syms.texi revision 1.5
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_UNDEFINED} or @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) 193 194 /* An ELF common symbol. */ 195#define BSF_ELF_COMMON (1 << 6) 196 197 /* A weak global symbol, overridable without warnings by 198 a regular global symbol of the same name. */ 199#define BSF_WEAK (1 << 7) 200 201 /* This symbol was created to point to a section, e.g. ELF's 202 STT_SECTION symbols. */ 203#define BSF_SECTION_SYM (1 << 8) 204 205 /* The symbol used to be a common symbol, but now it is 206 allocated. */ 207#define BSF_OLD_COMMON (1 << 9) 208 209 /* In some files the type of a symbol sometimes alters its 210 location in an output file - ie in coff a @code{ISFCN} symbol 211 which is also @code{C_EXT} symbol appears where it was 212 declared and not at the end of a section. This bit is set 213 by the target BFD part to convey this information. */ 214#define BSF_NOT_AT_END (1 << 10) 215 216 /* Signal that the symbol is the label of constructor section. */ 217#define BSF_CONSTRUCTOR (1 << 11) 218 219 /* Signal that the symbol is a warning symbol. The name is a 220 warning. The name of the next symbol is the one to warn about; 221 if a reference is made to a symbol with the same name as the next 222 symbol, a warning is issued by the linker. */ 223#define BSF_WARNING (1 << 12) 224 225 /* Signal that the symbol is indirect. This symbol is an indirect 226 pointer to the symbol with the same name as the next symbol. */ 227#define BSF_INDIRECT (1 << 13) 228 229 /* BSF_FILE marks symbols that contain a file name. This is used 230 for ELF STT_FILE symbols. */ 231#define BSF_FILE (1 << 14) 232 233 /* Symbol is from dynamic linking information. */ 234#define BSF_DYNAMIC (1 << 15) 235 236 /* The symbol denotes a data object. Used in ELF, and perhaps 237 others someday. */ 238#define BSF_OBJECT (1 << 16) 239 240 /* This symbol is a debugging symbol. The value is the offset 241 into the section of the data. BSF_DEBUGGING should be set 242 as well. */ 243#define BSF_DEBUGGING_RELOC (1 << 17) 244 245 /* This symbol is thread local. Used in ELF. */ 246#define BSF_THREAD_LOCAL (1 << 18) 247 248 /* This symbol represents a complex relocation expression, 249 with the expression tree serialized in the symbol name. */ 250#define BSF_RELC (1 << 19) 251 252 /* This symbol represents a signed complex relocation expression, 253 with the expression tree serialized in the symbol name. */ 254#define BSF_SRELC (1 << 20) 255 256 /* This symbol was created by bfd_get_synthetic_symtab. */ 257#define BSF_SYNTHETIC (1 << 21) 258 259 /* This symbol is an indirect code object. Unrelated to BSF_INDIRECT. 260 The dynamic linker will compute the value of this symbol by 261 calling the function that it points to. BSF_FUNCTION must 262 also be also set. */ 263#define BSF_GNU_INDIRECT_FUNCTION (1 << 22) 264 /* This symbol is a globally unique data object. The dynamic linker 265 will make sure that in the entire process there is just one symbol 266 with this name and type in use. BSF_OBJECT must also be set. */ 267#define BSF_GNU_UNIQUE (1 << 23) 268 269 flagword flags; 270 271 /* A pointer to the section to which this symbol is 272 relative. This will always be non NULL, there are special 273 sections for undefined and absolute symbols. */ 274 struct bfd_section *section; 275 276 /* Back end special data. */ 277 union 278 @{ 279 void *p; 280 bfd_vma i; 281 @} 282 udata; 283@} 284asymbol; 285 286@end example 287 288@node symbol handling functions, , typedef asymbol, Symbols 289@subsection Symbol handling functions 290 291 292@findex bfd_get_symtab_upper_bound 293@subsubsection @code{bfd_get_symtab_upper_bound} 294@strong{Description}@* 295Return the number of bytes required to store a vector of pointers 296to @code{asymbols} for all the symbols in the BFD @var{abfd}, 297including a terminal NULL pointer. If there are no symbols in 298the BFD, then return 0. If an error occurs, return -1. 299@example 300#define bfd_get_symtab_upper_bound(abfd) \ 301 BFD_SEND (abfd, _bfd_get_symtab_upper_bound, (abfd)) 302 303@end example 304 305@findex bfd_is_local_label 306@subsubsection @code{bfd_is_local_label} 307@strong{Synopsis} 308@example 309bfd_boolean bfd_is_local_label (bfd *abfd, asymbol *sym); 310@end example 311@strong{Description}@* 312Return TRUE if the given symbol @var{sym} in the BFD @var{abfd} is 313a compiler generated local label, else return FALSE. 314 315@findex bfd_is_local_label_name 316@subsubsection @code{bfd_is_local_label_name} 317@strong{Synopsis} 318@example 319bfd_boolean bfd_is_local_label_name (bfd *abfd, const char *name); 320@end example 321@strong{Description}@* 322Return TRUE if a symbol with the name @var{name} in the BFD 323@var{abfd} is a compiler generated local label, else return 324FALSE. This just checks whether the name has the form of a 325local label. 