syms.texi revision 1.3
1@section Symbols 2BFD tries to maintain as much symbol information as it can when 3it moves information from file to file. BFD passes information 4to applications though the @code{asymbol} structure. When the 5application requests the symbol table, BFD reads the table in 6the native form and translates parts of it into the internal 7format. To maintain more than the information passed to 8applications, some targets keep some information ``behind the 9scenes'' in a structure only the particular back end knows 10about. For example, the coff back end keeps the original 11symbol table structure as well as the canonical structure when 12a BFD is read in. On output, the coff back end can reconstruct 13the output symbol table so that no information is lost, even 14information unique to coff which BFD doesn't know or 15understand. If a coff symbol table were read, but were written 16through an a.out back end, all the coff specific information 17would be lost. The symbol table of a BFD 18is not necessarily read in until a canonicalize request is 19made. Then the BFD back end fills in a table provided by the 20application with pointers to the canonical information. To 21output symbols, the application provides BFD with a table of 22pointers to pointers to @code{asymbol}s. This allows applications 23like the linker to output a symbol as it was read, since the ``behind 24the 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 35There are two stages to reading a symbol table from a BFD: 36allocating storage, and the actual reading process. This is an 37excerpt 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 65All storage for the symbols themselves is in an objalloc 66connected 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 70Writing of a symbol table is automatic when a BFD open for 71writing is closed. The application attaches a vector of 72pointers to pointers to symbols to the BFD being written, and 73fills in the symbol count. The close and cleanup code reads 74through the table provided and performs all the necessary 75operations. The BFD output code must always be provided with an 76``owned'' symbol: one which has come from another BFD, or one 77which has been created using @code{bfd_make_empty_symbol}. Here is an 78example showing the creation of a symbol table with only one element: 79 80@example 81 #include "sysdep.h" 82 #include "bfd.h" 83 int main (void) 84 @{ 85 bfd *abfd; 86 asymbol *ptrs[2]; 87 asymbol *new; 88 89 abfd = bfd_openw ("foo","a.out-sunos-big"); 90 bfd_set_format (abfd, bfd_object); 91 new = bfd_make_empty_symbol (abfd); 92 new->name = "dummy_symbol"; 93 new->section = bfd_make_section_old_way (abfd, ".text"); 94 new->flags = BSF_GLOBAL; 95 new->value = 0x12345; 96 97 ptrs[0] = new; 98 ptrs[1] = 0; 99 100 bfd_set_symtab (abfd, ptrs, 1); 101 bfd_close (abfd); 102 return 0; 103 @} 104 105 ./makesym 106 nm foo 107 00012345 A dummy_symbol 108@end example 109 110Many formats cannot represent arbitrary symbol information; for 111instance, the @code{a.out} object format does not allow an 112arbitrary number of sections. A symbol pointing to a section 113which is not one of @code{.text}, @code{.data} or @code{.bss} cannot 114be described. 115 116@node Mini Symbols, typedef asymbol, Writing Symbols, Symbols 117@subsection Mini Symbols 118Mini symbols provide read-only access to the symbol table. 119They use less memory space, but require more time to access. 120They can be useful for tools like nm or objdump, which may 121have to handle symbol tables of extremely large executables. 122 123The @code{bfd_read_minisymbols} function will read the symbols 124into memory in an internal form. It will return a @code{void *} 125pointer to a block of memory, a symbol count, and the size of 126each symbol. The pointer is allocated using @code{malloc}, and 127should be freed by the caller when it is no longer needed. 128 129The function @code{bfd_minisymbol_to_symbol} will take a pointer 130to a minisymbol, and a pointer to a structure returned by 131@code{bfd_make_empty_symbol}, and return a @code{asymbol} structure. 