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# BFD front end

## `typedef bfd`

A BFD has type `bfd`; objects of this type are the cornerstone of any application using BFD. Using BFD consists of making references though the BFD and to data in the BFD.

Here is the structure that defines the type `bfd`. It contains the major data about the file and pointers to the rest of the data.
.

    struct _bfd
    {
        /* The filename the application opened the BFD with.  */
        CONST char *filename;

        /* A pointer to the target jump table.             */
        const struct bfd_target *xvec;

        /* To avoid dragging too many header files into every file that
           includes `bfd.h', IOSTREAM has been declared as a "char
           *", and MTIME as a "long".  Their correct types, to which they
           are cast when used, are "FILE *" and "time_t".    The iostream
           is the result of an fopen on the filename.  However, if the
           BFD_IN_MEMORY flag is set, then iostream is actually a pointer
           to a bfd_in_memory struct.  */
        PTR iostream;

        /* Is the file descriptor being cached?  That is, can it be closed as
           needed, and re-opened when accessed later?  */

        boolean cacheable;

        /* Marks whether there was a default target specified when the
           BFD was opened. This is used to select which matching algorithm
           to use to choose the back end. */

        boolean target_defaulted;

        /* The caching routines use these to maintain a
           least-recently-used list of BFDs */

        struct _bfd *lru_prev, *lru_next;

        /* When a file is closed by the caching routines, BFD retains
           state information on the file here: */

        file_ptr where;

        /* and here: ("once" means at least once) */

        boolean opened_once;

        /* Set if we have a locally maintained mtime value, rather than
           getting it from the file each time: */

        boolean mtime_set;

        /* File modified time, if mtime_set is true: */

        long mtime;

        /* Reserved for an unimplemented file locking extension.*/

        int ifd;

        /* The format which belongs to the BFD. (object, core, etc.) */

        bfd_format format;

        /* The direction the BFD was opened with*/

        enum bfd_direction {no_direction = 0,
                            read_direction = 1,
                            write_direction = 2,
                            both_direction = 3} direction;

        /* Format_specific flags*/

        flagword flags;

        /* Currently my_archive is tested before adding origin to
           anything. I believe that this can become always an add of
           origin, with origin set to 0 for non archive files.   */

        file_ptr origin;

        /* Remember when output has begun, to stop strange things
           from happening. */
        boolean output_has_begun;

        /* Pointer to linked list of sections*/
        struct sec  *sections;

        /* The number of sections */
        unsigned int section_count;

        /* Stuff only useful for object files:
           The start address. */
        bfd_vma start_address;

        /* Used for input and output*/
        unsigned int symcount;

        /* Symbol table for output BFD (with symcount entries) */
        struct symbol_cache_entry  **outsymbols;

        /* Pointer to structure which contains architecture information*/
        const struct bfd_arch_info *arch_info;

        /* Stuff only useful for archives:*/
        PTR arelt_data;
        struct _bfd *my_archive;     /* The containing archive BFD.  */
        struct _bfd *next;           /* The next BFD in the archive.  */
        struct _bfd *archive_head;   /* The first BFD in the archive.  */
        boolean has_armap;

        /* A chain of BFD structures involved in a link.  */
        struct _bfd *link_next;

        /* A field used by _bfd_generic_link_add_archive_symbols.  This will
           be used only for archive elements.  */
        int archive_pass;

