Targets
=======

Description

Each port of BFD to a different machine requries the creation of a target back end. All the back end provides to the root part of BFD is a structure containing pointers to functions which perform certain low level operations on files. BFD translates the applications's requests through a pointer into calls to the back end routines.

When a file is opened with bfd_openr, its format and target are unknown. BFD uses various mechanisms to determine how to interpret the file. The operations performed are:

  * Create a BFD by calling the internal routine _bfd_new_bfd, then call bfd_find_target with the target string supplied to bfd_openr and the new BFD pointer.

  * If a null target string was provided to bfd_find_target, look up the environment variable GNUTARGET and use that as the target string.

  * If the target string is still NULL, or the target string is default, then use the first item in the target vector as the target type, and set target_defaulted in the BFD to cause bfd_check_format to loop through all the targets. See bfd_target.  See Formats.

  * Otherwise, inspect the elements in the target vector one by one, until a match on target name is found. When found, use it.

  * Otherwise return the error bfd_error_invalid_target to bfd_openr.

  * bfd_openr attempts to open the file using bfd_open_file, and returns the BFD.

Once the BFD has been opened and the target selected, the file format may be determined. This is done by calling bfd_check_format on the BFD with a suggested format. If target_defaulted has been set, each possible target type is tried to see if it recognizes the specified format. bfd_check_format returns true when the caller guesses right.

bfd_target
----------

Description

This structure contains everything that BFD knows about a target. It includes things like its byte order, name, and which routines to call to do various operations.

Every BFD points to a target structure with its xvec member.

The macros below are used to dispatch to functions through the bfd_target vector. They are used in a number of macros further down in bfd.h, and are also used when calling various routines by hand inside the BFD implementation.  The ARGLIST argument must be parenthesized; it contains all the arguments to the called function.

They make the documentation (more) unpleasant to read, so if someone wants to fix this and not break the above, please do.

     #define BFD_SEND(bfd, message, arglist) \
                    ((*((bfd)->xvec->message)) arglist)

     #ifdef DEBUG_BFD_SEND
     #undef BFD_SEND
     #define BFD_SEND(bfd, message, arglist) \
       (((bfd) && (bfd)->xvec && (bfd)->xvec->message) ? \
         ((*((bfd)->xvec->message)) arglist) : \
         (bfd_assert (__FILE__,__LINE__), NULL))
     #endif

For operations which index on the BFD format:

     #define BFD_SEND_FMT(bfd, message, arglist) \
                 (((bfd)->xvec->message[(int)((bfd)->format)]) arglist)

     #ifdef DEBUG_BFD_SEND
     #undef BFD_SEND_FMT
     #define BFD_SEND_FMT(bfd, message, arglist) \
       (((bfd) && (bfd)->xvec && (bfd)->xvec->message) ? \
        (((bfd)->xvec->message[(int)((bfd)->format)]) arglist) : \
        (bfd_assert (__FILE__,__LINE__), NULL))
     #endif

This is the structure which defines the type of BFD this is.  The xvec member of the struct bfd itself points here.  Each module that implements access to a different target under BFD, defines one of these.

FIXME, these names should be rationalised with the names of the entry points which call them. Too bad we can't have one macro to define them both!

     enum bfd_flavour {
       bfd_target_unknown_flavour,
       bfd_target_aout_flavour,
       bfd_target_coff_flavour,
       bfd_target_ecoff_flavour,
       bfd_target_elf_flavour,
       bfd_target_ieee_flavour,
       bfd_target_nlm_flavour,
       bfd_target_oasys_flavour,
       bfd_target_tekhex_flavour,
       bfd_target_srec_flavour,
       bfd_target_ihex_flavour,
       bfd_target_som_flavour,
       bfd_target_os9k_flavour,
       bfd_target_versados_flavour,
       bfd_target_msdos_flavour,
       bfd_target_ovax_flavour,
       bfd_target_evax_flavour
     };

     enum bfd_endian { BFD_ENDIAN_BIG, BFD_ENDIAN_LITTLE, BFD_ENDIAN_UNKNOWN };

      /* Forward declaration.  */
     typedef struct bfd_link_info _bfd_link_info;

     typedef struct bfd_target
     {

Identifies the kind of target, e.g., SunOS4, Ultrix, etc.

       char *name;

