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diff --git a/bfd/doc/syms.texi b/bfd/doc/syms.texi new file mode 100644 index 0000000000..d5dc6599f2 --- /dev/null +++ b/bfd/doc/syms.texi @@ -0,0 +1,479 @@ +@section Symbols +BFD tries to maintain as much symbol information as it can when +it moves information from file to file. BFD passes information +to applications though the @code{asymbol} structure. When the +application requests the symbol table, BFD reads the table in +the native form and translates parts of it into the internal +format. To maintain more than the information passed to +applications, some targets keep some information ``behind the +scenes'' in a structure only the particular back end knows +about. For example, the coff back end keeps the original +symbol table structure as well as the canonical structure when +a BFD is read in. On output, the coff back end can reconstruct +the output symbol table so that no information is lost, even +information unique to coff which BFD doesn't know or +understand. If a coff symbol table were read, but were written +through an a.out back end, all the coff specific information +would be lost. The symbol table of a BFD +is not necessarily read in until a canonicalize request is +made. Then the BFD back end fills in a table provided by the +application with pointers to the canonical information. To +output symbols, the application provides BFD with a table of +pointers to pointers to @code{asymbol}s. This allows applications +like the linker to output a symbol as it was read, since the ``behind +the scenes'' information will be still available. +@menu +* Reading Symbols:: +* Writing Symbols:: +* Mini Symbols:: +* typedef asymbol:: +* symbol handling functions:: +@end menu + +@node Reading Symbols, Writing Symbols, Symbols, Symbols +@subsection Reading symbols +There are two stages to reading a symbol table from a BFD: +allocating storage, and the actual reading process. This is an +excerpt from an application which reads the symbol table: + +@example + long storage_needed; + asymbol **symbol_table; + long number_of_symbols; + long i; + + storage_needed = bfd_get_symtab_upper_bound (abfd); + + if (storage_needed < 0) + FAIL + + if (storage_needed == 0) + return; + + symbol_table = xmalloc (storage_needed); + ... + number_of_symbols = + bfd_canonicalize_symtab (abfd, symbol_table); + + if (number_of_symbols < 0) + FAIL + + for (i = 0; i < number_of_symbols; i++) + process_symbol (symbol_table[i]); +@end example + +All storage for the symbols themselves is in an objalloc +connected to the BFD; it is freed when the BFD is closed. + +@node Writing Symbols, Mini Symbols, Reading Symbols, Symbols +@subsection Writing symbols +Writing of a symbol table is automatic when a BFD open for +writing is closed. The application attaches a vector of +pointers to pointers to symbols to the BFD being written, and +fills in the symbol count. The close and cleanup code reads +through the table provided and performs all the necessary +operations. The BFD output code must always be provided with an +``owned'' symbol: one which has come from another BFD, or one +which has been created using @code{bfd_make_empty_symbol}. Here is an +example showing the creation of a symbol table with only one element: + +@example + #include "bfd.h" + int main (void) + @{ + bfd *abfd; + asymbol *ptrs[2]; + asymbol *new; + + abfd = bfd_openw ("foo","a.out-sunos-big"); + bfd_set_format (abfd, bfd_object); + new = bfd_make_empty_symbol (abfd); + new->name = "dummy_symbol"; + new->section = bfd_make_section_old_way (abfd, ".text"); + new->flags = BSF_GLOBAL; + new->value = 0x12345; + + ptrs[0] = new; + ptrs[1] = 0; + + bfd_set_symtab (abfd, ptrs, 1); + bfd_close (abfd); + return 0; + @} + + ./makesym + nm foo + 00012345 A dummy_symbol +@end example + +Many formats cannot represent arbitrary symbol information; for +instance, the @code{a.out} object format does not allow an +arbitrary number of sections. A symbol pointing to a section +which is not one of @code{.text}, @code{.data} or @code{.bss} cannot +be described. + +@node