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1 24 jeremybenn
@section Symbols
2
BFD tries to maintain as much symbol information as it can when
3
it moves information from file to file. BFD passes information
4
to applications though the @code{asymbol} structure. When the
5
application requests the symbol table, BFD reads the table in
6
the native form and translates parts of it into the internal
7
format. To maintain more than the information passed to
8
applications, some targets keep some information ``behind the
9
scenes'' in a structure only the particular back end knows
10
about. For example, the coff back end keeps the original
11
symbol table structure as well as the canonical structure when
12
a BFD is read in. On output, the coff back end can reconstruct
13
the output symbol table so that no information is lost, even
14
information unique to coff which BFD doesn't know or
15
understand. If a coff symbol table were read, but were written
16
through an a.out back end, all the coff specific information
17
would be lost. The symbol table of a BFD
18
is not necessarily read in until a canonicalize request is
19
made. Then the BFD back end fills in a table provided by the
20
application with pointers to the canonical information.  To
21
output symbols, the application provides BFD with a table of
22
pointers to pointers to @code{asymbol}s. This allows applications
23
like the linker to output a symbol as it was read, since the ``behind
24
the scenes'' information will be still available.
25
@menu
26
* Reading Symbols::
27
* Writing Symbols::
28
* Mini Symbols::
29
* typedef asymbol::
30
* symbol handling functions::
31
@end menu
32
 
33
@node Reading Symbols, Writing Symbols, Symbols, Symbols
34
@subsection Reading symbols
35
There are two stages to reading a symbol table from a BFD:
36
allocating storage, and the actual reading process. This is an
37
excerpt from an application which reads the symbol table:
38
 
39
@example
40
         long storage_needed;
41
         asymbol **symbol_table;
42
         long number_of_symbols;
43
         long i;
44
 
45
         storage_needed = bfd_get_symtab_upper_bound (abfd);
46
 
47
         if (storage_needed < 0)
48
           FAIL
49
 
50
         if (storage_needed == 0)
51
           return;
52
 
53
         symbol_table = xmalloc (storage_needed);
54
           ...
55
         number_of_symbols =
56
            bfd_canonicalize_symtab (abfd, symbol_table);
57
 
58
         if (number_of_symbols < 0)
59
           FAIL
60
 
61
         for (i = 0; i < number_of_symbols; i++)
62
           process_symbol (symbol_table[i]);
63
@end example
64
 
65
All storage for the symbols themselves is in an objalloc
66
connected to the BFD; it is freed when the BFD is closed.
67
 
68
@node Writing Symbols, Mini Symbols, Reading Symbols, Symbols
69
@subsection Writing symbols
70
Writing of a symbol table is automatic when a BFD open for
71
writing is closed. The application attaches a vector of
72
pointers to pointers to symbols to the BFD being written, and
73
fills in the symbol count. The close and cleanup code reads
74
through the table provided and performs all the necessary
75
operations. The BFD output code must always be provided with an
76
``owned'' symbol: one which has come from another BFD, or one
77
which has been created using @code{bfd_make_empty_symbol}.  Here is an
78
example showing the creation of a symbol table with only one element:
79
 
80
@example
81
       #include "bfd.h"
82
       int main (void)
83
       @{
84
         bfd *abfd;
85
         asymbol *ptrs[2];
86
         asymbol *new;
87
 
88
         abfd = bfd_openw ("foo","a.out-sunos-big");
89
         bfd_set_format (abfd, bfd_object);
90
         new = bfd_make_empty_symbol (abfd);
91
         new->name = "dummy_symbol";
92
         new->section = bfd_make_section_old_way (abfd, ".text");
93
         new->flags = BSF_GLOBAL;
94
         new->value = 0x12345;
95
 
96
         ptrs[0] = new;
97
         ptrs[1] = 0;
98
 
99
         bfd_set_symtab (abfd, ptrs, 1);
100
         bfd_close (abfd);
101
         return 0;
102
       @}
103
 
104
       ./makesym
105
       nm foo
106
       00012345 A dummy_symbol
107
@end example
108
 