132The return value may or may not be the same as the value from 133@code{bfd_make_empty_symbol} which was passed in. 134 135 136@node typedef asymbol, symbol handling functions, Mini Symbols, Symbols 137@subsection typedef asymbol 138An @code{asymbol} has the form: 139 140 141@example 142 143typedef struct bfd_symbol 144@{ 145 /* A pointer to the BFD which owns the symbol. This information 146 is necessary so that a back end can work out what additional 147 information (invisible to the application writer) is carried 148 with the symbol. 149 150 This field is *almost* redundant, since you can use section->owner 151 instead, except that some symbols point to the global sections 152 bfd_@{abs,com,und@}_section. This could be fixed by making 153 these globals be per-bfd (or per-target-flavor). FIXME. */ 154 struct bfd *the_bfd; /* Use bfd_asymbol_bfd(sym) to access this field. */ 155 156 /* The text of the symbol. The name is left alone, and not copied; the 157 application may not alter it. */ 158 const char *name; 159 160 /* The value of the symbol. This really should be a union of a 161 numeric value with a pointer, since some flags indicate that 162 a pointer to another symbol is stored here. */ 163 symvalue value; 164 165 /* Attributes of a symbol. */ 166#define BSF_NO_FLAGS 0x00 167 168 /* The symbol has local scope; @code{static} in @code{C}. The value 169 is the offset into the section of the data. */ 170#define BSF_LOCAL (1 << 0) 171 172 /* The symbol has global scope; initialized data in @code{C}. The 173 value is the offset into the section of the data. */ 174#define BSF_GLOBAL (1 << 1) 175 176 /* The symbol has global scope and is exported. The value is 177 the offset into the section of the data. */ 178#define BSF_EXPORT BSF_GLOBAL /* No real difference. */ 179 180 /* A normal C symbol would be one of: 181 @code{BSF_LOCAL}, @code{BSF_COMMON}, @code{BSF_UNDEFINED} or 182 @code{BSF_GLOBAL}. */ 183 184 /* The symbol is a debugging record. The value has an arbitrary 185 meaning, unless BSF_DEBUGGING_RELOC is also set. */ 186#define BSF_DEBUGGING (1 << 2) 187 188 /* The symbol denotes a function entry point. Used in ELF, 189 perhaps others someday. */ 190#define BSF_FUNCTION (1 << 3) 191 192 /* Used by the linker. */ 193#define BSF_KEEP (1 << 5) 194#define BSF_KEEP_G (1 << 6) 195 196 /* A weak global symbol, overridable without warnings by 197 a regular global symbol of the same name. */ 198#define BSF_WEAK (1 << 7) 199 200 /* This symbol was created to point to a section, e.g. ELF's 201 STT_SECTION symbols. */ 202#define BSF_SECTION_SYM (1 << 8) 203 204 /* The symbol used to be a common symbol, but now it is 205 allocated. */ 206#define BSF_OLD_COMMON (1 << 9) 207 208 /* In some files the type of a symbol sometimes alters its 209 location in an output file - ie in coff a @code{ISFCN} symbol 210 which is also @code{C_EXT} symbol appears where it was 211 declared and not at the end of a section. This bit is set 212 by the target BFD part to convey this information. */ 213#define BSF_NOT_AT_END (1 << 10) 214 215 /* Signal that the symbol is the label of constructor section. */ 216#define BSF_CONSTRUCTOR (1 << 11) 217 218 /* Signal that the symbol is a warning symbol. The name is a 219 warning. The name of the next symbol is the one to warn about; 220 if a reference is made to a symbol with the same name as the next 221 symbol, a warning is issued by the linker. */ 222#define BSF_WARNING (1 << 12) 223 224 /* Signal that the symbol is indirect. This symbol is an indirect 225 pointer to the symbol with the same name as the next symbol. */ 226#define BSF_INDIRECT (1 << 13) 227 228 /* BSF_FILE marks symbols that contain a file name. This is used 229 for ELF STT_FILE symbols. */ 230#define BSF_FILE (1 << 14) 231 232 /* Symbol is from dynamic linking information. */ 233#define BSF_DYNAMIC (1 << 15) 234 235 /* The symbol denotes a data object. Used in ELF, and perhaps 236 others someday. */ 237#define BSF_OBJECT (1 << 16) 238 239 /* This symbol is