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### 3.3 coff backends

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

Coff in all its varieties is implemented with a few common files and a number of implementation specific files. For example, The 88k bcs coff format is implemented in the file coff-m88k.c. This file `#include`s coff/m88k.h which defines the external structure of the coff format for the 88k, and coff/internal.h which defines the internal structure. coff-m88k.c also defines the relocations used by the 88k format See Relocations.

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

#### 3.3.1 Porting to a new version of coff

The recommended method is to select from the existing implementations the version of coff which is most like the one you want to use. For example, we'll say that i386 coff is the one you select, and that your coff flavour is called foo. Copy i386coff.c to foocoff.c, copy ../include/coff/i386.h to ../include/coff/foo.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/coff/foo.h so that they match what you need. You will probably also have to add `#ifdef`s to the code in coff/internal.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 and your host system's 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.

#### 3.3.2 How the coff backend works

##### 3.3.2.1 File layout

The Coff backend is split into generic routines that are applicable to any Coff target and routines that are specific to a particular target. The target-specific routines are further split into ones which are basically the same for all Coff targets except that they use the external symbol format or use different values for certain constants.

The generic routines are in coffgen.c. These routines work for any Coff target. They use some hooks into the target specific code; the hooks are in a `bfd_coff_backend_data` structure, one of which exists for each target.

The essentially similar target-specific routines are in coffcode.h. This header file includes executable C code. The various Coff targets first include the appropriate Coff header file, make any special defines that are needed, and then include coffcode.h.

Some of the Coff targets then also have additional routines in the target source file itself.

For example, coff-i960.c includes coff/internal.h and coff/i960.h. It then defines a few constants, such as `I960`, and includes coffcode.h. Since the i960 has complex relocation types, coff-i960.c also includes some code to manipulate the i960 relocs. This code is not in coffcode.h because it would not be used by any other target.

##### 3.3.2.2 Bit twiddling

Each flavour of coff supported in BFD has its own header file describing the external layout of the structures. There is also an internal description of the coff layout, in coff/internal.h. 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. E.g., the size of line number entry in coff is sometimes 16 bits, and sometimes 32 bits. `#define`ing `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 to 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_lineno_in`. `GDB` reads the symbol table on its own, but uses BFD to fix things up. More of the bit twiddlers are exported for `gas`; `coff_swap_aux_out`, `coff_swap_sym_out`, `coff_swap_lineno_out`, `coff_swap_reloc_out`, `coff_swap_filehdr_out`, `coff_swap_aouthdr_out`, `coff_swap_scnhdr_out`. `Gas` currently keeps track of all the symbol table and reloc drudgery itself, thereby saving the internal BFD overhead, but uses BFD to swap things on the way out, making cross ports much safer. Doing so also allows BFD (and thus the linker) to use the same header files as `gas`, which makes one avenue to disaster disappear.

##### 3.3.2.3 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 problem by keeping the original symbol table around, "behind the scenes".

When a symbol table is requested (through a call to `bfd_canonicalize_symtab`), a request gets through to `coff_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 change 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 at 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.

##### 3.3.2.4 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 the symbol was taken, 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.

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. 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 hierarchy required by coff. It changes each pointer to a symbol into the index into the symbol table of the asymbol.

- `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 table to the file.

##### 3.3.2.5 `coff_symbol_type`

Description
The hidden information for an `asymbol` is described in a `combined_entry_type`:

         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 value of this symbol be renumbered.  Used for
              XCOFF C_BSTAT symbols.  Set by coff_slurp_symbol_table.  */
           unsigned int fix_value : 1;

           /* Should the tag field of this symbol be renumbered.
              Created by coff_pointerize_aux. */
           unsigned int fix_tag : 1;

           /* Should the endidx field of this symbol be renumbered.
              Created by coff_pointerize_aux. */
           unsigned int fix_end : 1;