        /* Used by the back end to hold private data. */

        union
          {
          struct aout_data_struct *aout_data;
          struct artdata *aout_ar_data;
          struct _oasys_data *oasys_obj_data;
          struct _oasys_ar_data *oasys_ar_data;
          struct coff_tdata *coff_obj_data;
          struct pe_tdata *pe_obj_data;
          struct xcoff_tdata *xcoff_obj_data;
          struct ecoff_tdata *ecoff_obj_data;
          struct ieee_data_struct *ieee_data;
          struct ieee_ar_data_struct *ieee_ar_data;
          struct srec_data_struct *srec_data;
          struct ihex_data_struct *ihex_data;
          struct tekhex_data_struct *tekhex_data;
          struct elf_obj_tdata *elf_obj_data;
          struct nlm_obj_tdata *nlm_obj_data;
          struct bout_data_struct *bout_data;
          struct sun_core_struct *sun_core_data;
          struct trad_core_struct *trad_core_data;
          struct som_data_struct *som_data;
          struct hpux_core_struct *hpux_core_data;
          struct hppabsd_core_struct *hppabsd_core_data;
          struct sgi_core_struct *sgi_core_data;
          struct lynx_core_struct *lynx_core_data;
          struct osf_core_struct *osf_core_data;
          struct cisco_core_struct *cisco_core_data;
          struct versados_data_struct *versados_data;
          struct netbsd_core_struct *netbsd_core_data;
          PTR any;
          } tdata;

        /* Used by the application to hold private data*/
        PTR usrdata;

      /* Where all the allocated stuff under this BFD goes.  This is a
         struct objalloc *, but we use PTR to avoid requiring the inclusion of
         objalloc.h.  */
        PTR memory;
    };

## Error reporting

Most BFD functions return nonzero on success (check their individual documentation for precise semantics). On an error, they call `bfd_set_error` to set an error condition that callers can check by calling `bfd_get_error`. If that returns `bfd_error_system_call`, then check `errno`.

The easiest way to report a BFD error to the user is to use `bfd_perror`.

### Type `bfd_error_type`

The values returned by `bfd_get_error` are defined by the enumerated type `bfd_error_type`.
.

    typedef enum bfd_error
    {
      bfd_error_no_error = 0,
      bfd_error_system_call,
      bfd_error_invalid_target,
      bfd_error_wrong_format,
      bfd_error_invalid_operation,
      bfd_error_no_memory,
      bfd_error_no_symbols,
      bfd_error_no_armap,
      bfd_error_no_more_archived_files,
      bfd_error_malformed_archive,
      bfd_error_file_not_recognized,
      bfd_error_file_ambiguously_recognized,
      bfd_error_no_contents,
      bfd_error_nonrepresentable_section,
      bfd_error_no_debug_section,
      bfd_error_bad_value,
      bfd_error_file_truncated,
      bfd_error_file_too_big,
      bfd_error_invalid_error_code
    } bfd_error_type;

#### `bfd_get_error`

Synopsis

    bfd_error_type bfd_get_error (void);

Description
Return the current BFD error condition.

#### `bfd_set_error`

Synopsis

    void bfd_set_error (bfd_error_type error_tag);

Description
Set the BFD error condition to be error_tag.

#### `bfd_errmsg`

Synopsis

    CONST char *bfd_errmsg (bfd_error_type error_tag);

Description
Return a string describing the error error_tag, or the system error if error_tag is `bfd_error_system_call`.

#### `bfd_perror`

Synopsis

    void bfd_perror (CONST char *message);

Description
Print to the standard error stream a string describing the last BFD error that occurred, or the last system error if the last BFD error was a system call failure. If message is non-NULL and non-empty, the error string printed is preceded by message, a colon, and a space. It is followed by a newline.

### BFD error handler

Some BFD functions want to print messages describing the problem. They call a BFD error handler function. This function may be overriden by the program.

The BFD error handler acts like printf.
.

    typedef void (*bfd_error_handler_type) PARAMS ((const char *, ...));

#### `bfd_set_error_handler`

Synopsis

    bfd_error_handler_type bfd_set_error_handler (bfd_error_handler_type);

Description
Set the BFD error handler function. Returns the previous function.

#### `bfd_set_error_program_name`

Synopsis

    void bfd_set_error_program_name (const char *);

Description
Set the program name to use when printing a BFD error. This is printed before the error message followed by a colon and space. The string must not be changed after it is passed to this function.

