syms.texi

这个是LINUX下的GDB调度工具的源码

@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      0x01

  /* The symbol has global scope; initialized data in @code{C}. The
     value is the offset into the section of the data.  */
#define BSF_GLOBAL     0x02

  /* 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_FORT_COMM},  @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  0x08

  /* The symbol denotes a function entry point.  Used in ELF,
     perhaps others someday.  */
#define BSF_FUNCTION    0x10

  /* Used by the linker.  */
#define BSF_KEEP        0x20
#define BSF_KEEP_G      0x40

  /* A weak global symbol, overridable without warnings by
     a regular global symbol of the same name.  */
#define BSF_WEAK        0x80

  /* This symbol was created to point to a section, e.g. ELF's
     STT_SECTION symbols.  */
#define BSF_SECTION_SYM 0x100

  /* The symbol used to be a common symbol, but now it is
     allocated.  */
#define BSF_OLD_COMMON  0x200

  /* The default value for common data.  */
#define BFD_FORT_COMM_DEFAULT_VALUE 0

  /* 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    0x400

  /* Signal that the symbol is the label of constructor section.  */
#define BSF_CONSTRUCTOR   0x800

  /* 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       0x1000

  /* 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      0x2000

  /* BSF_FILE marks symbols that contain a file name.  This is used
     for ELF STT_FILE symbols.  */