           /* Should the x_csect.x_scnlen field be renumbered.
              Created by coff_pointerize_aux. */
           unsigned int fix_scnlen : 1;

           /* Fix up an XCOFF C_BINCL/C_EINCL symbol.  The value is the
              index into the line number entries.  Set by coff_slurp_symbol_table.  */
           unsigned int fix_line : 1;

           /* 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;

           /* Have the line numbers been relocated yet ? */
           bfd_boolean done_lineno;
         } coff_symbol_type;

##### 3.3.2.6 `bfd_coff_backend_data`

         /* COFF symbol classifications.  */

         enum coff_symbol_classification
         {
           /* Global symbol.  */
           COFF_SYMBOL_GLOBAL,
           /* Common symbol.  */
           COFF_SYMBOL_COMMON,
           /* Undefined symbol.  */
           COFF_SYMBOL_UNDEFINED,
           /* Local symbol.  */
           COFF_SYMBOL_LOCAL,
           /* PE section symbol.  */
           COFF_SYMBOL_PE_SECTION
         };

Special entry points for gdb to swap in coff symbol table parts:

         typedef struct
         {
           void (*_bfd_coff_swap_aux_in)
             (bfd *, void *, int, int, int, int, void *);

           void (*_bfd_coff_swap_sym_in)
             (bfd *, void *, void *);

           void (*_bfd_coff_swap_lineno_in)
             (bfd *, void *, void *);

           unsigned int (*_bfd_coff_swap_aux_out)
             (bfd *, void *, int, int, int, int, void *);

           unsigned int (*_bfd_coff_swap_sym_out)
             (bfd *, void *, void *);

           unsigned int (*_bfd_coff_swap_lineno_out)
             (bfd *, void *, void *);

           unsigned int (*_bfd_coff_swap_reloc_out)
             (bfd *, void *, void *);

           unsigned int (*_bfd_coff_swap_filehdr_out)
             (bfd *, void *, void *);

           unsigned int (*_bfd_coff_swap_aouthdr_out)
             (bfd *, void *, void *);

           unsigned int (*_bfd_coff_swap_scnhdr_out)
             (bfd *, void *, void *);

           unsigned int _bfd_filhsz;
           unsigned int _bfd_aoutsz;
           unsigned int _bfd_scnhsz;
           unsigned int _bfd_symesz;
           unsigned int _bfd_auxesz;
           unsigned int _bfd_relsz;
           unsigned int _bfd_linesz;
           unsigned int _bfd_filnmlen;
           bfd_boolean _bfd_coff_long_filenames;
           bfd_boolean _bfd_coff_long_section_names;
           unsigned int _bfd_coff_default_section_alignment_power;
           bfd_boolean _bfd_coff_force_symnames_in_strings;
           unsigned int _bfd_coff_debug_string_prefix_length;

           void (*_bfd_coff_swap_filehdr_in)
             (bfd *, void *, void *);

           void (*_bfd_coff_swap_aouthdr_in)
             (bfd *, void *, void *);

           void (*_bfd_coff_swap_scnhdr_in)
             (bfd *, void *, void *);

           void (*_bfd_coff_swap_reloc_in)
             (bfd *abfd, void *, void *);

           bfd_boolean (*_bfd_coff_bad_format_hook)
             (bfd *, void *);

           bfd_boolean (*_bfd_coff_set_arch_mach_hook)
             (bfd *, void *);

           void * (*_bfd_coff_mkobject_hook)
             (bfd *, void *, void *);

           bfd_boolean (*_bfd_styp_to_sec_flags_hook)
             (bfd *, void *, const char *, asection *, flagword *);

           void (*_bfd_set_alignment_hook)
             (bfd *, asection *, void *);

           bfd_boolean (*_bfd_coff_slurp_symbol_table)
             (bfd *);

           bfd_boolean (*_bfd_coff_symname_in_debug)
             (bfd *, struct internal_syment *);