The "flavour" of a back end is a general indication about the contents of a file.

       enum bfd_flavour flavour;

The order of bytes within the data area of a file.

       enum bfd_endian byteorder;

The order of bytes within the header parts of a file.

       enum bfd_endian header_byteorder;

A mask of all the flags which an executable may have set - from the set BFD_NO_FLAGS, HAS_RELOC, ...D_PAGED.

       flagword object_flags;

A mask of all the flags which a section may have set - from the set SEC_NO_FLAGS, SEC_ALLOC, ...SET_NEVER_LOAD.

       flagword section_flags;

The character normally found at the front of a symbol (if any), perhaps `_'.

       char symbol_leading_char;

The pad character for file names within an archive header.

       char ar_pad_char;

The maximum number of characters in an archive header.

       unsigned short ar_max_namelen;

Entries for byte swapping for data. These are different from the other entry points, since they don't take a BFD asthe first argument. Certain other handlers could do the same.

       bfd_vma      (*bfd_getx64) PARAMS ((const bfd_byte *));
       bfd_signed_vma (*bfd_getx_signed_64) PARAMS ((const bfd_byte *));
       void         (*bfd_putx64) PARAMS ((bfd_vma, bfd_byte *));
       bfd_vma      (*bfd_getx32) PARAMS ((const bfd_byte *));
       bfd_signed_vma (*bfd_getx_signed_32) PARAMS ((const bfd_byte *));
       void         (*bfd_putx32) PARAMS ((bfd_vma, bfd_byte *));
       bfd_vma      (*bfd_getx16) PARAMS ((const bfd_byte *));
       bfd_signed_vma (*bfd_getx_signed_16) PARAMS ((const bfd_byte *));
       void         (*bfd_putx16) PARAMS ((bfd_vma, bfd_byte *));

Byte swapping for the headers

       bfd_vma      (*bfd_h_getx64) PARAMS ((const bfd_byte *));
       bfd_signed_vma (*bfd_h_getx_signed_64) PARAMS ((const bfd_byte *));
       void         (*bfd_h_putx64) PARAMS ((bfd_vma, bfd_byte *));
       bfd_vma      (*bfd_h_getx32) PARAMS ((const bfd_byte *));
       bfd_signed_vma (*bfd_h_getx_signed_32) PARAMS ((const bfd_byte *));
       void         (*bfd_h_putx32) PARAMS ((bfd_vma, bfd_byte *));
       bfd_vma      (*bfd_h_getx16) PARAMS ((const bfd_byte *));
       bfd_signed_vma (*bfd_h_getx_signed_16) PARAMS ((const bfd_byte *));
       void         (*bfd_h_putx16) PARAMS ((bfd_vma, bfd_byte *));

Format dependent routines: these are vectors of entry points within the target vector structure, one for each format to check.

Check the format of a file being read.  Return a bfd_target * or zero.

       const struct bfd_target *(*_bfd_check_format[bfd_type_end]) PARAMS ((bfd *));

Set the format of a file being written.

       boolean             (*_bfd_set_format[bfd_type_end]) PARAMS ((bfd *));

Write cached information into a file being written, at bfd_close.

       boolean             (*_bfd_write_contents[bfd_type_end]) PARAMS ((bfd *));

The general target vector.  These vectors are initialized using the BFD_JUMP_TABLE macros.

        /* Generic entry points.  */
     #define BFD_JUMP_TABLE_GENERIC(NAME)\
     CAT(NAME,_close_and_cleanup),\
     CAT(NAME,_bfd_free_cached_info),\
     CAT(NAME,_new_section_hook),\
     CAT(NAME,_get_section_contents),\
     CAT(NAME,_get_section_contents_in_window)

        /* Called when the BFD is being closed to do any necessary cleanup.  */
       boolean       (*_close_and_cleanup) PARAMS ((bfd *));
        /* Ask the BFD to free all cached information.  */
       boolean (*_bfd_free_cached_info) PARAMS ((bfd *));
        /* Called when a new section is created.  */
       boolean       (*_new_section_hook) PARAMS ((bfd *, sec_ptr));
        /* Read the contents of a section.  */
       boolean       (*_bfd_get_section_contents) PARAMS ((bfd *, sec_ptr, PTR,
                                                 file_ptr, bfd_size_type));
       boolean       (*_bfd_get_section_contents_in_window)
                               PARAMS ((bfd *, sec_ptr, bfd_window *,
                                        file_ptr, bfd_size_type));

