRINGS 0x40000000
318 /* This section contains data about section groups. */
319 #define SEC_GROUP 0x80000000
321 /* End of section flags. */
323 /* Some internal packed boolean fields. */
325 /* See the vma field. */
326 unsigned int user_set_vma : 1;
328 /* Whether relocations have been processed. */
329 unsigned int reloc_done : 1;
331 /* A mark flag used by some of the linker backends. */
332 unsigned int linker_mark : 1;
334 /* Another mark flag used by some of the linker backends. Set for
335 output sections that have an input section. */
336 unsigned int linker_has_input : 1;
338 /* A mark flag used by some linker backends for garbage collection. */
339 unsigned int gc_mark : 1;
341 /* Used by the ELF code to mark sections which have been allocated
343 unsigned int segment_mark : 1;
345 /* End of internal packed boolean fields. */
347 /* The virtual memory address of the section - where it will be
348 at run time. The symbols are relocated against this. The
349 user_set_vma flag is maintained by bfd; if it's not set, the
350 backend can assign addresses (for example, in @code{a.out}, where
351 the default address for @code{.data} is dependent on the specific
352 target and various flags). */
355 /* The load address of the section - where it would be in a
356 rom image; really only used for writing section header
360 /* The size of the section in octets, as it will be output.
361 Contains a value even if the section has no contents (e.g., the
362 size of @code{.bss}). This will be filled in after relocation. */
363 bfd_size_type _cooked_size;
365 /* The original size on disk of the section, in octets. Normally this
366 value is the same as the size, but if some relaxing has
367 been done, then this value will be bigger. */
368 bfd_size_type _raw_size;
370 /* If this section is going to be output, then this value is the
371 offset in *bytes* into the output section of the first byte in the
372 input section (byte ==> smallest addressable unit on the
373 target). In most cases, if this was going to start at the
374 100th octet (8-bit quantity) in the output section, this value
375 would be 100. However, if the target byte size is 16 bits
376 (bfd_octets_per_byte is "2"), this value would be 50. */
377 bfd_vma output_offset;
379 /* The output section through which to map on output. */
380 struct sec *output_section;
382 /* The alignment requirement of the section, as an exponent of 2 -
383 e.g., 3 aligns to 2^3 (or 8). */
384 unsigned int alignment_power;
386 /* If an input section, a pointer to a vector of relocation
387 records for the data in this section. */
388 struct reloc_cache_entry *relocation;
390 /* If an output section, a pointer to a vector of pointers to
391 relocation records for the data in this section. */
392 struct reloc_cache_entry **orelocation;
394 /* The number of relocation records in one of the above. */
395 unsigned reloc_count;
397 /* Information below is back end specific - and not always used
400 /* File position of section data. */
403 /* File position of relocation info. */
404 file_ptr rel_filepos;
406 /* File position of line data. */
407 file_ptr line_filepos;
409 /* Pointer to data for applications. */
412 /* If the SEC_IN_MEMORY flag is set, this points to the actual
414 unsigned char *contents;
416 /* Attached line number information. */
419 /* Number of line number records. */
420 unsigned int lineno_count;
422 /* Entity size for merging purposes. */
423 unsigned int entsize;
425 /* Optional information about a COMDAT entry; NULL if not COMDAT. */
426 struct bfd_comdat_info *comdat;
428 /* When a section is being output, this value changes as more
429 linenumbers are written out. */
430 file_ptr moving_line_filepos;
432 /* What the section number is in the target world. */
437 /* If this is a constructor section then here is a list of the
438 relocations created to relocate items within it. */
439 struct relent_chain *constructor_chain;
441 /* The BFD which owns the section. */
444 /* A symbol which points at this section only. */
445 struct symbol_cache_entry *symbol;
446 struct symbol_cache_entry **symbol_ptr_ptr;
448 struct bfd_link_order *link_order_head;
449 struct bfd_link_order *link_order_tail;
452 /* These sections are global, and are managed by BFD. The application
453 and target back end are not permitted to change the values in
454 these sections. New code should use the section_ptr macros rather
455 than referring directly to the const sections. The const sections
456 may eventually vanish. */
457 #define BFD_ABS_SECTION_NAME "*ABS*"
458 #define BFD_UND_SECTION_NAME "*UND*"
459 #define BFD_COM_SECTION_NAME "*COM*"
460 #define BFD_IND_SECTION_NAME "*IND*"
462 /* The absolute section. */