           bfd_boolean (*_bfd_coff_pointerize_aux_hook)
             (bfd *, combined_entry_type *, combined_entry_type *,
                     unsigned int, combined_entry_type *);

           bfd_boolean (*_bfd_coff_print_aux)
             (bfd *, FILE *, combined_entry_type *, combined_entry_type *,
                     combined_entry_type *, unsigned int);

           void (*_bfd_coff_reloc16_extra_cases)
             (bfd *, struct bfd_link_info *, struct bfd_link_order *, arelent *,
                    bfd_byte *, unsigned int *, unsigned int *);

           int (*_bfd_coff_reloc16_estimate)
             (bfd *, asection *, arelent *, unsigned int,
                     struct bfd_link_info *);

           enum coff_symbol_classification (*_bfd_coff_classify_symbol)
             (bfd *, struct internal_syment *);

           bfd_boolean (*_bfd_coff_compute_section_file_positions)
             (bfd *);

           bfd_boolean (*_bfd_coff_start_final_link)
             (bfd *, struct bfd_link_info *);

           bfd_boolean (*_bfd_coff_relocate_section)
             (bfd *, struct bfd_link_info *, bfd *, asection *, bfd_byte *,
                     struct internal_reloc *, struct internal_syment *, asection **);

           reloc_howto_type *(*_bfd_coff_rtype_to_howto)
             (bfd *, asection *, struct internal_reloc *,
                     struct coff_link_hash_entry *, struct internal_syment *,
                     bfd_vma *);

           bfd_boolean (*_bfd_coff_adjust_symndx)
             (bfd *, struct bfd_link_info *, bfd *, asection *,
                     struct internal_reloc *, bfd_boolean *);

           bfd_boolean (*_bfd_coff_link_add_one_symbol)
             (struct bfd_link_info *, bfd *, const char *, flagword,
                     asection *, bfd_vma, const char *, bfd_boolean, bfd_boolean,
                     struct bfd_link_hash_entry **);

           bfd_boolean (*_bfd_coff_link_output_has_begun)
             (bfd *, struct coff_final_link_info *);

           bfd_boolean (*_bfd_coff_final_link_postscript)
             (bfd *, struct coff_final_link_info *);

           bfd_boolean (*_bfd_coff_print_pdata)
             (bfd *, void *);

         } bfd_coff_backend_data;

         #define coff_backend_info(abfd) \
           ((bfd_coff_backend_data *) (abfd)->xvec->backend_data)

         #define bfd_coff_swap_aux_in(a,e,t,c,ind,num,i) \
           ((coff_backend_info (a)->_bfd_coff_swap_aux_in) (a,e,t,c,ind,num,i))

         #define bfd_coff_swap_sym_in(a,e,i) \
           ((coff_backend_info (a)->_bfd_coff_swap_sym_in) (a,e,i))

         #define bfd_coff_swap_lineno_in(a,e,i) \
           ((coff_backend_info ( a)->_bfd_coff_swap_lineno_in) (a,e,i))

         #define bfd_coff_swap_reloc_out(abfd, i, o) \
           ((coff_backend_info (abfd)->_bfd_coff_swap_reloc_out) (abfd, i, o))

         #define bfd_coff_swap_lineno_out(abfd, i, o) \
           ((coff_backend_info (abfd)->_bfd_coff_swap_lineno_out) (abfd, i, o))

         #define bfd_coff_swap_aux_out(a,i,t,c,ind,num,o) \
           ((coff_backend_info (a)->_bfd_coff_swap_aux_out) (a,i,t,c,ind,num,o))

         #define bfd_coff_swap_sym_out(abfd, i,o) \
           ((coff_backend_info (abfd)->_bfd_coff_swap_sym_out) (abfd, i, o))

         #define bfd_coff_swap_scnhdr_out(abfd, i,o) \
           ((coff_backend_info (abfd)->_bfd_coff_swap_scnhdr_out) (abfd, i, o))