        /* Entry points to copy private data.  */
     #define BFD_JUMP_TABLE_COPY(NAME)\
     CAT(NAME,_bfd_copy_private_bfd_data),\
     CAT(NAME,_bfd_merge_private_bfd_data),\
     CAT(NAME,_bfd_copy_private_section_data),\
     CAT(NAME,_bfd_copy_private_symbol_data),\
     CAT(NAME,_bfd_set_private_flags),\
     CAT(NAME,_bfd_print_private_bfd_data)\
        /* Called to copy BFD general private data from one object file
          to another.  */
       boolean       (*_bfd_copy_private_bfd_data) PARAMS ((bfd *, bfd *));
        /* Called to merge BFD general private data from one object file
          to a common output file when linking.  */
       boolean       (*_bfd_merge_private_bfd_data) PARAMS ((bfd *, bfd *));
        /* Called to copy BFD private section data from one object file
          to another.  */
       boolean       (*_bfd_copy_private_section_data) PARAMS ((bfd *, sec_ptr,
                                                            bfd *, sec_ptr));
        /* Called to copy BFD private symbol data from one symbol
          to another.  */
       boolean       (*_bfd_copy_private_symbol_data) PARAMS ((bfd *, asymbol *,
                                                               bfd *, asymbol *));
        /* Called to set private backend flags */
       boolean       (*_bfd_set_private_flags) PARAMS ((bfd *, flagword));

        /* Called to print private BFD data */
       boolean       (*_bfd_print_private_bfd_data) PARAMS ((bfd *, PTR));

        /* Core file entry points.  */
     #define BFD_JUMP_TABLE_CORE(NAME)\
     CAT(NAME,_core_file_failing_command),\
     CAT(NAME,_core_file_failing_signal),\
     CAT(NAME,_core_file_matches_executable_p)
       char *   (*_core_file_failing_command) PARAMS ((bfd *));
       int      (*_core_file_failing_signal) PARAMS ((bfd *));
       boolean  (*_core_file_matches_executable_p) PARAMS ((bfd *, bfd *));

        /* Archive entry points.  */
     #define BFD_JUMP_TABLE_ARCHIVE(NAME)\
     CAT(NAME,_slurp_armap),\
     CAT(NAME,_slurp_extended_name_table),\
     CAT(NAME,_construct_extended_name_table),\
     CAT(NAME,_truncate_arname),\
     CAT(NAME,_write_armap),\
     CAT(NAME,_read_ar_hdr),\
     CAT(NAME,_openr_next_archived_file),\
     CAT(NAME,_get_elt_at_index),\
     CAT(NAME,_generic_stat_arch_elt),\
     CAT(NAME,_update_armap_timestamp)
       boolean  (*_bfd_slurp_armap) PARAMS ((bfd *));
       boolean  (*_bfd_slurp_extended_name_table) PARAMS ((bfd *));
       boolean  (*_bfd_construct_extended_name_table)
                  PARAMS ((bfd *, char **, bfd_size_type *, const char **));
       void     (*_bfd_truncate_arname) PARAMS ((bfd *, CONST char *, char *));
       boolean  (*write_armap) PARAMS ((bfd *arch,
                                   unsigned int elength,
                                   struct orl *map,
                                   unsigned int orl_count,
                                   int stridx));
       PTR (*_bfd_read_ar_hdr_fn) PARAMS ((bfd *));
       bfd *    (*openr_next_archived_file) PARAMS ((bfd *arch, bfd *prev));
     #define bfd_get_elt_at_index(b,i) BFD_SEND(b, _bfd_get_elt_at_index, (b,i))
       bfd *    (*_bfd_get_elt_at_index) PARAMS ((bfd *, symindex));
       int      (*_bfd_stat_arch_elt) PARAMS ((bfd *, struct stat *));
       boolean  (*_bfd_update_armap_timestamp) PARAMS ((bfd *));