463 extern const asection bfd_abs_section;
464 #define bfd_abs_section_ptr ((asection *) &bfd_abs_section)
465 #define bfd_is_abs_section(sec) ((sec) == bfd_abs_section_ptr)
466 /* Pointer to the undefined section. */
467 extern const asection bfd_und_section;
468 #define bfd_und_section_ptr ((asection *) &bfd_und_section)
469 #define bfd_is_und_section(sec) ((sec) == bfd_und_section_ptr)
470 /* Pointer to the common section. */
471 extern const asection bfd_com_section;
472 #define bfd_com_section_ptr ((asection *) &bfd_com_section)
473 /* Pointer to the indirect section. */
474 extern const asection bfd_ind_section;
475 #define bfd_ind_section_ptr ((asection *) &bfd_ind_section)
476 #define bfd_is_ind_section(sec) ((sec) == bfd_ind_section_ptr)
478 #define bfd_is_const_section(SEC) \
479 ( ((SEC) == bfd_abs_section_ptr) \
480 || ((SEC) == bfd_und_section_ptr) \
481 || ((SEC) == bfd_com_section_ptr) \
482 || ((SEC) == bfd_ind_section_ptr))
484 extern const struct symbol_cache_entry * const bfd_abs_symbol;
485 extern const struct symbol_cache_entry * const bfd_com_symbol;
486 extern const struct symbol_cache_entry * const bfd_und_symbol;
487 extern const struct symbol_cache_entry * const bfd_ind_symbol;
488 #define bfd_get_section_size_before_reloc(section) \
489 ((section)->reloc_done ? (abort (), (bfd_size_type) 1) \
490 : (section)->_raw_size)
491 #define bfd_get_section_size_after_reloc(section) \
492 ((section)->reloc_done ? (section)->_cooked_size \
493 : (abort (), (bfd_size_type) 1))
495 /* Macros to handle insertion and deletion of a bfd's sections. These
496 only handle the list pointers, ie. do not adjust section_count,
498 #define bfd_section_list_remove(ABFD, PS) \
501 asection **_ps = PS; \
502 asection *_s = *_ps; \
504 if (_s->next == NULL) \
505 (ABFD)->section_tail = _ps; \
508 #define bfd_section_list_insert(ABFD, PS, S) \
511 asection **_ps = PS; \
515 if (_s->next == NULL) \
516 (ABFD)->section_tail = &_s->next; \
522 @node section prototypes, , typedef asection, Sections
523 @subsection Section prototypes
524 These are the functions exported by the section handling part of BFD.
526 @findex bfd_section_list_clear
527 @subsubsection @code{bfd_section_list_clear}
530 void bfd_section_list_clear (bfd *);
532 @strong{Description}@*
533 Clears the section list, and also resets the section count and
536 @findex bfd_get_section_by_name
537 @subsubsection @code{bfd_get_section_by_name}
540 asection *bfd_get_section_by_name(bfd *abfd, const char *name);
542 @strong{Description}@*
543 Run through @var{abfd} and return the one of the
544 @code{asection}s whose name matches @var{name}, otherwise @code{NULL}.
545 @xref{Sections}, for more information.
547 This should only be used in special cases; the normal way to process
548 all sections of a given name is to use @code{bfd_map_over_sections} and
549 @code{strcmp} on the name (or better yet, base it on the section flags
550 or something else) for each section.
552 @findex bfd_get_unique_section_name
553 @subsubsection @code{bfd_get_unique_section_name}
556 char *bfd_get_unique_section_name(bfd *abfd,
560 @strong{Description}@*
561 Invent a section name that is unique in @var{abfd} by tacking
562 a dot and a digit suffix onto the original @var{templat}. If
563 @var{count} is non-NULL, then it specifies the first number
564 tried as a suffix to generate a unique name. The value
565 pointed to by @var{count} will be incremented in this case.
567 @findex bfd_make_section_old_way
568 @subsubsection @code{bfd_make_section_old_way}
571 asection *bfd_make_section_old_way(bfd *abfd, const char *name);
573 @strong{Description}@*
574 Create a new empty section called @var{name}
575 and attach it to the end of the chain of sections for the
576 BFD @var{abfd}. An attempt to create a section with a name which
577 is already in use returns its pointer without changing the
580 It has the funny name since this is the way it used to be
581 before it was rewritten....
587 @code{bfd_error_invalid_operation} -
588 If output has already started for this BFD.
590 @code{bfd_error_no_memory} -
591 If memory allocation fails.
594 @findex bfd_make_section_anyway
595 @subsubsection @code{bfd_make_section_anyway}
598 asection *bfd_make_section_anyway(bfd *abfd, const char *name);
600 @strong{Description}@*
601 Create a new empty section called @var{name} and attach it to the end of
602 the chain of sections for @var{abfd}. Create a new section even if there
603 is already a section with that name.
605 Return @code{NULL} and set @code{bfd_error} on error; possible errors are:
609 @code{bfd_error_invalid_operation} - If output has already started for @var{abfd}.
611 @code{bfd_error_no_memory} - If memory allocation fails.
