syms.texi revision 1.1.1.8
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 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 0 166 167 /* 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_UNDEFINED} or @code{BSF_GLOBAL}. */ 181 182 /* The symbol is a debugging record. The value has an arbitrary 183 meaning, unless BSF_DEBUGGING_RELOC is also set. */ 184#define BSF_DEBUGGING (1 << 2) 185 186 /* The symbol denotes a function entry point. Used in ELF, 187 perhaps others someday. */ 188#define BSF_FUNCTION (1 << 3) 189 190 /* Used by the linker. */ 191#define BSF_KEEP (1 << 5) 192 193 /* An ELF common symbol. */ 194#define BSF_ELF_COMMON (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 /* This section symbol should be included in the symbol table. */ 269#define BSF_SECTION_SYM_USED (1 << 24) 270 271 flagword flags; 272 273 /* A pointer to the section to which this symbol is 274 relative. This will always be non NULL, there are special 275 sections for undefined and absolute symbols. */ 276 struct bfd_section *section; 277 278 /* Back end special data. */ 279 union 280 @{ 281 void *p; 282 bfd_vma i; 283 @} 284 udata; 285@} 286asymbol; 287 288@end example 289 290@node symbol handling functions, , typedef asymbol, Symbols 291@subsection Symbol handling functions 292 293 294@findex bfd_get_symtab_upper_bound 295@subsubsection @code{bfd_get_symtab_upper_bound} 296Return the number of bytes required to store a vector of pointers 297to @code{asymbols} for all the symbols in the BFD @var{abfd}, 298including a terminal NULL pointer. If there are no symbols in 299the BFD, then return 0. If an error occurs, return -1. 300@example 301#define bfd_get_symtab_upper_bound(abfd) \ 302 BFD_SEND (abfd, _bfd_get_symtab_upper_bound, (abfd)) 303 304@end example 305 306@findex bfd_is_local_label 307@subsubsection @code{bfd_is_local_label} 308@deftypefn {Function} bool bfd_is_local_label (bfd *abfd, asymbol *sym); 309Return TRUE if the given symbol @var{sym} in the BFD @var{abfd} is 310a compiler generated local label, else return FALSE. 311 312@end deftypefn 313@findex bfd_is_local_label_name 314@subsubsection @code{bfd_is_local_label_name} 315@deftypefn {Function} bool bfd_is_local_label_name (bfd *abfd, const char *name); 316Return TRUE if a symbol with the name @var{name} in the BFD 317@var{abfd} is a compiler generated local label, else return 318FALSE. This just checks whether the name has the form of a 319local label. 320@example 321#define bfd_is_local_label_name(abfd, name) \ 322 BFD_SEND (abfd, _bfd_is_local_label_name, (abfd, name)) 323 324@end example 325 326@end deftypefn 327@findex bfd_is_target_special_symbol 328@subsubsection @code{bfd_is_target_special_symbol} 329@deftypefn {Function} bool bfd_is_target_special_symbol (bfd *abfd, asymbol *sym); 330Return TRUE iff a symbol @var{sym} in the BFD @var{abfd} is something 331special to the particular target represented by the BFD. Such symbols 332should normally not be mentioned to the user. 333@example 334#define bfd_is_target_special_symbol(abfd, sym) \ 335 BFD_SEND (abfd, _bfd_is_target_special_symbol, (abfd, sym)) 336 337@end example 338 339@end deftypefn 340@findex bfd_canonicalize_symtab 341@subsubsection @code{bfd_canonicalize_symtab} 342Read the symbols from the BFD @var{abfd}, and fills in 343the vector @var{location} with pointers to the symbols and 344a trailing NULL. 345Return the actual number of symbol pointers, not 346including the NULL. 347@example 348#define bfd_canonicalize_symtab(abfd, location) \ 349 BFD_SEND (abfd, _bfd_canonicalize_symtab, (abfd, location)) 350 351@end example 352 353@findex bfd_set_symtab 354@subsubsection @code{bfd_set_symtab} 355@deftypefn {Function} bool bfd_set_symtab (bfd *abfd, asymbol **location, unsigned int count); 356Arrange that when the output BFD @var{abfd} is closed, 357the table @var{location} of @var{count} pointers to symbols 358will be written. 