syms.texi revision 1.1.1.2
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 "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 109Many formats cannot represent arbitrary symbol information; for 110instance, the @code{a.out} object format does not allow an 111arbitrary number of sections. A symbol pointing to a section 112which is not one of @code{.text}, @code{.data} or @code{.bss} cannot 113be described. 114 115@node Mini Symbols, typedef asymbol, Writing Symbols, Symbols 116@subsection Mini Symbols 117Mini symbols provide read-only access to the symbol table. 118They use less memory space, but require more time to access. 119They can be useful for tools like nm or objdump, which may 120have to handle symbol tables of extremely large executables. 121 122The @code{bfd_read_minisymbols} function will read the symbols 123into memory in an internal form. It will return a @code{void *} 124pointer to a block of memory, a symbol count, and the size of 125each symbol. The pointer is allocated using @code{malloc}, and 126should be freed by the caller when it is no longer needed. 127 128The function @code{bfd_minisymbol_to_symbol} will take a pointer 129to a minisymbol, and a pointer to a structure returned by 130@code{bfd_make_empty_symbol}, and return a @code{asymbol} structure. 131The 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 137An @code{asymbol} has the form: 138 139 140@example 141 142typedef 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#define BSF_NO_FLAGS 0x00 166 167 /* The symbol has local scope; @code{static} in @code{C}. The value 168 is the offset into the section of the data. */ 169#define BSF_LOCAL (1 << 0) 170 171 /* The symbol has global scope; initialized data in @code{C}. The 172 value is the offset into the section of the data. */ 173#define BSF_GLOBAL (1 << 1) 174 175 /* 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 @code{BSF_LOCAL}, @code{BSF_COMMON}, @code{BSF_UNDEFINED} or 181 @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#define BSF_DEBUGGING (1 << 2) 186 187 /* The symbol denotes a function entry point. Used in ELF, 188 perhaps others someday. */ 189#define BSF_FUNCTION (1 << 3) 190 191 /* Used by the linker. */ 192#define BSF_KEEP (1 << 5) 193#define BSF_KEEP_G (1 << 6) 194 195 /* A weak global symbol, overridable without warnings by 196 a regular global symbol of the same name. */ 197#define BSF_WEAK (1 << 7) 198 199 /* This symbol was created to point to a section, e.g. ELF's 200 STT_SECTION symbols. */ 201#define BSF_SECTION_SYM (1 << 8) 202 203 /* The symbol used to be a common symbol, but now it is 204 allocated. */ 205#define BSF_OLD_COMMON (1 << 9) 206 207 /* 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#define BSF_NOT_AT_END (1 << 10) 213 214 /* Signal that the symbol is the label of constructor section. */ 215#define BSF_CONSTRUCTOR (1 << 11) 216 217 /* 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#define BSF_WARNING (1 << 12) 222 223 /* 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#define BSF_INDIRECT (1 << 13) 226 227 /* BSF_FILE marks symbols that contain a file name. This is used 228 for ELF STT_FILE symbols. */ 229#define BSF_FILE (1 << 14) 230 231 /* Symbol is from dynamic linking information. */ 232#define BSF_DYNAMIC (1 << 15) 233 234 /* The symbol denotes a data object. Used in ELF, and perhaps 235 others someday. */ 236#define BSF_OBJECT (1 << 16) 237 238 /* 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#define BSF_DEBUGGING_RELOC (1 << 17) 242 243 /* This symbol is thread local. Used in ELF. */ 244#define BSF_THREAD_LOCAL (1 << 18) 245 246 /* This symbol represents a complex relocation expression, 247 with the expression tree serialized in the symbol name. */ 248#define BSF_RELC (1 << 19) 249 250 /* This symbol represents a signed complex relocation expression, 251 with the expression tree serialized in the symbol name. */ 252#define BSF_SRELC (1 << 20) 253 254 /* 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 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@} 282asymbol; 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}@* 293Return the number of bytes required to store a vector of pointers 294to @code{asymbols} for all the symbols in the BFD @var{abfd}, 295including a terminal NULL pointer. If there