         #define bfd_coff_swap_filehdr_out(abfd, i,o) \
           ((coff_backend_info (abfd)->_bfd_coff_swap_filehdr_out) (abfd, i, o))

         #define bfd_coff_swap_aouthdr_out(abfd, i,o) \
           ((coff_backend_info (abfd)->_bfd_coff_swap_aouthdr_out) (abfd, i, o))

         #define bfd_coff_filhsz(abfd) (coff_backend_info (abfd)->_bfd_filhsz)
         #define bfd_coff_aoutsz(abfd) (coff_backend_info (abfd)->_bfd_aoutsz)
         #define bfd_coff_scnhsz(abfd) (coff_backend_info (abfd)->_bfd_scnhsz)
         #define bfd_coff_symesz(abfd) (coff_backend_info (abfd)->_bfd_symesz)
         #define bfd_coff_auxesz(abfd) (coff_backend_info (abfd)->_bfd_auxesz)
         #define bfd_coff_relsz(abfd)  (coff_backend_info (abfd)->_bfd_relsz)
         #define bfd_coff_linesz(abfd) (coff_backend_info (abfd)->_bfd_linesz)
         #define bfd_coff_filnmlen(abfd) (coff_backend_info (abfd)->_bfd_filnmlen)
         #define bfd_coff_long_filenames(abfd) \
           (coff_backend_info (abfd)->_bfd_coff_long_filenames)
         #define bfd_coff_long_section_names(abfd) \
           (coff_backend_info (abfd)->_bfd_coff_long_section_names)
         #define bfd_coff_default_section_alignment_power(abfd) \
           (coff_backend_info (abfd)->_bfd_coff_default_section_alignment_power)
         #define bfd_coff_swap_filehdr_in(abfd, i,o) \
           ((coff_backend_info (abfd)->_bfd_coff_swap_filehdr_in) (abfd, i, o))

         #define bfd_coff_swap_aouthdr_in(abfd, i,o) \
           ((coff_backend_info (abfd)->_bfd_coff_swap_aouthdr_in) (abfd, i, o))

         #define bfd_coff_swap_scnhdr_in(abfd, i,o) \
           ((coff_backend_info (abfd)->_bfd_coff_swap_scnhdr_in) (abfd, i, o))

         #define bfd_coff_swap_reloc_in(abfd, i, o) \
           ((coff_backend_info (abfd)->_bfd_coff_swap_reloc_in) (abfd, i, o))

         #define bfd_coff_bad_format_hook(abfd, filehdr) \
           ((coff_backend_info (abfd)->_bfd_coff_bad_format_hook) (abfd, filehdr))

         #define bfd_coff_set_arch_mach_hook(abfd, filehdr)\
           ((coff_backend_info (abfd)->_bfd_coff_set_arch_mach_hook) (abfd, filehdr))
         #define bfd_coff_mkobject_hook(abfd, filehdr, aouthdr)\
           ((coff_backend_info (abfd)->_bfd_coff_mkobject_hook)\
            (abfd, filehdr, aouthdr))

         #define bfd_coff_styp_to_sec_flags_hook(abfd, scnhdr, name, section, flags_ptr)\
           ((coff_backend_info (abfd)->_bfd_styp_to_sec_flags_hook)\
            (abfd, scnhdr, name, section, flags_ptr))

         #define bfd_coff_set_alignment_hook(abfd, sec, scnhdr)\
           ((coff_backend_info (abfd)->_bfd_set_alignment_hook) (abfd, sec, scnhdr))

         #define bfd_coff_slurp_symbol_table(abfd)\
           ((coff_backend_info (abfd)->_bfd_coff_slurp_symbol_table) (abfd))

         #define bfd_coff_symname_in_debug(abfd, sym)\
           ((coff_backend_info (abfd)->_bfd_coff_symname_in_debug) (abfd, sym))

