binutils-gdb/bfd/syms.c

/* Generic symbol-table support for the BFD library.
   Copyright (C) 1990-1991 Free Software Foundation, Inc.
   Written by Cygnus Support.

This file is part of BFD, the Binary File Descriptor library.

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.  */

/*doc*
@section Symbols
BFD trys 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 infomation passed to applications some targets keep
some information 'behind the sceans', 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 was read, but was written through an a.out back end,
all the coff specific information would be lost. (.. until BFD 2 :).

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 read, since the 'behind the sceens'
information will be still available.

@menu
* Reading Symbols::
* Writing 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
appliction which reads the symbol table:

*+
  unsigned int storage_needed;
  asymbol **symbol_table;
  unsigned int number_of_symbols;
  unsigned int i;

  storage_needed = get_symtab_upper_bound (abfd);

  if (storage_needed == 0) {
     return ;
  }
  symbol_table = (asymbol **) malloc (storage_needed);
    ...
  number_of_symbols = 
     bfd_canonicalize_symtab (abfd, symbol_table); 

  for (i = 0; i < number_of_symbols; i++) {
     process_symbol (symbol_table[i]);
  }
*-

All storage for the symbols themselves is in an obstack connected to
the BFD, and is freed when the BFD is closed.

@node Writing Symbols, typedef asymbol, 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 outputing 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}. 

An example showing the creation of a symbol table with only one
element:

*+
#include "bfd.h"
main() 
{
  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 = (asection *)0;
  new->flags = BSF_ABSOLUTE | BSF_GLOBAL;
  new->value = 0x12345;

  ptrs[0] = new;
  ptrs[1] = (asymbol *)0;
  
  bfd_set_symtab(abfd, ptrs, 1);
  bfd_close(abfd);
}

./makesym 
nm foo
00012345 A dummy_symbol


*-

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


/*doc*
@node typedef asymbol, symbol handling functions, Writing Symbols, Symbols

*/
/*proto*
@subsection typedef asymbol
An @code{asymbol} has the form:

*+++

$typedef struct symbol_cache_entry 
${
A pointer to the BFD which owns the symbol. This information is
necessary so that a back end can work out what additional (invisible to
the application writer) information is carried with the symbol. 

$  struct _bfd *the_bfd;

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.

$   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

Obsolete

$#define BSF_IMPORT	0x04

The symbol has global scope, and is exported. The value is the offset
into the section of the data.

$#define BSF_EXPORT	0x08

The symbol is undefined. @code{extern} in @code{C}. The value has no meaning.

$#define BSF_UNDEFINED	0x10	

The symbol is common, initialized to zero; default in @code{C}. The
value is the size of the object in bytes.

$#define BSF_FORT_COMM	0x20	

A normal @code{C} symbol would be one of:
@code{BSF_LOCAL}, @code{BSF_FORT_COMM},  @code{BSF_UNDEFINED} or @code{BSF_EXPORT|BSD_GLOBAL}

The symbol is a debugging record. The value has an arbitary meaning.

$#define BSF_DEBUGGING	0x40

The symbol has no section attached, any value is the actual value and
is not a relative offset to a section.

$#define BSF_ABSOLUTE	0x80

Used by the linker

$#define BSF_KEEP        0x10000
$#define BSF_KEEP_G      0x80000

Unused

$#define BSF_WEAK        0x100000
$#define BSF_CTOR        0x200000 
$#define BSF_FAKE        0x400000 

The symbol used to be a common symbol, but now it is allocated.

$#define BSF_OLD_COMMON  0x800000  

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    0x40000

Signal that the symbol is the label of constructor section.

$#define BSF_CONSTRUCTOR   0x1000000

Signal that the symbol is a warning symbol. If the symbol is a warning
symbol, then the value field (I know this is tacky) will point to the
asymbol which when referenced will cause the warning.

$#define BSF_WARNING       0x2000000

Signal that the symbol is indirect. The value of the symbol is a
pointer to an undefined asymbol which contains the name to use
instead.

$#define BSF_INDIRECT     0x4000000

$  flagword flags;

Aointer to the section to which this symbol is relative, or 0 if the
symbol is absolute or undefined. Note that it is not sufficient to set
this location to 0 to mark a symbol as absolute - the flag
@code{BSF_ABSOLUTE} must be set also.

$  struct sec *section;

Back end special data. This is being phased out in favour of making
this a union.

$  PTR udata;	
$} asymbol;
*---

*/

#include "sysdep.h"
#include "bfd.h"
#include "libbfd.h"
 
/*doc*
@node symbol handling functions, Symbols, typedef asymbol, Symbols
@subsection Symbol Handling Functions

*/

/*proto* get_symtab_upper_bound
Returns the number of bytes required in a vector of pointers to
@code{asymbols} for all the symbols in the supplied BFD, including a
terminal NULL pointer. If there are no symbols in the BFD, then 0 is
returned.
*+
#define get_symtab_upper_bound(abfd) \
     BFD_SEND (abfd, _get_symtab_upper_bound, (abfd))
*-

*/

/*proto* bfd_canonicalize_symtab
Supplied a BFD and a pointer to an uninitialized vector of pointers.
This reads in the symbols from the BFD, and fills in the table with
pointers to the symbols, and a trailing NULL. The routine returns the
actual number of symbol pointers not including the NULL.

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

*-
*/


/*proto* bfd_set_symtab
Provided a table of pointers to to symbols and a count, writes to the
output BFD the symbols when closed.

*; PROTO(boolean, bfd_set_symtab, (bfd *, asymbol **, unsigned int ));
*/

boolean
bfd_set_symtab (abfd, location, symcount)
     bfd *abfd;
     asymbol **location;
     unsigned int symcount;
{
  if ((abfd->format != bfd_object) || (bfd_read_p (abfd))) {
    bfd_error = invalid_operation;
    return false;
  }

  bfd_get_outsymbols (abfd) = location;
  bfd_get_symcount (abfd) = symcount;
  return true;
}

/*proto* bfd_print_symbol_vandf
Prints the value and flags of the symbol supplied to the stream file.

*; PROTO(void, bfd_print_symbol_vandf, (PTR file, asymbol *symbol));
*/
void
DEFUN(bfd_print_symbol_vandf,(file, symbol),
PTR file AND
asymbol *symbol)
{
  flagword type = symbol->flags;
  if (symbol->section != (asection *)NULL)
      {
	fprintf_vma(file, symbol->value+symbol->section->vma);
      }
  else 
      {
	fprintf_vma(file, symbol->value);
      }
  fprintf(file," %c%c%c%c%c%c%c%c%c%c",
	  (type & BSF_LOCAL)  ? 'l':' ',
	  (type & BSF_GLOBAL) ? 'g' : ' ',
	  (type & BSF_IMPORT) ? 'i' : ' ',
	  (type & BSF_EXPORT) ? 'e' : ' ',
	  (type & BSF_UNDEFINED) ? 'u' : ' ',
	  (type & BSF_FORT_COMM) ? 'c' : ' ',
	  (type & BSF_CONSTRUCTOR) ? 'C' : ' ',
	  (type & BSF_WARNING) ? 'W' : ' ',
	  (type & BSF_INDIRECT) ? 'I' : ' ',
	  (type & BSF_DEBUGGING) ? 'd' :' ');

}


/*proto*  bfd_make_empty_symbol
This function creates a new @code{asymbol} structure for the BFD, and
returns a pointer to it.

This routine is necessary, since 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.
*+
#define bfd_make_empty_symbol(abfd) \
     BFD_SEND (abfd, _bfd_make_empty_symbol, (abfd))
*-
*/