a debugging symbol. The value is the offset 240 into the section of the data. BSF_DEBUGGING should be set 241 as well. */ 242#define BSF_DEBUGGING_RELOC (1 << 17) 243 244 /* This symbol is thread local. Used in ELF. */ 245#define BSF_THREAD_LOCAL (1 << 18) 246 247 /* This symbol represents a complex relocation expression, 248 with the expression tree serialized in the symbol name. */ 249#define BSF_RELC (1 << 19) 250 251 /* This symbol represents a signed complex relocation expression, 252 with the expression tree serialized in the symbol name. */ 253#define BSF_SRELC (1 << 20) 254 255 /* This symbol was created by bfd_get_synthetic_symtab. */ 256#define BSF_SYNTHETIC (1 << 21) 257 258 /* This symbol is an indirect code object. Unrelated to BSF_INDIRECT. 259 The dynamic linker will compute the value of this symbol by 260 calling the function that it points to. BSF_FUNCTION must 261 also be also set. */ 262#define BSF_GNU_INDIRECT_FUNCTION (1 << 22) 263 /* This symbol is a globally unique data object. The dynamic linker 264 will make sure that in the entire process there is just one symbol 265 with this name and type in use. BSF_OBJECT must also be set. */ 266#define BSF_GNU_UNIQUE (1 << 23) 267 268 flagword flags; 269 270 /* A pointer to the section to which this symbol is 271 relative. This will always be non NULL, there are special 272 sections for undefined and absolute symbols. */ 273 struct bfd_section *section; 274 275 /* Back end special data. */ 276 union 277 @{ 278 void *p; 279 bfd_vma i; 280 @} 281 udata; 282@} 283asymbol; 284 285@end example 286 287@node symbol handling functions, , typedef asymbol, Symbols 288@subsection Symbol handling functions 289 290 291@findex bfd_get_symtab_upper_bound 292@subsubsection @code{bfd_get_symtab_upper_bound} 293@strong{Description}@* 294Return the number of bytes required to store a vector of pointers 295to @code{asymbols} for all the symbols in the BFD @var{abfd}, 296including a terminal NULL pointer. If there are no symbols in 297the BFD, then return 0. If an error occurs, return -1. 298@example 299#define bfd_get_symtab_upper_bound(abfd) \ 300 BFD_SEND (abfd, _bfd_get_symtab_upper_bound, (abfd)) 301 302@end example 303 304@findex bfd_is_local_label 305@subsubsection @code{bfd_is_local_label} 306@strong{Synopsis} 307@example 308bfd_boolean bfd_is_local_label (bfd *abfd, asymbol *sym); 309@end example 310@strong{Description}@* 311Return TRUE if the given symbol @var{sym} in the BFD @var{abfd} is 312a compiler generated local label, else return FALSE. 313 314@findex bfd_is_local_label_name 315@subsubsection @code{bfd_is_local_label_name} 316@strong{Synopsis} 317@example 318bfd_boolean bfd_is_local_label_name (bfd *abfd, const char *name); 319@end example 320@strong{Description}@* 321Return TRUE if a symbol with the name @var{name} in the BFD 322@var{abfd} is a compiler generated local label, else return 323FALSE. This just checks whether the name has the form of a 324local label. 325@example 326#define bfd_is_local_label_name(abfd, name) \ 327 BFD_SEND (abfd, _bfd_is_local_label_name, (abfd, name)) 328 329@end example 330 331@findex bfd_is_target_special_symbol 332@subsubsection @code{bfd_is_target_special_symbol} 333@strong{Synopsis} 334@example 335bfd_boolean bfd_is_target_special_symbol (bfd *abfd, asymbol *sym); 336@end example 337@strong{Description}@* 338Return TRUE iff a symbol @var{sym} in the BFD @var{abfd} is something 339special to the particular target represented by the BFD. Such symbols 340should normally not be mentioned to the user. 341@example 342#define bfd_is_target_special_symbol(abfd, sym) \ 343 BFD_SEND (abfd, _bfd_is_target_special_symbol, (abfd, sym)) 344 345@end example 346 347@findex bfd_canonicalize_symtab 348@subsubsection @code{bfd_canonicalize_symtab} 349@strong{Description}@* 350Read the symbols from the BFD @var{abfd}, and fills in 351the vector @var{location} with pointers to the symbols and 352a trailing NULL. 353Return the actual number of symbol pointers, not 354including the NULL. 355@example 356#define bfd_canonicalize_symtab(abfd, location) \ 357 BFD_SEND (abfd, _bfd_canonicalize_symtab, (abfd, location)) 358 359@end example 360 361@findex bfd_set_symtab 362@subsubsection @code{bfd_set_symtab} 363@strong{Synopsis} 364@example 365bfd_boolean bfd_set_symtab 366 (bfd *abfd, asymbol **location, unsigned int count); 367@end example 368@strong{Description}@* 369Arrange that when the output BFD @var{abfd} is closed, 370the table @var{location} of @var{count} pointers to symbols 371will be written. 