## Symbols

#### `bfd_get_reloc_upper_bound`

Synopsis

    long bfd_get_reloc_upper_bound(bfd *abfd, asection *sect);

Description
Return the number of bytes required to store the relocation information associated with section sect attached to bfd abfd. If an error occurs, return -1.

#### `bfd_canonicalize_reloc`

Synopsis

    long bfd_canonicalize_reloc
       (bfd *abfd,
        asection *sec,
        arelent **loc,
        asymbol	**syms);

Description
Call the back end associated with the open BFD abfd and translate the external form of the relocation information attached to sec into the internal canonical form. Place the table into memory at loc, which has been preallocated, usually by a call to `bfd_get_reloc_upper_bound`. Returns the number of relocs, or -1 on error.

The syms table is also needed for horrible internal magic reasons.

#### `bfd_set_reloc`

Synopsis

    void bfd_set_reloc
       (bfd *abfd, asection *sec, arelent **rel, unsigned int count)

Description
Set the relocation pointer and count within section sec to the values rel and count. The argument abfd is ignored.

#### `bfd_set_file_flags`

Synopsis

    boolean bfd_set_file_flags(bfd *abfd, flagword flags);

Description
Set the flag word in the BFD abfd to the value flags.

Possible errors are:

- `bfd_error_wrong_format` - The target bfd was not of object format.
- `bfd_error_invalid_operation` - The target bfd was open for reading.
- `bfd_error_invalid_operation` - The flag word contained a bit which was not applicable to the type of file. E.g., an attempt was made to set the `D_PAGED` bit on a BFD format which does not support demand paging.

#### `bfd_set_start_address`

Synopsis

    boolean bfd_set_start_address(bfd *abfd, bfd_vma vma);

Description
Make vma the entry point of output BFD abfd.
Returns
Returns `true` on success, `false` otherwise.

#### `bfd_get_mtime`

Synopsis

    long bfd_get_mtime(bfd *abfd);

Description
Return the file modification time (as read from the file system, or from the archive header for archive members).

#### `bfd_get_size`

Synopsis

    long bfd_get_size(bfd *abfd);

Description
Return the file size (as read from file system) for the file associated with BFD abfd.

The initial motivation for, and use of, this routine is not so we can get the exact size of the object the BFD applies to, since that might not be generally possible (archive members for example). It would be ideal if someone could eventually modify it so that such results were guaranteed.

Instead, we want to ask questions like "is this NNN byte sized object I'm about to try read from file offset YYY reasonable?" As as example of where we might do this, some object formats use string tables for which the first `sizeof(long)` bytes of the table contain the size of the table itself, including the size bytes. If an application tries to read what it thinks is one of these string tables, without some way to validate the size, and for some reason the size is wrong (byte swapping error, wrong location for the string table, etc.), the only clue is likely to be a read error when it tries to read the table, or a "virtual memory exhausted" error when it tries to allocate 15 bazillon bytes of space for the 15 bazillon byte table it is about to read. This function at least allows us to answer the quesion, "is the size reasonable?".

#### `bfd_get_gp_size`

Synopsis

    int bfd_get_gp_size(bfd *abfd);

Description
Return the maximum size of objects to be optimized using the GP register under MIPS ECOFF. This is typically set by the `-G` argument to the compiler, assembler or linker.

#### `bfd_set_gp_size`

Synopsis

    void bfd_set_gp_size(bfd *abfd, int i);

Description
Set the maximum size of objects to be optimized using the GP register under ECOFF or MIPS ELF. This is typically set by the `-G` argument to the compiler, assembler or linker.

#### `bfd_scan_vma`

Synopsis

    bfd_vma bfd_scan_vma(CONST char *string, CONST char **end, int base);

Description
Convert, like `strtoul`, a numerical expression string into a `bfd_vma` integer, and return that integer. (Though without as many bells and whistles as `strtoul`.) The expression is assumed to be unsigned (i.e., positive). If given a base, it is used as the base for conversion. A base of 0 causes the function to interpret the string in hex if a leading "0x" or "0X" is found, otherwise in octal if a leading zero is found, otherwise in decimal.