        /* Entry points used for symbols.  */
     #define BFD_JUMP_TABLE_SYMBOLS(NAME)\
     CAT(NAME,_get_symtab_upper_bound),\
     CAT(NAME,_get_symtab),\
     CAT(NAME,_make_empty_symbol),\
     CAT(NAME,_print_symbol),\
     CAT(NAME,_get_symbol_info),\
     CAT(NAME,_bfd_is_local_label_name),\
     CAT(NAME,_get_lineno),\
     CAT(NAME,_find_nearest_line),\
     CAT(NAME,_bfd_make_debug_symbol),\
     CAT(NAME,_read_minisymbols),\
     CAT(NAME,_minisymbol_to_symbol)
       long  (*_bfd_get_symtab_upper_bound) PARAMS ((bfd *));
       long  (*_bfd_canonicalize_symtab) PARAMS ((bfd *,
                                                  struct symbol_cache_entry **));
       struct symbol_cache_entry  *
                     (*_bfd_make_empty_symbol) PARAMS ((bfd *));
       void          (*_bfd_print_symbol) PARAMS ((bfd *, PTR,
                                           struct symbol_cache_entry *,
                                           bfd_print_symbol_type));
     #define bfd_print_symbol(b,p,s,e) BFD_SEND(b, _bfd_print_symbol, (b,p,s,e))
       void          (*_bfd_get_symbol_info) PARAMS ((bfd *,
                                           struct symbol_cache_entry *,
                                           symbol_info *));
     #define bfd_get_symbol_info(b,p,e) BFD_SEND(b, _bfd_get_symbol_info, (b,p,e))
       boolean       (*_bfd_is_local_label_name) PARAMS ((bfd *, const char *));

       alent *    (*_get_lineno) PARAMS ((bfd *, struct symbol_cache_entry *));
       boolean    (*_bfd_find_nearest_line) PARAMS ((bfd *abfd,
                         struct sec *section, struct symbol_cache_entry **symbols,
                         bfd_vma offset, CONST char **file, CONST char **func,
                         unsigned int *line));
       /* Back-door to allow format-aware applications to create debug symbols
         while using BFD for everything else.  Currently used by the assembler
         when creating COFF files.  */
       asymbol *  (*_bfd_make_debug_symbol) PARAMS ((
            bfd *abfd,
            void *ptr,
            unsigned long size));
     #define bfd_read_minisymbols(b, d, m, s) \
       BFD_SEND (b, _read_minisymbols, (b, d, m, s))
       long  (*_read_minisymbols) PARAMS ((bfd *, boolean, PTR *,
                                           unsigned int *));
     #define bfd_minisymbol_to_symbol(b, d, m, f) \
       BFD_SEND (b, _minisymbol_to_symbol, (b, d, m, f))
       asymbol *(*_minisymbol_to_symbol) PARAMS ((bfd *, boolean, const PTR,
                                                  asymbol *));

        /* Routines for relocs.  */
     #define BFD_JUMP_TABLE_RELOCS(NAME)\
     CAT(NAME,_get_reloc_upper_bound),\
     CAT(NAME,_canonicalize_reloc),\
     CAT(NAME,_bfd_reloc_type_lookup)
       long  (*_get_reloc_upper_bound) PARAMS ((bfd *, sec_ptr));
       long  (*_bfd_canonicalize_reloc) PARAMS ((bfd *, sec_ptr, arelent **,
                                                 struct symbol_cache_entry **));
        /* See documentation on reloc types.  */
       reloc_howto_type *
            (*reloc_type_lookup) PARAMS ((bfd *abfd,
                                          bfd_reloc_code_real_type code));

        /* Routines used when writing an object file.  */
     #define BFD_JUMP_TABLE_WRITE(NAME)\
     CAT(NAME,_set_arch_mach),\
     CAT(NAME,_set_section_contents)
       boolean    (*_bfd_set_arch_mach) PARAMS ((bfd *, enum bfd_architecture,
                         unsigned long));
       boolean       (*_bfd_set_section_contents) PARAMS ((bfd *, sec_ptr, PTR,
                                                 file_ptr, bfd_size_type));