372 373@findex bfd_print_symbol_vandf 374@subsubsection @code{bfd_print_symbol_vandf} 375@strong{Synopsis} 376@example 377void bfd_print_symbol_vandf (bfd *abfd, void *file, asymbol *symbol); 378@end example 379@strong{Description}@* 380Print the value and flags of the @var{symbol} supplied to the 381stream @var{file}. 382 383@findex bfd_make_empty_symbol 384@subsubsection @code{bfd_make_empty_symbol} 385@strong{Description}@* 386Create a new @code{asymbol} structure for the BFD @var{abfd} 387and return a pointer to it. 388 389This routine is necessary because each back end has private 390information surrounding the @code{asymbol}. Building your own 391@code{asymbol} and pointing to it will not create the private 392information, and will cause problems later on. 393@example 394#define bfd_make_empty_symbol(abfd) \ 395 BFD_SEND (abfd, _bfd_make_empty_symbol, (abfd)) 396 397@end example 398 399@findex _bfd_generic_make_empty_symbol 400@subsubsection @code{_bfd_generic_make_empty_symbol} 401@strong{Synopsis} 402@example 403asymbol *_bfd_generic_make_empty_symbol (bfd *); 404@end example 405@strong{Description}@* 406Create a new @code{asymbol} structure for the BFD @var{abfd} 407and return a pointer to it. Used by core file routines, 408binary back-end and anywhere else where no private info 409is needed. 410 411@findex bfd_make_debug_symbol 412@subsubsection @code{bfd_make_debug_symbol} 413@strong{Description}@* 414Create a new @code{asymbol} structure for the BFD @var{abfd}, 415to be used as a debugging symbol. Further details of its use have 416yet to be worked out. 417@example 418#define bfd_make_debug_symbol(abfd,ptr,size) \ 419 BFD_SEND (abfd, _bfd_make_debug_symbol, (abfd, ptr, size)) 420 421@end example 422 423@findex bfd_decode_symclass 424@subsubsection @code{bfd_decode_symclass} 425@strong{Description}@* 426Return a character corresponding to the symbol 427class of @var{symbol}, or '?' for an unknown class. 428 429@strong{Synopsis} 430@example 431int bfd_decode_symclass (asymbol *symbol); 432@end example 433@findex bfd_is_undefined_symclass 434@subsubsection @code{bfd_is_undefined_symclass} 435@strong{Description}@* 436Returns non-zero if the class symbol returned by 437bfd_decode_symclass represents an undefined symbol. 438Returns zero otherwise. 439 440@strong{Synopsis} 441@example 442bfd_boolean bfd_is_undefined_symclass (int symclass); 443@end example 444@findex bfd_symbol_info 445@subsubsection @code{bfd_symbol_info} 446@strong{Description}@* 447Fill in the basic info about symbol that nm needs. 448Additional info may be added by the back-ends after 449calling this function. 450 451@strong{Synopsis} 452@example 453void bfd_symbol_info (asymbol *symbol, symbol_info *ret); 454@end example 455@findex bfd_copy_private_symbol_data 456@subsubsection @code{bfd_copy_private_symbol_data} 457@strong{Synopsis} 458@example 459bfd_boolean bfd_copy_private_symbol_data 460 (bfd *ibfd, asymbol *isym, bfd *obfd, asymbol *osym); 461@end example 462@strong{Description}@* 463Copy private symbol information from @var{isym} in the BFD 464@var{ibfd} to the symbol @var{osym} in the BFD @var{obfd}. 465Return @code{TRUE} on success, @code{FALSE} on error. Possible error 466returns are: 467 468@itemize @bullet 469 470@item 471@code{bfd_error_no_memory} - 472Not enough memory exists to create private data for @var{osec}. 473@end itemize 474@example 475#define bfd_copy_private_symbol_data(ibfd, isymbol, obfd, osymbol) \ 476 BFD_SEND (obfd, _bfd_copy_private_symbol_data, \ 477 (ibfd, isymbol, obfd, osymbol)) 478 479@end example 480 481