Overflow is not detected.

#### `bfd_copy_private_bfd_data`

Synopsis

    boolean bfd_copy_private_bfd_data(bfd *ibfd, bfd *obfd);

Description
Copy private BFD information from the BFD ibfd to the the BFD obfd. Return `true` on success, `false` on error. Possible error returns are:

- `bfd_error_no_memory` - Not enough memory exists to create private data for obfd.

    #define bfd_copy_private_bfd_data(ibfd, obfd) \
         BFD_SEND (obfd, _bfd_copy_private_bfd_data, \
    		(ibfd, obfd))

#### `bfd_merge_private_bfd_data`

Synopsis

    boolean bfd_merge_private_bfd_data(bfd *ibfd, bfd *obfd);

Description
Merge private BFD information from the BFD ibfd to the the output file BFD obfd when linking. Return `true` on success, `false` on error. Possible error returns are:

- `bfd_error_no_memory` - Not enough memory exists to create private data for obfd.

    #define bfd_merge_private_bfd_data(ibfd, obfd) \
         BFD_SEND (obfd, _bfd_merge_private_bfd_data, \
    		(ibfd, obfd))

#### `bfd_set_private_flags`

Synopsis

    boolean bfd_set_private_flags(bfd *abfd, flagword flags);

Description
Set private BFD flag information in the BFD abfd. Return `true` on success, `false` on error. Possible error returns are:

- `bfd_error_no_memory` - Not enough memory exists to create private data for obfd.

    #define bfd_set_private_flags(abfd, flags) \
         BFD_SEND (abfd, _bfd_set_private_flags, \
    		(abfd, flags))

#### `stuff`

Description
Stuff which should be documented:

    #define bfd_sizeof_headers(abfd, reloc) \
         BFD_SEND (abfd, _bfd_sizeof_headers, (abfd, reloc))

    #define bfd_find_nearest_line(abfd, sec, syms, off, file, func, line) \
         BFD_SEND (abfd, _bfd_find_nearest_line,  (abfd, sec, syms, off, file, func, line))

            /* Do these three do anything useful at all, for any back end?  */
    #define bfd_debug_info_start(abfd) \
            BFD_SEND (abfd, _bfd_debug_info_start, (abfd))

    #define bfd_debug_info_end(abfd) \
            BFD_SEND (abfd, _bfd_debug_info_end, (abfd))

    #define bfd_debug_info_accumulate(abfd, section) \
            BFD_SEND (abfd, _bfd_debug_info_accumulate, (abfd, section))

    #define bfd_stat_arch_elt(abfd, stat) \
            BFD_SEND (abfd, _bfd_stat_arch_elt,(abfd, stat))

    #define bfd_update_armap_timestamp(abfd) \
            BFD_SEND (abfd, _bfd_update_armap_timestamp, (abfd))

    #define bfd_set_arch_mach(abfd, arch, mach)\
            BFD_SEND ( abfd, _bfd_set_arch_mach, (abfd, arch, mach))

    #define bfd_relax_section(abfd, section, link_info, again) \
           BFD_SEND (abfd, _bfd_relax_section, (abfd, section, link_info, again))

    #define bfd_link_hash_table_create(abfd) \
    	BFD_SEND (abfd, _bfd_link_hash_table_create, (abfd))

    #define bfd_link_add_symbols(abfd, info) \
    	BFD_SEND (abfd, _bfd_link_add_symbols, (abfd, info))

    #define bfd_final_link(abfd, info) \
    	BFD_SEND (abfd, _bfd_final_link, (abfd, info))

    #define bfd_free_cached_info(abfd) \
           BFD_SEND (abfd, _bfd_free_cached_info, (abfd))

