coff backends
=============

BFD supports a number of different flavours of coff format. The major difference between formats are the sizes and alignments of fields in structures on disk, and the occasional extra field.

Coff in all its varieties is implimented with a few common files and a number of implementation specific files. For example, The 88k bcs coff format is implemented in the file m88k-bcs.c. This file #includes m88k-bcs.h which defines the external structure of the coff format for the 88k, and internalcoff.h which defines the internal structure. m88k-bcs.c also defines the relocations used by the 88k format see Relocations. Then the major portion of coff code is included (coffcode.h) which defines the methods used to act upon the types defined in m88k-bcs.h and internalcoff.h.

The Intel i960 processor version of coff is implemented in icoff.c. This file has the same structure as m88k-bcs.c, except that it includes intel-coff.h rather than m88k-bcs.h.

Porting To A New Version of Coff
--------------------------------

The recommended method is to select from the existing implimentations the version of coff which is most like the one you want to use, for our purposes, we'll say that i386 coff is the one you select, and that your coff flavour is called foo. Copy the i386coff.c to foocoff.c, copy ../include/i386coff.h to ../include/foocoff.h and add the lines to targets.c and Makefile.in so that your new back end is used.

Alter the shapes of the structures in ../include/foocoff.h so that they match what you need. You will probably also have to add #ifdefs to the code in internalcoff.h and coffcode.h if your version of coff is too wild.

You can verify that your new BFD backend works quite simply by building objdump from the binutils directory, and making sure that its version of what's going on at your host systems idea (assuming it has the pretty standard coff dump utility (usually called att-dump or just dump)) are the same.

Then clean up your code, and send what you've done to Cygnus. Then your stuff will be in the next release, and you won't have to keep integrating it.

How The Coff Backend Works
--------------------------

Bit Twiddling
.............

Each flavour of coff supported in BFD has its own header file descibing the external layout of the structures. There is also an internal description of the coff layout (in internalcoff.h) file. A major function of the coff backend is swapping the bytes and twiddling the bits to translate the external form of the structures into the normal internal form. This is all performed in the bfd_swap_thing_direction routines. Some elements are different sizes between different versions of coff, it is the duty of the coff version specific include file to override the definitions of various packing routines in coffcode.h. Eg the size of line number entry in coff is sometimes 16 bits, and sometimes 32 bits. #defineing PUT_LNSZ_LNNO and GET_LNSZ_LNNO will select the correct one. No doubt, some day someone will find a version of coff which has a varying field size not catered for at the moment. To port BFD, that person will have to add more #defines.

Three of the bit twiddling routines are exported to gdb; coff_swap_aux_in, coff_swap_sym_in and coff_swap_linno_in. GDB reads the symbol table on its own, but uses BFD to fix things up.

Symbol Reading
..............

The simple canonical form for symbols used by BFD is not rich enough to keep all the information available in a coff symbol table. The back end gets around this by keeping the original symbol table around, "behind the sceens".

When a symbol table is requested (through a call to bfd_canonicalize_symtab, a request gets through to get_normalized_symtab. This reads the symbol table from the coff file and swaps all the structures inside into the internal form. It also fixes up all the pointers in the table (represented in the file by offsets from the first symbol in the table) into physical pointers to elements in the new internal table. This involves some work since the meanings of fields changes depending upon context; a field that is a pointer to another structure in the symbol table at one moment may be the size in bytes of a structure in the next.

Another pass is made over the table. All symbols which mark file names (C_FILE symbols) are modified so that the internal string points to the value in the auxent (the real filename) rather than the normal text associated with the symbol (".file").

At this time the symbol names are moved around. Coff stores all symbols less than nine characters long physically within the symbol table, longer strings are kept at the end of the file in the string table. This pass moves all strings into memory, and replaces them with pointers to the strings.

The symbol table is massaged once again, this time to create the canonical table used by the BFD application. Each symbol is inspected in turn, and a decision made (using the sclass field) about the various flags to set in the asymbol see Symbols. The generated canonical table shares strings with the hidden internal symbol table.

