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 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 asymbols. This allows applications like the linker to output a symbol as it was read, since the ``behind the scenes'' information will be still available.

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:

              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]);

All storage for the symbols themselves is in an objalloc connected to the BFD; it is freed when the BFD is closed.

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 bfd_make_empty_symbol.  Here is an example showing the creation of a symbol table with only one element:

            #include "sysdep.h"
            #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

Many formats cannot represent arbitrary symbol information; for instance, the a.out object format does not allow an arbitrary number of sections. A symbol pointing to a section which is not one  of .text, .data or .bss cannot be described.

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 bfd_read_minisymbols function will read the symbols into memory in an internal form.  It will return a void * pointer to a block of memory, a symbol count, and the size of each symbol.  The pointer is allocated using malloc, and should be freed by the caller when it is no longer needed.

The function bfd_minisymbol_to_symbol will take a pointer to a minisymbol, and a pointer to a structure returned by bfd_make_empty_symbol, and return a asymbol structure. The return value may or may not be the same as the value from bfd_make_empty_symbol which was passed in.

typedef asymbol
---------------

An asymbol has the form:

     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            0

       /* The symbol has local scope; static in 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 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:
          BSF_LOCAL, BSF_UNDEFINED or 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)

       /* An ELF common symbol.  */
     #define BSF_ELF_COMMON          (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 ISFCN symbol
          which is also 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;

Symbol handling functions
-------------------------

bfd_get_symtab_upper_bound
..........................

Description

Return the number of bytes required to store a vector of pointers to asymbols for all the symbols in the BFD ABFD, including a terminal NULL pointer. If there are no symbols in the BFD, then return 0.  If an error occurs, return -1.

     #define bfd_get_symtab_upper_bound(abfd) \
            BFD_SEND (abfd, _bfd_get_symtab_upper_bound, (abfd))

bfd_is_local_label
..................

Synopsis

     bfd_boolean bfd_is_local_label (bfd *abfd, asymbol *sym);

Description

Return TRUE if the given symbol SYM in the BFD ABFD is a compiler generated local label, else return FALSE.

bfd_is_local_label_name
.......................

Synopsis

     bfd_boolean bfd_is_local_label_name (bfd *abfd, const char *name);

Description

Return TRUE if a symbol with the name NAME in the BFD ABFD is a compiler generated local label, else return FALSE.  This just checks whether the name has the form of a local label.

     #define bfd_is_local_label_name(abfd, name) \
            BFD_SEND (abfd, _bfd_is_local_label_name, (abfd, name))

bfd_is_target_special_symbol
............................

Synopsis

     bfd_boolean bfd_is_target_special_symbol (bfd *abfd, asymbol *sym);

Description

Return TRUE iff a symbol SYM in the BFD ABFD is something special to the particular target represented by the BFD.  Such symbols should normally not be mentioned to the user.

     #define bfd_is_target_special_symbol(abfd, sym) \
            BFD_SEND (abfd, _bfd_is_target_special_symbol, (abfd, sym))

bfd_canonicalize_symtab
.......................

Description

Read the symbols from the BFD ABFD, and fills in the vector LOCATION with pointers to the symbols and a trailing NULL. Return the actual number of symbol pointers, not including the NULL.

     #define bfd_canonicalize_symtab(abfd, location) \
            BFD_SEND (abfd, _bfd_canonicalize_symtab, (abfd, location))

bfd_set_symtab
..............

Synopsis

     bfd_boolean bfd_set_symtab
        (bfd *abfd, asymbol **location, unsigned int count);

Description

Arrange that when the output BFD ABFD is closed, the table LOCATION of COUNT pointers to symbols will be written.

bfd_print_symbol_vandf
......................

Synopsis

     void bfd_print_symbol_vandf (bfd *abfd, void *file, asymbol *symbol);

Description

Print the value and flags of the SYMBOL supplied to the stream FILE.

bfd_make_empty_symbol
.....................

Description

Create a new asymbol structure for the BFD ABFD and return a pointer to it.

This routine is necessary because each back end has private information surrounding the asymbol. Building your own asymbol and pointing to it will not create the private information, and will cause problems later on.

     #define bfd_make_empty_symbol(abfd) \
            BFD_SEND (abfd, _bfd_make_empty_symbol, (abfd))

_bfd_generic_make_empty_symbol
..............................

Synopsis

     asymbol *_bfd_generic_make_empty_symbol (bfd *);

Description

Create a new asymbol structure for the BFD ABFD and return a pointer to it.  Used by core file routines, binary back-end and anywhere else where no private info is needed.

bfd_make_debug_symbol
.....................

Description

Create a new asymbol structure for the BFD ABFD, to be used as a debugging symbol.  Further details of its use have yet to be worked out.

     #define bfd_make_debug_symbol(abfd,ptr,size) \
            BFD_SEND (abfd, _bfd_make_debug_symbol, (abfd, ptr, size))

bfd_decode_symclass
...................

Description

Return a character corresponding to the symbol class of SYMBOL, or '?' for an unknown class.

Synopsis

     int bfd_decode_symclass (asymbol *symbol);

bfd_is_undefined_symclass
.........................

Description

Returns non-zero if the class symbol returned by bfd_decode_symclass represents an undefined symbol. Returns zero otherwise.

Synopsis

     bfd_boolean bfd_is_undefined_symclass (int symclass);

bfd_symbol_info
...............

Description

Fill in the basic info about symbol that nm needs. Additional info may be added by the back-ends after calling this function.

Synopsis

     void bfd_symbol_info (asymbol *symbol, symbol_info *ret);

bfd_copy_private_symbol_data
............................

Synopsis

     bfd_boolean bfd_copy_private_symbol_data
        (bfd *ibfd, asymbol *isym, bfd *obfd, asymbol *osym);

Description

Copy private symbol information from ISYM in the BFD IBFD to the symbol OSYM in the BFD OBFD. Return TRUE on success, FALSE on error.  Possible error returns are:

  * bfd_error_no_memory - Not enough memory exists to create private data for OSEC.

     #define bfd_copy_private_symbol_data(ibfd, isymbol, obfd, osymbol) \
            BFD_SEND (obfd, _bfd_copy_private_symbol_data, \
                      (ibfd, isymbol, obfd, osymbol))