    #define bfd_get_dynamic_symtab_upper_bound(abfd) \
    	BFD_SEND (abfd, _bfd_get_dynamic_symtab_upper_bound, (abfd))

    #define bfd_print_private_bfd_data(abfd, file)\
    	BFD_SEND (abfd, _bfd_print_private_bfd_data, (abfd, file))

    #define bfd_canonicalize_dynamic_symtab(abfd, asymbols) \
    	BFD_SEND (abfd, _bfd_canonicalize_dynamic_symtab, (abfd, asymbols))

    #define bfd_get_dynamic_reloc_upper_bound(abfd) \
    	BFD_SEND (abfd, _bfd_get_dynamic_reloc_upper_bound, (abfd))

    #define bfd_canonicalize_dynamic_reloc(abfd, arels, asyms) \
    	BFD_SEND (abfd, _bfd_canonicalize_dynamic_reloc, (abfd, arels, asyms))

    extern bfd_byte *bfd_get_relocated_section_contents
    	PARAMS ((bfd *, struct bfd_link_info *,
    		  struct bfd_link_order *, bfd_byte *,
    		  boolean, asymbol **));

- Memory Usage
- Initialization
- Sections
- Symbols
- Archives
- Formats
- Relocations
- Core Files
- Targets
- Architectures
- Opening and Closing
- Internal
- File Caching
- Linker Functions
- Hash Tables

## Memory usage

BFD keeps all of its internal structures in obstacks. There is one obstack per open BFD file, into which the current state is stored. When a BFD is closed, the obstack is deleted, and so everything which has been allocated by BFD for the closing file is thrown away.

BFD does not free anything created by an application, but pointers into `bfd` structures become invalid on a `bfd_close`; for example, after a `bfd_close` the vector passed to `bfd_canonicalize_symtab` is still around, since it has been allocated by the application, but the data that it pointed to are lost.

The general rule is to not close a BFD until all operations dependent upon data from the BFD have been completed, or all the data from within the file has been copied. To help with the management of memory, there is a function (`bfd_alloc_size`) which returns the number of bytes in obstacks associated with the supplied BFD. This could be used to select the greediest open BFD, close it to reclaim the memory, perform some operation and reopen the BFD again, to get a fresh copy of the data structures.

## Initialization

These are the functions that handle initializing a BFD.

#### `bfd_init`

Synopsis

    void bfd_init(void);

Description
This routine must be called before any other BFD function to initialize magical internal data structures.

## Sections

The raw data contained within a BFD is maintained through the section abstraction. A single BFD may have any number of sections. It keeps hold of them by pointing to the first; each one points to the next in the list.

Sections are supported in BFD in `section.c`.

- Section Input
- Section Output
- typedef asection
- section prototypes

### Section input

When a BFD is opened for reading, the section structures are created and attached to the BFD.

Each section has a name which describes the section in the outside world--for example, `a.out` would contain at least three sections, called `.text`, `.data` and `.bss`.

Names need not be unique; for example a COFF file may have several sections named `.data`.

Sometimes a BFD will contain more than the "natural" number of sections. A back end may attach other sections containing constructor data, or an application may add a section (using `bfd_make_section`) to the sections attached to an already open BFD. For example, the linker creates an extra section `COMMON` for each input file's BFD to hold information about common storage.

The raw data is not necessarily read in when the section descriptor is created. Some targets may leave the data in place until a `bfd_get_section_contents` call is made. Other back ends may read in all the data at once. For example, an S-record file has to be read once to determine the size of the data. An IEEE-695 file doesn't contain raw data in sections, but data and relocation expressions intermixed, so the data area has to be parsed to get out the data and relocations.

### Section output

To write a new object style BFD, the various sections to be written have to be created. They are attached to the BFD in the same way as input sections; data is written to the sections using `bfd_set_section_contents`.

Any program that creates or combines sections (e.g., the assembler and linker) must use the `asection` fields `output_section` and `output_offset` to indicate the file sections to which each section must be written. (If the section is being created from scratch, `output_section` should probably point to the section itself and `output_offset` should probably be zero.)