109
Many formats cannot represent arbitrary symbol information; for
110
instance, the @code{a.out} object format does not allow an
111
arbitrary number of sections. A symbol pointing to a section
112
which is not one  of @code{.text}, @code{.data} or @code{.bss} cannot
113
be described.
114
 
115
@node Mini Symbols, typedef asymbol, Writing Symbols, Symbols
116
@subsection Mini Symbols
117
Mini symbols provide read-only access to the symbol table.
118
They use less memory space, but require more time to access.
119
They can be useful for tools like nm or objdump, which may
120
have to handle symbol tables of extremely large executables.
121
 
122
The @code{bfd_read_minisymbols} function will read the symbols
123
into memory in an internal form.  It will return a @code{void *}
124
pointer to a block of memory, a symbol count, and the size of
125
each symbol.  The pointer is allocated using @code{malloc}, and
126
should be freed by the caller when it is no longer needed.
127
 
128
The function @code{bfd_minisymbol_to_symbol} will take a pointer
129
to a minisymbol, and a pointer to a structure returned by
130
@code{bfd_make_empty_symbol}, and return a @code{asymbol} structure.
131
The return value may or may not be the same as the value from
132
@code{bfd_make_empty_symbol} which was passed in.
133
 
134
 
135
@node typedef asymbol, symbol handling functions, Mini Symbols, Symbols
136
@subsection typedef asymbol
137
An @code{asymbol} has the form:
138
 
139
 
140
@example
141
 
142
typedef struct bfd_symbol
143
@{
144
  /* A pointer to the BFD which owns the symbol. This information
145
     is necessary so that a back end can work out what additional
146
     information (invisible to the application writer) is carried
147
     with the symbol.
148
 
149
     This field is *almost* redundant, since you can use section->owner
150
     instead, except that some symbols point to the global sections
151
     bfd_@{abs,com,und@}_section.  This could be fixed by making
152
     these globals be per-bfd (or per-target-flavor).  FIXME.  */
153
  struct bfd *the_bfd; /* Use bfd_asymbol_bfd(sym) to access this field.  */
154
 
155
  /* The text of the symbol. The name is left alone, and not copied; the
156
     application may not alter it.  */
157
  const char *name;
158
 
159
  /* The value of the symbol.  This really should be a union of a
160
     numeric value with a pointer, since some flags indicate that
161
     a pointer to another symbol is stored here.  */
162
  symvalue value;
163
 
164
  /* Attributes of a symbol.  */
165 225 jeremybenn
#define BSF_NO_FLAGS           0x00
166 24 jeremybenn
 
167
  /* The symbol has local scope; @code{static} in @code{C}. The value
168
     is the offset into the section of the data.  */
169 225 jeremybenn
#define BSF_LOCAL              (1 << 0)
170 24 jeremybenn
 
171
  /* The symbol has global scope; initialized data in @code{C}. The
172
     value is the offset into the section of the data.  */
173 225 jeremybenn
#define BSF_GLOBAL             (1 << 1)
174 24 jeremybenn
 
175
  /* The symbol has global scope and is exported. The value is
176
     the offset into the section of the data.  */
177
#define BSF_EXPORT     BSF_GLOBAL /* No real difference.  */
178
 
179
  /* A normal C symbol would be one of:
180 225 jeremybenn
     @code{BSF_LOCAL}, @code{BSF_COMMON},  @code{BSF_UNDEFINED} or
181 24 jeremybenn
     @code{BSF_GLOBAL}.  */
182
 
183
  /* The symbol is a debugging record. The value has an arbitrary
184
     meaning, unless BSF_DEBUGGING_RELOC is also set.  */
185 225 jeremybenn
#define BSF_DEBUGGING          (1 << 2)
186 24 jeremybenn
 
187
  /* The symbol denotes a function entry point.  Used in ELF,
188
     perhaps others someday.  */
189 225 jeremybenn
#define BSF_FUNCTION           (1 << 3)
190 24 jeremybenn
 
191
  /* Used by the linker.  */
192 225 jeremybenn
#define BSF_KEEP               (1 << 5)
193
#define BSF_KEEP_G             (1 << 6)
194 24 jeremybenn
 