Any linenumbers are read from the coff file too, and attached to the symbols which own the functions the linenumbers belong to.

Symbol Writing
..............

Writing a symbol to a coff file which didn't come from a coff file will lose any debugging information. The asymbol structure remembers the BFD from which was born, and on output the back end makes sure that the same destination target as source target is present.

When the symbols have come from a coff file then all the debugging information is preserved.

Symbol tables are provided for writing to the back end in a vector of pointers to pointers. This allows applications like the linker to accumulate and output large symbol tables without having to do too much byte copying.

The symbol table is not output to a writable BFD until it is closed. The order of operations on the canonical symbol table at that point are:

coff_renumber_symbols
     This function runs through the provided symbol table and patches each symbol marked as a file place holder (C_FILE) to point to the next file place holder in the list. It also marks each offset field in the list with the offset from the first symbol of the current symbol.

     Another function of this procedure is to turn the canonical value form of BFD into the form used by coff. Internally, BFD expects symbol values to be offsets from a section base; so a symbol physically at 0x120, but in a section starting at 0x100, would have the value 0x20. Coff expects symbols to contain their final value, so symbols have their values changed at this point to reflect their sum with their owning section. Note that this transformation uses the output_section field of the asymbol's asection see Sections.

coff_mangle_symbols
     This routine runs though the provided symbol table and uses the offsets generated by the previous pass and the pointers generated when the symbol table was read in to create the structured hierachy required by coff. It changes each pointer to a symbol to an index into the symbol table of the symbol being referenced.

coff_write_symbols
     This routine runs through the symbol table and patches up the symbols from their internal form into the coff way, calls the bit twiddlers and writes out the tabel to the file.

The hidden information for an asymbol is:

      typedef struct coff_ptr_struct
      {

Remembers the offset from the first symbol in the file for this symbol. Generated by coff_renumber_symbols.

        unsigned int offset;

Should the tag field of this symbol be renumbered. Created by coff_pointerize_aux.

        char fix_tag;

Should the endidx field of this symbol be renumbered. Created by coff_pointerize_aux.

        char fix_end;

The container for the symbol structure as read and translated from the file.

        union {
          union internal_auxent auxent;
          struct internal_syment syment;
        } u;
      } combined_entry_type;

Each canonical asymbol really looks like this:

      typedef struct coff_symbol_struct
      {

The actual symbol which the rest of BFD works with

        asymbol symbol;

A pointer to the hidden information for this symbol

        combined_entry_type *native;

A pointer to the linenumber information for this symbol

        struct lineno_cache_entry *lineno;
      } coff_symbol_type;

Writing Relocations
...................

To write a relocations, all the back end does is step though the canonical relocation table, and create an internal_reloc. The symbol index to use is removed from the offset field in the symbol table supplied, the address comes directly from the sum of the section base address and the relocation offset and the type is dug directly from the howto field.

Then the internal_reloc is swapped into the shape of an external_reloc and written out to disk.

Reading Linenumbers
...................

Createing the linenumber table is done by reading in the entire coff linenumber table, and creating another table for internal use.

A coff line number table is structured so that each function is marked as having a line number of 0. Each line within the function is an offset from the first line in the function. The base of the line number information for the table is stored in the symbol associated with the function.

The information is copied from the external to the internal table, and each symbol which marks a function is marked by pointing its...

**How does this work **

Reading Relocations
...................

Coff relocations are easily transformed into the internal BFD form (arelent).

Reading a coff relocation table is done in the following stages:

  * The entire coff relocation table is read into memory.

  * Each relocation is processed in turn, first it is swapped from the external to the internal form.

  * The symbol referenced in the relocation's symbol index is turned into a pointer into the canonical symbol table. Note that this table is the same as the one returned by a call to bfd_canonicalize_symtab. The back end will call the routine and save the result if a canonicalization hasn't been done.

  * The reloc index is turned into a pointer to a howto structure, in a back end specific way. For instance, the 386 and 960 use the r_type to directly produce an index into a howto table vector; the 88k subtracts a number from the r_type field and creates an addend field.