359 360@end deftypefn 361@findex bfd_print_symbol_vandf 362@subsubsection @code{bfd_print_symbol_vandf} 363@deftypefn {Function} void bfd_print_symbol_vandf (bfd *abfd, void *file, asymbol *symbol); 364Print the value and flags of the @var{symbol} supplied to the 365stream @var{file}. 366 367@end deftypefn 368@findex bfd_make_empty_symbol 369@subsubsection @code{bfd_make_empty_symbol} 370Create a new @code{asymbol} structure for the BFD @var{abfd} 371and return a pointer to it. 372 373This routine is necessary because each back end has private 374information surrounding the @code{asymbol}. Building your own 375@code{asymbol} and pointing to it will not create the private 376information, and will cause problems later on. 377@example 378#define bfd_make_empty_symbol(abfd) \ 379 BFD_SEND (abfd, _bfd_make_empty_symbol, (abfd)) 380 381@end example 382 383@findex _bfd_generic_make_empty_symbol 384@subsubsection @code{_bfd_generic_make_empty_symbol} 385@deftypefn {Function} asymbol *_bfd_generic_make_empty_symbol (bfd *); 386Create a new @code{asymbol} structure for the BFD @var{abfd} 387and return a pointer to it. Used by core file routines, 388binary back-end and anywhere else where no private info 389is needed. 390 391@end deftypefn 392@findex bfd_make_debug_symbol 393@subsubsection @code{bfd_make_debug_symbol} 394Create a new @code{asymbol} structure for the BFD @var{abfd}, 395to be used as a debugging symbol. 396@example 397#define bfd_make_debug_symbol(abfd) \ 398 BFD_SEND (abfd, _bfd_make_debug_symbol, (abfd)) 399 400@end example 401 402@findex bfd_decode_symclass 403@subsubsection @code{bfd_decode_symclass} 404@deftypefn {Function} int bfd_decode_symclass (asymbol *symbol); 405Return a character corresponding to the symbol 406class of @var{symbol}, or '?' for an unknown class. 407 408@end deftypefn 409@findex bfd_is_undefined_symclass 410@subsubsection @code{bfd_is_undefined_symclass} 411@deftypefn {Function} bool bfd_is_undefined_symclass (int symclass); 412Returns non-zero if the class symbol returned by 413bfd_decode_symclass represents an undefined symbol. 414Returns zero otherwise. 415 416@end deftypefn 417@findex bfd_symbol_info 418@subsubsection @code{bfd_symbol_info} 419@deftypefn {Function} void bfd_symbol_info (asymbol *symbol, symbol_info *ret); 420Fill in the basic info about symbol that nm needs. 421Additional info may be added by the back-ends after 422calling this function. 423 424@end deftypefn 425@findex bfd_copy_private_symbol_data 426@subsubsection @code{bfd_copy_private_symbol_data} 427@deftypefn {Function} bool bfd_copy_private_symbol_data (bfd *ibfd, asymbol *isym, bfd *obfd, asymbol *osym); 428Copy private symbol information from @var{isym} in the BFD 429@var{ibfd} to the symbol @var{osym} in the BFD @var{obfd}. 430Return @code{TRUE} on success, @code{FALSE} on error. Possible error 431returns are: 432 433@itemize @bullet 434 435@item 436@code{bfd_error_no_memory} - 437Not enough memory exists to create private data for @var{osec}. 438@end itemize 439@example 440#define bfd_copy_private_symbol_data(ibfd, isymbol, obfd, osymbol) \ 441 BFD_SEND (obfd, _bfd_copy_private_symbol_data, \ 442 (ibfd, isymbol, obfd, osymbol)) 443 444@end example 445 446@end deftypefn 447