The data to be written comes from input sections attached (via `output_section` pointers) to the output sections. The output section structure can be considered a filter for the input section: the output section determines the vma of the output data and the name, but the input section determines the offset into the output section of the data to be written.

E.g., to create a section "O", starting at 0x100, 0x123 long, containing two subsections, "A" at offset 0x0 (i.e., at vma 0x100) and "B" at offset 0x20 (i.e., at vma 0x120) the `asection` structures would look like:

       section name          "A"
         output_offset   0x00
         size            0x20
         output_section ----------->  section name    "O"
                                 |    vma             0x100
       section name          "B" |    size            0x123
         output_offset   0x20    |
         size            0x103   |
         output_section  --------|

### Link orders

The data within a section is stored in a link_order. These are much like the fixups in `gas`. The link_order abstraction allows a section to grow and shrink within itself.

A link_order knows how big it is, and which is the next link_order and where the raw data for it is; it also points to a list of relocations which apply to it.

The link_order is used by the linker to perform relaxing on final code. The compiler creates code which is as big as necessary to make it work without relaxing, and the user can select whether to relax. Sometimes relaxing takes a lot of time. The linker runs around the relocations to see if any are attached to data which can be shrunk, if so it does it on a link_order by link_order basis.

### typedef asection

Here is the section structure:
.

    typedef struct sec
    {
            /* The name of the section; the name isn't a copy, the pointer is
            the same as that passed to bfd_make_section. */

        CONST char *name;

            /* Which section is it; 0..nth.      */

       int index;

            /* The next section in the list belonging to the BFD, or NULL. */

        struct sec *next;

            /* The field flags contains attributes of the section. Some
               flags are read in from the object file, and some are
               synthesized from other information.  */

        flagword flags;

    #define SEC_NO_FLAGS   0x000

            /* Tells the OS to allocate space for this section when loading.
               This is clear for a section containing debug information
               only. */
    #define SEC_ALLOC      0x001

            /* Tells the OS to load the section from the file when loading.
               This is clear for a .bss section. */
    #define SEC_LOAD       0x002

            /* The section contains data still to be relocated, so there is
               some relocation information too. */
    #define SEC_RELOC      0x004

    #if 0   /* Obsolete ? */
    #define SEC_BALIGN     0x008
    #endif

            /* A signal to the OS that the section contains read only
              data. */
    #define SEC_READONLY   0x010

            /* The section contains code only. */
    #define SEC_CODE       0x020

            /* The section contains data only. */
    #define SEC_DATA       0x040

            /* The section will reside in ROM. */
    #define SEC_ROM        0x080

            /* The section contains constructor information. This section
               type is used by the linker to create lists of constructors and
               destructors used by g++. When a back end sees a symbol
               which should be used in a constructor list, it creates a new
               section for the type of name (e.g., __CTOR_LIST__), attaches
               the symbol to it, and builds a relocation. To build the lists
               of constructors, all the linker has to do is catenate all the
               sections called __CTOR_LIST__ and relocate the data
               contained within - exactly the operations it would peform on
               standard data. */
    #define SEC_CONSTRUCTOR 0x100

            /* The section is a constuctor, and should be placed at the
              end of the text, data, or bss section(?). */
    #define SEC_CONSTRUCTOR_TEXT 0x1100
    #define SEC_CONSTRUCTOR_DATA 0x2100
    #define SEC_CONSTRUCTOR_BSS  0x3100

            /* The section has contents - a data section could be
               SEC_ALLOC | SEC_HAS_CONTENTS; a debug section could be
               SEC_HAS_CONTENTS */
    #define SEC_HAS_CONTENTS 0x200

            /* An instruction to the linker to not output the section
               even if it has information which would normally be written. */
    #define SEC_NEVER_LOAD 0x400