         #define bfd_coff_force_symnames_in_strings(abfd)\
           (coff_backend_info (abfd)->_bfd_coff_force_symnames_in_strings)

         #define bfd_coff_debug_string_prefix_length(abfd)\
           (coff_backend_info (abfd)->_bfd_coff_debug_string_prefix_length)

         #define bfd_coff_print_aux(abfd, file, base, symbol, aux, indaux)\
           ((coff_backend_info (abfd)->_bfd_coff_print_aux)\
            (abfd, file, base, symbol, aux, indaux))

         #define bfd_coff_reloc16_extra_cases(abfd, link_info, link_order,\
                                              reloc, data, src_ptr, dst_ptr)\
           ((coff_backend_info (abfd)->_bfd_coff_reloc16_extra_cases)\
            (abfd, link_info, link_order, reloc, data, src_ptr, dst_ptr))

         #define bfd_coff_reloc16_estimate(abfd, section, reloc, shrink, link_info)\
           ((coff_backend_info (abfd)->_bfd_coff_reloc16_estimate)\
            (abfd, section, reloc, shrink, link_info))

         #define bfd_coff_classify_symbol(abfd, sym)\
           ((coff_backend_info (abfd)->_bfd_coff_classify_symbol)\
            (abfd, sym))

         #define bfd_coff_compute_section_file_positions(abfd)\
           ((coff_backend_info (abfd)->_bfd_coff_compute_section_file_positions)\
            (abfd))

         #define bfd_coff_start_final_link(obfd, info)\
           ((coff_backend_info (obfd)->_bfd_coff_start_final_link)\
            (obfd, info))
         #define bfd_coff_relocate_section(obfd,info,ibfd,o,con,rel,isyms,secs)\
           ((coff_backend_info (ibfd)->_bfd_coff_relocate_section)\
            (obfd, info, ibfd, o, con, rel, isyms, secs))
         #define bfd_coff_rtype_to_howto(abfd, sec, rel, h, sym, addendp)\
           ((coff_backend_info (abfd)->_bfd_coff_rtype_to_howto)\
            (abfd, sec, rel, h, sym, addendp))
         #define bfd_coff_adjust_symndx(obfd, info, ibfd, sec, rel, adjustedp)\
           ((coff_backend_info (abfd)->_bfd_coff_adjust_symndx)\
            (obfd, info, ibfd, sec, rel, adjustedp))
         #define bfd_coff_link_add_one_symbol(info, abfd, name, flags, section,\
                                              value, string, cp, coll, hashp)\
           ((coff_backend_info (abfd)->_bfd_coff_link_add_one_symbol)\
            (info, abfd, name, flags, section, value, string, cp, coll, hashp))

         #define bfd_coff_link_output_has_begun(a,p) \
           ((coff_backend_info (a)->_bfd_coff_link_output_has_begun) (a, p))
         #define bfd_coff_final_link_postscript(a,p) \
           ((coff_backend_info (a)->_bfd_coff_final_link_postscript) (a, p))

         #define bfd_coff_have_print_pdata(a) \
           (coff_backend_info (a)->_bfd_coff_print_pdata)
         #define bfd_coff_print_pdata(a,p) \
           ((coff_backend_info (a)->_bfd_coff_print_pdata) (a, p))

##### 3.3.2.7 Writing relocations

To write relocations, the back end steps 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; 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.

##### 3.3.2.8 Reading linenumbers

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

A coff linenumber 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.

Note: The PE format uses line number 0 for a flag indicating a new source file.

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 ?

##### 3.3.2.9 Reading relocations

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

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

- Read the entire coff relocation table into memory.
- Process each relocation in turn; first swap it from the external to the internal form.
- Turn the symbol referenced in the relocation's symbol index into a pointer into the canonical symbol table. This table is the same as the one returned by a call to `bfd_canonicalize_symtab`. The back end will call that 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.