614 @findex bfd_make_section
615 @subsubsection @code{bfd_make_section}
618 asection *bfd_make_section(bfd *, const char *name);
620 @strong{Description}@*
621 Like @code{bfd_make_section_anyway}, but return @code{NULL} (without calling
622 bfd_set_error ()) without changing the section chain if there is already a
623 section named @var{name}. If there is an error, return @code{NULL} and set
626 @findex bfd_set_section_flags
627 @subsubsection @code{bfd_set_section_flags}
630 boolean bfd_set_section_flags(bfd *abfd, asection *sec, flagword flags);
632 @strong{Description}@*
633 Set the attributes of the section @var{sec} in the BFD
634 @var{abfd} to the value @var{flags}. Return @code{true} on success,
635 @code{false} on error. Possible error returns are:
640 @code{bfd_error_invalid_operation} -
641 The section cannot have one or more of the attributes
642 requested. For example, a .bss section in @code{a.out} may not
643 have the @code{SEC_HAS_CONTENTS} field set.
646 @findex bfd_map_over_sections
647 @subsubsection @code{bfd_map_over_sections}
650 void bfd_map_over_sections(bfd *abfd,
651 void (*func) (bfd *abfd,
656 @strong{Description}@*
657 Call the provided function @var{func} for each section
658 attached to the BFD @var{abfd}, passing @var{obj} as an
659 argument. The function will be called as if by
662 func(abfd, the_section, obj);
665 This is the prefered method for iterating over sections; an
666 alternative would be to use a loop:
670 for (p = abfd->sections; p != NULL; p = p->next)
674 @findex bfd_set_section_size
675 @subsubsection @code{bfd_set_section_size}
678 boolean bfd_set_section_size(bfd *abfd, asection *sec, bfd_size_type val);
680 @strong{Description}@*
681 Set @var{sec} to the size @var{val}. If the operation is
682 ok, then @code{true} is returned, else @code{false}.
684 Possible error returns:
688 @code{bfd_error_invalid_operation} -
689 Writing has started to the BFD, so setting the size is invalid.
692 @findex bfd_set_section_contents
693 @subsubsection @code{bfd_set_section_contents}
696 boolean bfd_set_section_contents (bfd *abfd, asection *section,
697 PTR data, file_ptr offset,
698 bfd_size_type count);
700 @strong{Description}@*
701 Sets the contents of the section @var{section} in BFD
702 @var{abfd} to the data starting in memory at @var{data}. The
703 data is written to the output section starting at offset
704 @var{offset} for @var{count} octets.
706 Normally @code{true} is returned, else @code{false}. Possible error
711 @code{bfd_error_no_contents} -
712 The output section does not have the @code{SEC_HAS_CONTENTS}
713 attribute, so nothing can be written to it.
717 This routine is front end to the back end function
718 @code{_bfd_set_section_contents}.
720 @findex bfd_get_section_contents
721 @subsubsection @code{bfd_get_section_contents}
724 boolean bfd_get_section_contents (bfd *abfd, asection *section,
725 PTR location, file_ptr offset,
726 bfd_size_type count);
728 @strong{Description}@*
729 Read data from @var{section} in BFD @var{abfd}
730 into memory starting at @var{location}. The data is read at an
731 offset of @var{offset} from the start of the input section,
732 and is read for @var{count} bytes.
734 If the contents of a constructor with the @code{SEC_CONSTRUCTOR}
735 flag set are requested or if the section does not have the
736 @code{SEC_HAS_CONTENTS} flag set, then the @var{location} is filled
737 with zeroes. If no errors occur, @code{true} is returned, else
740 @findex bfd_copy_private_section_data
741 @subsubsection @code{bfd_copy_private_section_data}
744 boolean bfd_copy_private_section_data (bfd *ibfd, asection *isec,
745 bfd *obfd, asection *osec);
747 @strong{Description}@*
748 Copy private section information from @var{isec} in the BFD
749 @var{ibfd} to the section @var{osec} in the BFD @var{obfd}.
750 Return @code{true} on success, @code{false} on error. Possible error
756 @code{bfd_error_no_memory} -
757 Not enough memory exists to create private data for @var{osec}.
760 #define bfd_copy_private_section_data(ibfd, isection, obfd, osection) \
761 BFD_SEND (obfd, _bfd_copy_private_section_data, \
762 (ibfd, isection, obfd, osection))
765 @findex _bfd_strip_section_from_output
766 @subsubsection @code{_bfd_strip_section_from_output}
769 void _bfd_strip_section_from_output
770 (struct bfd_link_info *info, asection *section);
772 @strong{Description}@*
773 Remove @var{section} from the output. If the output section
774 becomes empty, remove it from the output bfd. @var{info} may
775 be NULL; if it is not, it is used to decide whether the output