            /* The section is a COFF shared library section.  This flag is
               only for the linker.  If this type of section appears in
               the input file, the linker must copy it to the output file
               without changing the vma or size.  FIXME: Although this
               was originally intended to be general, it really is COFF
               specific (and the flag was renamed to indicate this).  It
               might be cleaner to have some more general mechanism to
               allow the back end to control what the linker does with
               sections. */
    #define SEC_COFF_SHARED_LIBRARY 0x800

            /* The section contains common symbols (symbols may be defined
               multiple times, the value of a symbol is the amount of
               space it requires, and the largest symbol value is the one
               used).  Most targets have exactly one of these (which we
    	    translate to bfd_com_section_ptr), but ECOFF has two. */
    #define SEC_IS_COMMON 0x8000

            /* The section contains only debugging information.  For
               example, this is set for ELF .debug and .stab sections.
               strip tests this flag to see if a section can be
               discarded. */
    #define SEC_DEBUGGING 0x10000

            /* The contents of this section are held in memory pointed to
               by the contents field.  This is checked by
               bfd_get_section_contents, and the data is retrieved from
               memory if appropriate.  */
    #define SEC_IN_MEMORY 0x20000

            /* The contents of this section are to be excluded by the
    	    linker for executable and shared objects unless those
    	    objects are to be further relocated.  */
    #define SEC_EXCLUDE 0x40000

    	/* The contents of this section are to be sorted by the
    	   based on the address specified in the associated symbol
    	   table.  */
    #define SEC_SORT_ENTRIES 0x80000

    	/* When linking, duplicate sections of the same name should be
    	   discarded, rather than being combined into a single section as
    	   is usually done.  This is similar to how common symbols are
    	   handled.  See SEC_LINK_DUPLICATES below.  */
    #define SEC_LINK_ONCE 0x100000

    	/* If SEC_LINK_ONCE is set, this bitfield describes how the linker
    	   should handle duplicate sections.  */
    #define SEC_LINK_DUPLICATES 0x600000

    	/* This value for SEC_LINK_DUPLICATES means that duplicate
    	   sections with the same name should simply be discarded. */
    #define SEC_LINK_DUPLICATES_DISCARD 0x0

    	/* This value for SEC_LINK_DUPLICATES means that the linker
    	   should warn if there are any duplicate sections, although
    	   it should still only link one copy.  */
    #define SEC_LINK_DUPLICATES_ONE_ONLY 0x200000

    	/* This value for SEC_LINK_DUPLICATES means that the linker
    	   should warn if any duplicate sections are a different size.  */
    #define SEC_LINK_DUPLICATES_SAME_SIZE 0x400000

    	/* This value for SEC_LINK_DUPLICATES means that the linker
    	   should warn if any duplicate sections contain different
    	   contents.  */
    #define SEC_LINK_DUPLICATES_SAME_CONTENTS 0x600000

    	/* This section was created by the linker as part of dynamic
    	   relocation or other arcane processing.  It is skipped when
    	   going through the first-pass output, trusting that someone
    	   else up the line will take care of it later.  */
    #define SEC_LINKER_CREATED 0x800000

    	/*  End of section flags.  */

    	/* Some internal packed boolean fields.  */

    	/* See the vma field.  */
    	unsigned int user_set_vma : 1;

    	/* Whether relocations have been processed.  */
    	unsigned int reloc_done : 1;

    	/* A mark flag used by some of the linker backends.  */
    	unsigned int linker_mark : 1;

    	/* End of internal packed boolean fields.  */

           /*  The virtual memory address of the section - where it will be
               at run time.  The symbols are relocated against this.  The
    	    user_set_vma flag is maintained by bfd; if it's not set, the
    	    backend can assign addresses (for example, in a.out, where
    	    the default address for .data is dependent on the specific
    	    target and various flags).  */

       bfd_vma vma;

           /*  The load address of the section - where it would be in a
               rom image; really only used for writing section header
    	    information. */

       bfd_vma lma;