        /* Routines used by the linker.  */
     #define BFD_JUMP_TABLE_LINK(NAME)\
     CAT(NAME,_sizeof_headers),\
     CAT(NAME,_bfd_get_relocated_section_contents),\
     CAT(NAME,_bfd_relax_section),\
     CAT(NAME,_bfd_link_hash_table_create),\
     CAT(NAME,_bfd_link_add_symbols),\
     CAT(NAME,_bfd_final_link),\
     CAT(NAME,_bfd_link_split_section),\
     CAT(NAME,_bfd_gc_sections)
       int        (*_bfd_sizeof_headers) PARAMS ((bfd *, boolean));
       bfd_byte * (*_bfd_get_relocated_section_contents) PARAMS ((bfd *,
                         struct bfd_link_info *, struct bfd_link_order *,
                         bfd_byte *data, boolean relocateable,
                         struct symbol_cache_entry **));

       boolean    (*_bfd_relax_section) PARAMS ((bfd *, struct sec *,
                         struct bfd_link_info *, boolean *again));

        /* Create a hash table for the linker.  Different backends store
          different information in this table.  */
       struct bfd_link_hash_table *(*_bfd_link_hash_table_create) PARAMS ((bfd *));

        /* Add symbols from this object file into the hash table.  */
       boolean (*_bfd_link_add_symbols) PARAMS ((bfd *, struct bfd_link_info *));

        /* Do a link based on the link_order structures attached to each
          section of the BFD.  */
       boolean (*_bfd_final_link) PARAMS ((bfd *, struct bfd_link_info *));

        /* Should this section be split up into smaller pieces during linking.  */
       boolean (*_bfd_link_split_section) PARAMS ((bfd *, struct sec *));

        /* Remove sections that are not referenced from the output.  */
       boolean (*_bfd_gc_sections) PARAMS ((bfd *, struct bfd_link_info *));

        /* Routines to handle dynamic symbols and relocs.  */
     #define BFD_JUMP_TABLE_DYNAMIC(NAME)\
     CAT(NAME,_get_dynamic_symtab_upper_bound),\
     CAT(NAME,_canonicalize_dynamic_symtab),\
     CAT(NAME,_get_dynamic_reloc_upper_bound),\
     CAT(NAME,_canonicalize_dynamic_reloc)
        /* Get the amount of memory required to hold the dynamic symbols. */
       long  (*_bfd_get_dynamic_symtab_upper_bound) PARAMS ((bfd *));
        /* Read in the dynamic symbols.  */
       long  (*_bfd_canonicalize_dynamic_symtab)
         PARAMS ((bfd *, struct symbol_cache_entry **));
        /* Get the amount of memory required to hold the dynamic relocs.  */
       long  (*_bfd_get_dynamic_reloc_upper_bound) PARAMS ((bfd *));
        /* Read in the dynamic relocs.  */
       long  (*_bfd_canonicalize_dynamic_reloc)
         PARAMS ((bfd *, arelent **, struct symbol_cache_entry **));

A pointer to an alternative bfd_target in case the current one is not satisfactory.  This can happen when the target cpu supports both big and little endian code, and target chosen by the linker has the wrong endianness.  The function open_output() in ld/ldlang.c uses this field to find an alternative output format that is suitable.

       /* Opposite endian version of this target.  */
      const struct bfd_target * alternative_target;

Data for use by back-end routines, which isn't generic enough to belong in this structure.

      PTR backend_data;

     } bfd_target;

bfd_set_default_target
......................

Synopsis

     boolean bfd_set_default_target (const char *name);

Description

Set the default target vector to use when recognizing a BFD. This takes the name of the target, which may be a BFD target name or a configuration triplet.

bfd_find_target
...............

Synopsis

     const bfd_target *bfd_find_target(CONST char *target_name, bfd *abfd);

Description

Return a pointer to the transfer vector for the object target named TARGET_NAME.  If TARGET_NAME is NULL, choose the one in the environment variable GNUTARGET; if that is null or not defined, then choose the first entry in the target list. Passing in the string "default" or setting the environment variable to "default" will cause the first entry in the target list to be returned, and "target_defaulted" will be set in the BFD.  This causes bfd_check_format to loop over all the targets to find the one that matches the file being read.

bfd_target_list
...............

Synopsis

     const char **bfd_target_list(void);

Description

Return a freshly malloced NULL-terminated vector of the names of all the valid BFD targets. Do not modify the names.

bfd_seach_for_target
....................

Synopsis

     const bfd_target * bfd_search_for_target (int (* search_func)(const bfd_target *, void *), void *);

Description

Return a pointer to the first transfer vector in the list of transfer vectors maintained by BFD that produces a non-zero result when passed to the function SEARCH_FUNC.  The parameter DATA is passed, unexamined, to the search function.