326@example 327#define bfd_is_local_label_name(abfd, name) \ 328 BFD_SEND (abfd, _bfd_is_local_label_name, (abfd, name)) 329 330@end example 331 332@findex bfd_is_target_special_symbol 333@subsubsection @code{bfd_is_target_special_symbol} 334@strong{Synopsis} 335@example 336bfd_boolean bfd_is_target_special_symbol (bfd *abfd, asymbol *sym); 337@end example 338@strong{Description}@* 339Return TRUE iff a symbol @var{sym} in the BFD @var{abfd} is something 340special to the particular target represented by the BFD. Such symbols 341should normally not be mentioned to the user. 342@example 343#define bfd_is_target_special_symbol(abfd, sym) \ 344 BFD_SEND (abfd, _bfd_is_target_special_symbol, (abfd, sym)) 345 346@end example 347 348@findex bfd_canonicalize_symtab 349@subsubsection @code{bfd_canonicalize_symtab} 350@strong{Description}@* 351Read the symbols from the BFD @var{abfd}, and fills in 352the vector @var{location} with pointers to the symbols and 353a trailing NULL. 354Return the actual number of symbol pointers, not 355including the NULL. 356@example 357#define bfd_canonicalize_symtab(abfd, location) \ 358 BFD_SEND (abfd, _bfd_canonicalize_symtab, (abfd, location)) 359 360@end example 361 362@findex bfd_set_symtab 363@subsubsection @code{bfd_set_symtab} 364@strong{Synopsis} 365@example 366bfd_boolean bfd_set_symtab 367 (bfd *abfd, asymbol **location, unsigned int count); 368@end example 369@strong{Description}@* 370Arrange that when the output BFD @var{abfd} is closed, 371the table @var{location} of @var{count} pointers to symbols 372will be written. 373 374@findex bfd_print_symbol_vandf 375@subsubsection @code{bfd_print_symbol_vandf} 376@strong{Synopsis} 377@example 378void bfd_print_symbol_vandf (bfd *abfd, void *file, asymbol *symbol); 379@end example 380@strong{Description}@* 381Print the value and flags of the @var{symbol} supplied to the 382stream @var{file}. 383 384@findex bfd_make_empty_symbol 385@subsubsection @code{bfd_make_empty_symbol} 386@strong{Description}@* 387Create a new @code{asymbol} structure for the BFD @var{abfd} 388and return a pointer to it. 389 390This routine is necessary because each back end has private 391information surrounding the @code{asymbol}. Building your own 392@code{asymbol} and pointing to it will not create the private 393information, and will cause problems later on. 394@example 395#define bfd_make_empty_symbol(abfd) \ 396 BFD_SEND (abfd, _bfd_make_empty_symbol, (abfd)) 397 398@end example 399 400@findex _bfd_generic_make_empty_symbol 401@subsubsection @code{_bfd_generic_make_empty_symbol} 402@strong{Synopsis} 403@example 404asymbol *_bfd_generic_make_empty_symbol (bfd *); 405@end example 406@strong{Description}@* 407Create a new @code{asymbol} structure for the BFD @var{abfd} 408and return a pointer to it. Used by core file routines, 409binary back-end and anywhere else where no private info 410is needed. 411 412@findex bfd_make_debug_symbol 413@subsubsection @code{bfd_make_debug_symbol} 414@strong{Description}@* 415Create a new @code{asymbol} structure for the BFD @var{abfd}, 416to be used as a debugging symbol. Further details of its use have 417yet to be worked out. 418@example 419#define bfd_make_debug_symbol(abfd,ptr,size) \ 420 BFD_SEND (abfd, _bfd_make_debug_symbol, (abfd, ptr, size)) 421 422@end example 423 424@findex bfd_decode_symclass 425@subsubsection @code{bfd_decode_symclass} 426@strong{Description}@* 427Return a character corresponding to the symbol 428class of @var{symbol}, or '?' for an unknown class. 429 430@strong{Synopsis} 431@example 432int bfd_decode_symclass (asymbol *symbol); 433@end example 434@findex bfd_is_undefined_symclass 435@subsubsection @code{bfd_is_undefined_symclass} 436@strong{Description}@* 437Returns non-zero if the class symbol returned by 438bfd_decode_symclass represents an undefined symbol. 439Returns zero otherwise. 440 441@strong{Synopsis} 442@example 443bfd_boolean bfd_is_undefined_symclass (int symclass); 444@end example 445@findex bfd_symbol_info 446@subsubsection @code{bfd_symbol_info} 447@strong{Description}@* 448Fill in the basic info about symbol that nm needs. 449Additional info may be added by the back-ends after 450calling this function. 451 452@strong{Synopsis} 453@example 454void bfd_symbol_info (asymbol *symbol, symbol_info *ret); 455@end example 456@findex bfd_copy_private_symbol_data 457@subsubsection @code{bfd_copy_private_symbol_data} 458@strong{Synopsis} 459@example 460bfd_boolean bfd_copy_private_symbol_data 461 (bfd *ibfd, asymbol *isym, bfd *obfd, asymbol *osym); 462@end example 463@strong{Description}@* 464Copy private symbol information from @var{isym} in the BFD 465@var{ibfd} to the symbol @var{osym} in the BFD @var{obfd}. 466Return @code{TRUE} on success, @code{FALSE} on error. Possible error 467returns are: 468 469@itemize @bullet 470 471@item 472@code{bfd_error_no_memory} - 473Not enough memory exists to create private data for @var{osec}. 474@end itemize 475@example 476#define bfd_copy_private_symbol_data(ibfd, isymbol, obfd, osymbol) \ 477 BFD_SEND (obfd, _bfd_copy_private_symbol_data, \ 478 (ibfd, isymbol, obfd, osymbol)) 479 480@end example 481 482