are no symbols in 296the 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 307bfd_boolean bfd_is_local_label (bfd *abfd, asymbol *sym); 308@end example 309@strong{Description}@* 310Return TRUE if the given symbol @var{sym} in the BFD @var{abfd} is 311a 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 317bfd_boolean bfd_is_local_label_name (bfd *abfd, const char *name); 318@end example 319@strong{Description}@* 320Return TRUE if a symbol with the name @var{name} in the BFD 321@var{abfd} is a compiler generated local label, else return 322FALSE. This just checks whether the name has the form of a 323local 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 334bfd_boolean bfd_is_target_special_symbol (bfd *abfd, asymbol *sym); 335@end example 336@strong{Description}@* 337Return TRUE iff a symbol @var{sym} in the BFD @var{abfd} is something 338special to the particular target represented by the BFD. Such symbols 339should 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}@* 349Read the symbols from the BFD @var{abfd}, and fills in 350the vector @var{location} with pointers to the symbols and 351a trailing NULL. 352Return the actual number of symbol pointers, not 353including 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 364bfd_boolean bfd_set_symtab 365 (bfd *abfd, asymbol **location, unsigned int count); 366@end example 367@strong{Description}@* 368Arrange that when the output BFD @var{abfd} is closed, 369the table @var{location} of @var{count} pointers to symbols 370will be written. 371 372@findex bfd_print_symbol_vandf 373@subsubsection @code{bfd_print_symbol_vandf} 374@strong{Synopsis} 375@example 376void bfd_print_symbol_vandf (bfd *abfd, void *file, asymbol *symbol); 377@end example 378@strong{Description}@* 379Print the value and flags of the @var{symbol} supplied to the 380stream @var{file}. 381 382@findex bfd_make_empty_symbol 383@subsubsection @code{bfd_make_empty_symbol} 384@strong{Description}@* 385Create a new @code{asymbol} structure for the BFD @var{abfd} 386and return a pointer to it. 387 388This routine is necessary because each back end has private 389information surrounding the @code{asymbol}. Building your own 390@code{asymbol} and pointing to it will not create the private 391information, 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 402asymbol *_bfd_generic_make_empty_symbol (bfd *); 403@end example 404@strong{Description}@* 405Create a new @code{asymbol} structure for the BFD @var{abfd} 406and return a pointer to it. Used by core file routines, 407binary back-end and anywhere else where no private info 408is needed. 409 410@findex bfd_make_debug_symbol 411@subsubsection @code{bfd_make_debug_symbol} 412@strong{Description}@* 413Create a new @code{asymbol} structure for the BFD @var{abfd}, 414to be used as a debugging symbol. Further details of its use have 415yet 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}@* 425Return a character corresponding to the symbol 426class of @var{symbol}, or '?' for an unknown class. 427 428@strong{Synopsis} 429@example 430int bfd_decode_symclass (asymbol *symbol); 431@end example 432@findex bfd_is_undefined_symclass 433@subsubsection @code{bfd_is_undefined_symclass} 434@strong{Description}@* 435Returns non-zero if the class symbol returned by 436bfd_decode_symclass represents an undefined symbol. 437Returns zero otherwise. 438 439@strong{Synopsis} 440@example 441bfd_boolean bfd_is_undefined_symclass (int symclass); 442@end example 443@findex bfd_symbol_info 444@subsubsection @code{bfd_symbol_info} 445@strong{Description}@* 446Fill in the basic info about symbol that nm needs. 447Additional info may be added by the back-ends after 448calling this function. 449 450@strong{Synopsis} 451@example 452void 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 458bfd_boolean bfd_copy_private_symbol_data 459 (bfd *ibfd, asymbol *isym, bfd *obfd, asymbol *osym); 460@end example 461@strong{Description}@* 462Copy private symbol information from @var{isym} in the BFD 463@var{ibfd} to the symbol @var{osym} in the BFD @var{obfd}. 464Return @code{TRUE} on success, @code{FALSE} on error. Possible error 465returns are: 466 467@itemize @bullet 468 469@item 470@code{bfd_error_no_memory} - 471Not 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 479 480