195
  /* A weak global symbol, overridable without warnings by
196
     a regular global symbol of the same name.  */
197 225 jeremybenn
#define BSF_WEAK               (1 << 7)
198 24 jeremybenn
 
199
  /* This symbol was created to point to a section, e.g. ELF's
200
     STT_SECTION symbols.  */
201 225 jeremybenn
#define BSF_SECTION_SYM        (1 << 8)
202 24 jeremybenn
 
203
  /* The symbol used to be a common symbol, but now it is
204
     allocated.  */
205 225 jeremybenn
#define BSF_OLD_COMMON         (1 << 9)
206 24 jeremybenn
 
207
  /* In some files the type of a symbol sometimes alters its
208
     location in an output file - ie in coff a @code{ISFCN} symbol
209
     which is also @code{C_EXT} symbol appears where it was
210
     declared and not at the end of a section.  This bit is set
211
     by the target BFD part to convey this information.  */
212 225 jeremybenn
#define BSF_NOT_AT_END         (1 << 10)
213 24 jeremybenn
 
214
  /* Signal that the symbol is the label of constructor section.  */
215 225 jeremybenn
#define BSF_CONSTRUCTOR        (1 << 11)
216 24 jeremybenn
 
217
  /* Signal that the symbol is a warning symbol.  The name is a
218
     warning.  The name of the next symbol is the one to warn about;
219
     if a reference is made to a symbol with the same name as the next
220
     symbol, a warning is issued by the linker.  */
221 225 jeremybenn
#define BSF_WARNING            (1 << 12)
222 24 jeremybenn
 
223
  /* Signal that the symbol is indirect.  This symbol is an indirect
224
     pointer to the symbol with the same name as the next symbol.  */
225 225 jeremybenn
#define BSF_INDIRECT           (1 << 13)
226 24 jeremybenn
 
227
  /* BSF_FILE marks symbols that contain a file name.  This is used
228
     for ELF STT_FILE symbols.  */
229 225 jeremybenn
#define BSF_FILE               (1 << 14)
230 24 jeremybenn
 
231
  /* Symbol is from dynamic linking information.  */
232 225 jeremybenn
#define BSF_DYNAMIC            (1 << 15)
233 24 jeremybenn
 
234
  /* The symbol denotes a data object.  Used in ELF, and perhaps
235
     others someday.  */
236 225 jeremybenn
#define BSF_OBJECT             (1 << 16)
237 24 jeremybenn
 
238
  /* This symbol is a debugging symbol.  The value is the offset
239
     into the section of the data.  BSF_DEBUGGING should be set
240
     as well.  */
241 225 jeremybenn
#define BSF_DEBUGGING_RELOC    (1 << 17)
242 24 jeremybenn
 
243
  /* This symbol is thread local.  Used in ELF.  */
244 225 jeremybenn
#define BSF_THREAD_LOCAL       (1 << 18)
245 24 jeremybenn
 
246
  /* This symbol represents a complex relocation expression,
247
     with the expression tree serialized in the symbol name.  */
248 225 jeremybenn
#define BSF_RELC               (1 << 19)
249 24 jeremybenn
 
250
  /* This symbol represents a signed complex relocation expression,
251
     with the expression tree serialized in the symbol name.  */
252 225 jeremybenn
#define BSF_SRELC              (1 << 20)
253 24 jeremybenn
 
254 225 jeremybenn
  /* This symbol was created by bfd_get_synthetic_symtab.  */
255
#define BSF_SYNTHETIC          (1 << 21)
256
 
257
  /* This symbol is an indirect code object.  Unrelated to BSF_INDIRECT.
258
     The dynamic linker will compute the value of this symbol by
259
     calling the function that it points to.  BSF_FUNCTION must
260
     also be also set.  */
261
#define BSF_GNU_INDIRECT_FUNCTION (1 << 22)
262
  /* This symbol is a globally unique data object.  The dynamic linker
263
     will make sure that in the entire process there is just one symbol
264
     with this name and type in use.  BSF_OBJECT must also be set.  */
265
#define BSF_GNU_UNIQUE         (1 << 23)
266
 