Mini Symbols, typedef asymbol, Writing Symbols, Symbols +@subsection Mini Symbols +Mini symbols provide read-only access to the symbol table. +They use less memory space, but require more time to access. +They can be useful for tools like nm or objdump, which may +have to handle symbol tables of extremely large executables. + +The @code{bfd_read_minisymbols} function will read the symbols +into memory in an internal form. It will return a @code{void *} +pointer to a block of memory, a symbol count, and the size of +each symbol. The pointer is allocated using @code{malloc}, and +should be freed by the caller when it is no longer needed. + +The function @code{bfd_minisymbol_to_symbol} will take a pointer +to a minisymbol, and a pointer to a structure returned by +@code{bfd_make_empty_symbol}, and return a @code{asymbol} structure. +The return value may or may not be the same as the value from +@code{bfd_make_empty_symbol} which was passed in. + + +@node typedef asymbol, symbol handling functions, Mini Symbols, Symbols +@subsection typedef asymbol +An @code{asymbol} has the form: + + +@example + +typedef struct bfd_symbol +@{ + /* A pointer to the BFD which owns the symbol. This information + is necessary so that a back end can work out what additional + information (invisible to the application writer) is carried + with the symbol. + + This field is *almost* redundant, since you can use section->owner + instead, except that some symbols point to the global sections + bfd_@{abs,com,und@}_section. This could be fixed by making + these globals be per-bfd (or per-target-flavor). FIXME. */ + struct bfd *the_bfd; /* Use bfd_asymbol_bfd(sym) to access this field. */ + + /* The text of the symbol. The name is left alone, and not copied; the + application may not alter it. */ + const char *name; + + /* The value of the symbol. This really should be a union of a + numeric value with a pointer, since some flags indicate that + a pointer to another symbol is stored here. */ + symvalue value; + + /* Attributes of a symbol. */ +#define BSF_NO_FLAGS 0x00 + + /* The symbol has local scope; @code{static} in @code{C}. The value + is the offset into the section of the data. */ +#define BSF_LOCAL (1 << 0) + + /* The symbol has global scope; initialized data in @code{C}. The + value is the offset into the section of the data. */ +#define BSF_GLOBAL (1 << 1) + + /* The symbol has global scope and is exported. The value is + the offset into the section of the data. */ +#define BSF_EXPORT BSF_GLOBAL /* No real difference. */ + + /* A normal C symbol would be one of: + @code{BSF_LOCAL}, @code{BSF_COMMON}, @code{BSF_UNDEFINED} or + @code{BSF_GLOBAL}. */ + + /* The symbol is a debugging record. The value has an arbitrary + meaning, unless BSF_DEBUGGING_RELOC is also set. */ +#define BSF_DEBUGGING (1 << 2) + + /* The symbol denotes a function entry point. Used in ELF, + perhaps others someday. */ +#define BSF_FUNCTION (1 << 3) + + /* Used by the linker. */ +#define BSF_KEEP (1 << 5) +#define BSF_KEEP_G (1 << 6) + + /* A weak global symbol, overridable without warnings by + a regular global symbol of the same name. */ +#define BSF_WEAK (1 << 7) + + /* This symbol was created to point to a section, e.g. ELF's + STT_SECTION symbols. */ +#define BSF_SECTION_SYM (1 << 8) + + /* The symbol used to be a common symbol, but now it is + allocated. */ +#define BSF_OLD_COMMON (1 << 9) + + /* In some files the type of a symbol sometimes alters its + location in an output file - ie in coff a @code{ISFCN} symbol + which is also @code{C_EXT} symbol appears where it was + declared and not at the end of a section. This bit is set + by the target BFD part to convey this information. */ +#define BSF_NOT_AT_END (1 << 10) + + /* Signal that the symbol is the label of constructor section. */ +#define BSF_CONSTRUCTOR (1 << 11) + + /* Signal that the symbol is a warning symbol. The name is a + warning. The name of the next symbol is the one to warn about; + if a reference is made to a symbol with the same name as the next + symbol, a warning is issued by the linker. */ +#define BSF_WARNING (1 << 12) + + /* Signal that the symbol is indirect. This symbol is an indirect + pointer to the symbol with the same name as the next symbol. */ +#define BSF_INDIRECT (1 << 13) + + /* BSF_FILE marks symbols that contain a file name. This is used + for ELF STT_FILE symbols. */ +#define BSF_FILE (1 << 14) + + /* Symbol is from dynamic linking information. */ +#define BSF_DYNAMIC (1 << 15) + + /* The symbol denotes a data object. Used in ELF, and perhaps + others someday. */ +#define BSF_OBJECT (1 << 16) + + /* This symbol is a debugging symbol. The value is the offset + into the section of the data. BSF_DEBUGGING should be set + as well. */ +#define BSF_DEBUGGING_RELOC (1 << 17) + + /* This symbol is thread local. Used in ELF. */ +#define BSF_THREAD_LOCAL (1 << 18) + + /* This symbol represents a complex relocation expression, + with the expression tree serialized in the symbol name. */ +#define BSF_RELC (1 << 19) + + /* This symbol represents a signed complex relocation expression, + with the expression tree serialized in the symbol name. */ +#define BSF_SRELC (1 << 20) + + /* This symbol was created by bfd_get_synthetic_symtab. */ +#define BSF_SYNTHETIC (1 << 21) + + /* This symbol is an indirect code object. Unrelated to BSF_INDIRECT. + The dynamic linker will compute the value of this symbol by + calling the function that it points to. BSF_FUNCTION must + also be also set. */ +#define BSF_GNU_INDIRECT_FUNCTION (1 << 22) + /* This symbol is a globally unique data object. The dynamic linker + will make sure that in the entire process there is just one symbol + with this name and type in use. BSF_OBJECT must also be set. */ +#define BSF_GNU_UNIQUE (1 << 23) + + flagword flags; + + /* A pointer to the section to which this symbol is + relative. This will always be non NULL, there are special + sections for undefined and absolute symbols. */ + struct bfd_section *section; + + /* Back end special data. */ + union + @{ + void *p; + bfd_vma i; + @} + udata; +@} +asymbol; + +@end example + +@node symbol handling functions, , typedef asymbol, Symbols +@subsection Symbol handling functions + + +@findex bfd_get_symtab_upper_bound +@subsubsection @code{bfd_get_symtab_upper_bound} +@strong{Description}@* +Return the number of bytes required to store a vector of pointers +to @code{asymbols} for all the symbols in the BFD @var{abfd}, +including a terminal NULL pointer. If there are no symbols in +the BFD, then return 0. If an error occurs, return -1. +@example +#define bfd_get_symtab_upper_bound(abfd) \ + BFD_SEND (abfd, _bfd_get_symtab_upper_bound, (abfd)) + +@end example + +@findex bfd_is_local_label +@subsubsection @code{bfd_is_local_label} +@strong{Synopsis} +@example +bfd_boolean bfd_is_local_label (bfd *abfd, asymbol *sym); +@end example +@strong{Description}@* +Return TRUE if the given symbol @var{sym} in the BFD @var{abfd} is +a compiler generated local label, else return FALSE. + +@findex bfd_is_local_label_name +@subsubsection @code{bfd_is_local_label_name} +@strong{Synopsis} +@example +bfd_boolean bfd_is_local_label_name (bfd *abfd, const char *name); +@end example +@strong{Description}@* +Return TRUE if a symbol with the name @var{name} in the BFD +@var{abfd} is a compiler generated local label, else return +FALSE. This just checks whether the name has the form of a +local label. +@example +#define bfd_is_local_label_name(abfd, name) \ + BFD_SEND (abfd, _bfd_is_local_label_name, (abfd, name)) + +@end example + +@findex bfd_is_target_special_symbol +@subsubsection @code{bfd_is_target_special_symbol} +@strong{Synopsis} +@example +bfd_boolean bfd_is_target_special_symbol (bfd *abfd, asymbol *sym); +@end example +@strong{Description}@* +Return TRUE iff a symbol @var{sym} in the BFD @var{abfd} is something +special to the particular target represented by the BFD. Such symbols +should normally not be mentioned to the user. +@example +#define bfd_is_target_special_symbol(abfd, sym) \ + BFD_SEND (abfd, _bfd_is_target_special_symbol, (abfd, sym)) + +@end example + +@findex bfd_canonicalize_symtab +@subsubsection @code{bfd_canonicalize_symtab} +@strong{Description}@* +Read the symbols from the BFD @var{abfd}, and fills in +the vector @var{location} with pointers to the symbols and +a trailing NULL. +Return the actual number of symbol pointers, not +including the NULL. +@example +#define bfd_canonicalize_symtab(abfd, location) \ + BFD_SEND (abfd, _bfd_canonicalize_symtab, (abfd, location)) + +@end example + +@findex bfd_set_symtab +@subsubsection @code{bfd_set_symtab} +@strong{Synopsis} +@example +bfd_boolean bfd_set_symtab + (bfd *abfd, asymbol **location, unsigned int count); +@end example +@strong{Description}@* +Arrange that when the output BFD @var{abfd} is closed, +the table @var{location} of @var{count} pointers to symbols +will be written. + +@findex bfd_print_symbol_vandf +@subsubsection @code{bfd_print_symbol_vandf} +@strong{Synopsis} +@example +void bfd_print_symbol_vandf (bfd *abfd, void *file, asymbol *symbol); +@end example +@strong{Description}@* +Print the value and flags of the @var{symbol} supplied to the +stream @var{file}. + +@findex bfd_make_empty_symbol +@subsubsection @code{bfd_make_empty_symbol} +@strong{Description}@* +Create a new @code{asymbol} structure for the BFD @var{abfd} +and return a pointer to it. + +This routine is necessary because each back end has private +information surrounding the @code{asymbol}. Building your own +@code{asymbol} and pointing to it will not create the private +information, and will cause problems later on. +@example +#define bfd_make_empty_symbol(abfd) \ + BFD_SEND (abfd, _bfd_make_empty_symbol, (abfd)) + +@end example + +@findex _bfd_generic_make_empty_symbol +@subsubsection @code{_bfd_generic_make_empty_symbol} +@strong{Synopsis} +@example +asymbol *_bfd_generic_make_empty_symbol (bfd *); +@end example +@strong{Description}@* +Create a new @code{asymbol} structure for the BFD @var{abfd} +and return a pointer to it. Used by core file routines, +binary back-end and anywhere else where no private info +is needed. + +@findex bfd_make_debug_symbol +@subsubsection @code{bfd_make_debug_symbol} +@strong{Description}@* +Create a new @code{asymbol} structure for the BFD @var{abfd}, +to be used as a debugging symbol. Further details of its use have +yet to be worked out. +@example +#define bfd_make_debug_symbol(abfd,ptr,size) \ + BFD_SEND (abfd, _bfd_make_debug_symbol, (abfd, ptr, size)) + +@end example + +@findex bfd_decode_symclass +@subsubsection @code{bfd_decode_symclass} +@strong{Description}@* +Return a character corresponding to the symbol +class of @var{symbol}, or '?' for an unknown class. + +@strong{Synopsis} +@example +int bfd_decode_symclass (asymbol *symbol); +@end example +@findex bfd_is_undefined_symclass +@subsubsection @code{bfd_is_undefined_symclass} +@strong{Description}@* +Returns non-zero if the class symbol returned by +bfd_decode_symclass represents an undefined symbol. +Returns zero otherwise. + +@strong{Synopsis} +@example +bfd_boolean bfd_is_undefined_symclass (int symclass); +@end example +@findex bfd_symbol_info +@subsubsection @code{bfd_symbol_info} +@strong{Description}@* +Fill in the basic info about symbol that nm needs. +Additional info may be added by the back-ends after +calling this function. + +@strong{Synopsis} +@example +void bfd_symbol_info (asymbol *symbol, symbol_info *ret); +@end example +@findex bfd_copy_private_symbol_data +@subsubsection @code{bfd_copy_private_symbol_data} +@strong{Synopsis} +@example +bfd_boolean bfd_copy_private_symbol_data + (bfd *ibfd, asymbol *isym, bfd *obfd, asymbol *osym); +@end example +@strong{Description}@* +Copy private symbol information from @var{isym} in the BFD +@var{ibfd} to the symbol @var{osym} in the BFD @var{obfd}. +Return @code{TRUE} on success, @code{FALSE} on error. Possible error +returns are: + +@itemize @bullet + +@item +@code{bfd_error_no_memory} - +Not enough memory exists to create private data for @var{osec}. +@end itemize +@example +#define bfd_copy_private_symbol_data(ibfd, isymbol, obfd, osymbol) \ + BFD_SEND (obfd, _bfd_copy_private_symbol_data, \ + (ibfd, isymbol, obfd, osymbol)) + +@end example + |