267 24 jeremybenn
  flagword flags;
268
 
269
  /* A pointer to the section to which this symbol is
270
     relative.  This will always be non NULL, there are special
271
     sections for undefined and absolute symbols.  */
272
  struct bfd_section *section;
273
 
274
  /* Back end special data.  */
275
  union
276
    @{
277
      void *p;
278
      bfd_vma i;
279
    @}
280
  udata;
281
@}
282
asymbol;
283
 
284
@end example
285
 
286
@node symbol handling functions,  , typedef asymbol, Symbols
287
@subsection Symbol handling functions
288
 
289
 
290
@findex bfd_get_symtab_upper_bound
291
@subsubsection @code{bfd_get_symtab_upper_bound}
292
@strong{Description}@*
293
Return the number of bytes required to store a vector of pointers
294
to @code{asymbols} for all the symbols in the BFD @var{abfd},
295
including a terminal NULL pointer. If there are no symbols in
296
the BFD, then return 0.  If an error occurs, return -1.
297
@example
298
#define bfd_get_symtab_upper_bound(abfd) \
299
     BFD_SEND (abfd, _bfd_get_symtab_upper_bound, (abfd))
300
 
301
@end example
302
 
303
@findex bfd_is_local_label
304
@subsubsection @code{bfd_is_local_label}
305
@strong{Synopsis}
306
@example
307
bfd_boolean bfd_is_local_label (bfd *abfd, asymbol *sym);
308
@end example
309
@strong{Description}@*
310
Return TRUE if the given symbol @var{sym} in the BFD @var{abfd} is
311
a compiler generated local label, else return FALSE.
312
 
313
@findex bfd_is_local_label_name
314
@subsubsection @code{bfd_is_local_label_name}
315
@strong{Synopsis}
316
@example
317
bfd_boolean bfd_is_local_label_name (bfd *abfd, const char *name);
318
@end example
319
@strong{Description}@*
320
Return TRUE if a symbol with the name @var{name} in the BFD
321
@var{abfd} is a compiler generated local label, else return
322
FALSE.  This just checks whether the name has the form of a
323
local label.
324
@example
325
#define bfd_is_local_label_name(abfd, name) \
326
  BFD_SEND (abfd, _bfd_is_local_label_name, (abfd, name))
327
 
328
@end example
329
 
330
@findex bfd_is_target_special_symbol
331
@subsubsection @code{bfd_is_target_special_symbol}
332
@strong{Synopsis}
333
@example
334
bfd_boolean bfd_is_target_special_symbol (bfd *abfd, asymbol *sym);
335
@end example
336
@strong{Description}@*
337
Return TRUE iff a symbol @var{sym} in the BFD @var{abfd} is something
338
special to the particular target represented by the BFD.  Such symbols
339
should normally not be mentioned to the user.
340
@example
341
#define bfd_is_target_special_symbol(abfd, sym) \
342
  BFD_SEND (abfd, _bfd_is_target_special_symbol, (abfd, sym))
343
 
344
@end example
345
 
346
@findex bfd_canonicalize_symtab
347
@subsubsection @code{bfd_canonicalize_symtab}
348
@strong{Description}@*
349
Read the symbols from the BFD @var{abfd}, and fills in
350
the vector @var{location} with pointers to the symbols and
351
a trailing NULL.
352
Return the actual number of symbol pointers, not
353
including the NULL.
354
@example
355
#define bfd_canonicalize_symtab(abfd, location) \
356
  BFD_SEND (abfd, _bfd_canonicalize_symtab, (abfd, location))
357
 
358
@end example
359
 
360
@findex bfd_set_symtab
361
@subsubsection @code{bfd_set_symtab}
362
@strong{Synopsis}
363
@example
364
bfd_boolean bfd_set_symtab
365
   (bfd *abfd, asymbol **location, unsigned int count);
366
@end example
367
@strong{Description}@*
368
Arrange that when the output BFD @var{abfd} is closed,
369
the table @var{location} of @var{count} pointers to symbols
370
will be written.
371
 
372
@findex bfd_print_symbol_vandf
373
@subsubsection @code{bfd_print_symbol_vandf}
374
@strong{Synopsis}
375
@example
376
void bfd_print_symbol_vandf (bfd *abfd, void *file, asymbol *symbol);
377
@end example
378
@strong{Description}@*
379
Print the value and flags of the @var{symbol} supplied to the
380
stream @var{file}.
381
 
382
@findex bfd_make_empty_symbol
383
@subsubsection @code{bfd_make_empty_symbol}
384
@strong{Description}@*
385
Create a new @code{asymbol} structure for the BFD @var{abfd}
386
and return a pointer to it.
387
 
388
This routine is necessary because each back end has private
389
information surrounding the @code{asymbol}. Building your own
390
@code{asymbol} and pointing to it will not create the private
391
information, and will cause problems later on.
392
@example
393
#define bfd_make_empty_symbol(abfd) \
394
  BFD_SEND (abfd, _bfd_make_empty_symbol, (abfd))
395
 
396
@end example
397
 
398
@findex _bfd_generic_make_empty_symbol
399
@subsubsection @code{_bfd_generic_make_empty_symbol}
400
@strong{Synopsis}
401
@example
402
asymbol *_bfd_generic_make_empty_symbol (bfd *);
403
@end example
404
@strong{Description}@*
405
Create a new @code{asymbol} structure for the BFD @var{abfd}
406
and return a pointer to it.  Used by core file routines,
407
binary back-end and anywhere else where no private info
408
is needed.
409
 
410
@findex bfd_make_debug_symbol
411
@subsubsection @code{bfd_make_debug_symbol}
412
@strong{Description}@*
413
Create a new @code{asymbol} structure for the BFD @var{abfd},
414
to be used as a debugging symbol.  Further details of its use have
415
yet to be worked out.
416
@example
417
#define bfd_make_debug_symbol(abfd,ptr,size) \
418
  BFD_SEND (abfd, _bfd_make_debug_symbol, (abfd, ptr, size))
419
 
420
@end example
421
 
422
@findex bfd_decode_symclass
423
@subsubsection @code{bfd_decode_symclass}
424
@strong{Description}@*
425
Return a character corresponding to the symbol
426
class of @var{symbol}, or '?' for an unknown class.
427
 
428
@strong{Synopsis}
429
@example
430
int bfd_decode_symclass (asymbol *symbol);
431
@end example
432
@findex bfd_is_undefined_symclass
433
@subsubsection @code{bfd_is_undefined_symclass}
434
@strong{Description}@*
435
Returns non-zero if the class symbol returned by
436
bfd_decode_symclass represents an undefined symbol.
437
Returns zero otherwise.
438
 
439
@strong{Synopsis}
440
@example
441
bfd_boolean bfd_is_undefined_symclass (int symclass);
442
@end example
443
@findex bfd_symbol_info
444
@subsubsection @code{bfd_symbol_info}
445
@strong{Description}@*
446
Fill in the basic info about symbol that nm needs.
447
Additional info may be added by the back-ends after
448
calling this function.
449
 
450
@strong{Synopsis}
451
@example
452
void bfd_symbol_info (asymbol *symbol, symbol_info *ret);
453
@end example
454
@findex bfd_copy_private_symbol_data
455
@subsubsection @code{bfd_copy_private_symbol_data}
456
@strong{Synopsis}
457
@example
458
bfd_boolean bfd_copy_private_symbol_data
459
   (bfd *ibfd, asymbol *isym, bfd *obfd, asymbol *osym);
460
@end example
461
@strong{Description}@*
462
Copy private symbol information from @var{isym} in the BFD
463
@var{ibfd} to the symbol @var{osym} in the BFD @var{obfd}.
464
Return @code{TRUE} on success, @code{FALSE} on error.  Possible error
465
returns are:
466
 
467
@itemize @bullet
468
 
469
@item
470
@code{bfd_error_no_memory} -
471
Not enough memory exists to create private data for @var{osec}.
472
@end itemize
473
@example
474
#define bfd_copy_private_symbol_data(ibfd, isymbol, obfd, osymbol) \
475
  BFD_SEND (obfd, _bfd_copy_private_symbol_data, \
476
            (ibfd, isymbol, obfd, osymbol))
477
 
478
@end example
479
 

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