            /* The size of the section in bytes, as it will be output.
               contains a value even if the section has no contents (e.g., the
               size of .bss). This will be filled in after relocation */

       bfd_size_type _cooked_size;

            /* The original size on disk of the section, in bytes.  Normally this
    	    value is the same as the size, but if some relaxing has
    	    been done, then this value will be bigger.  */

       bfd_size_type _raw_size;

            /* If this section is going to be output, then this value is the
               offset into the output section of the first byte in the input
               section. E.g., if this was going to start at the 100th byte in
               the output section, this value would be 100. */

       bfd_vma output_offset;

            /* The output section through which to map on output. */

       struct sec *output_section;

            /* The alignment requirement of the section, as an exponent of 2 -
               e.g., 3 aligns to 2^3 (or 8). */

       unsigned int alignment_power;

            /* If an input section, a pointer to a vector of relocation
               records for the data in this section. */

       struct reloc_cache_entry *relocation;

            /* If an output section, a pointer to a vector of pointers to
               relocation records for the data in this section. */

       struct reloc_cache_entry **orelocation;

            /* The number of relocation records in one of the above  */

       unsigned reloc_count;

            /* Information below is back end specific - and not always used
               or updated.  */

            /* File position of section data    */

       file_ptr filepos;

            /* File position of relocation info */

       file_ptr rel_filepos;

            /* File position of line data       */

       file_ptr line_filepos;

            /* Pointer to data for applications */

       PTR userdata;

            /* If the SEC_IN_MEMORY flag is set, this points to the actual
               contents.  */
       unsigned char *contents;

            /* Attached line number information */

       alent *lineno;

            /* Number of line number records   */

       unsigned int lineno_count;

            /* When a section is being output, this value changes as more
               linenumbers are written out */

       file_ptr moving_line_filepos;

            /* What the section number is in the target world  */

       int target_index;

       PTR used_by_bfd;

            /* If this is a constructor section then here is a list of the
               relocations created to relocate items within it. */

       struct relent_chain *constructor_chain;

            /* The BFD which owns the section. */

       bfd *owner;

    	 /* A symbol which points at this section only */
       struct symbol_cache_entry *symbol;
       struct symbol_cache_entry **symbol_ptr_ptr;

       struct bfd_link_order *link_order_head;
       struct bfd_link_order *link_order_tail;
    } asection ;

        /* These sections are global, and are managed by BFD.  The application
           and target back end are not permitted to change the values in
    	these sections.  New code should use the section_ptr macros rather
           than referring directly to the const sections.  The const sections
           may eventually vanish.  */
    #define BFD_ABS_SECTION_NAME "*ABS*"
    #define BFD_UND_SECTION_NAME "*UND*"
    #define BFD_COM_SECTION_NAME "*COM*"
    #define BFD_IND_SECTION_NAME "*IND*"

        /* the absolute section */
    extern const asection bfd_abs_section;
    #define bfd_abs_section_ptr ((asection *) &bfd_abs_section)
    #define bfd_is_abs_section(sec) ((sec) == bfd_abs_section_ptr)
        /* Pointer to the undefined section */
    extern const asection bfd_und_section;
    #define bfd_und_section_ptr ((asection *) &bfd_und_section)
    #define bfd_is_und_section(sec) ((sec) == bfd_und_section_ptr)
        /* Pointer to the common section */
    extern const asection bfd_com_section;
    #define bfd_com_section_ptr ((asection *) &bfd_com_section)
        /* Pointer to the indirect section */
    extern const asection bfd_ind_section;
    #define bfd_ind_section_ptr ((asection *) &bfd_ind_section)
    #define bfd_is_ind_section(sec) ((sec) == bfd_ind_section_ptr)

    extern const struct symbol_cache_entry * const bfd_abs_symbol;
    extern const struct symbol_cache_entry * const bfd_com_symbol;
    extern const struct symbol_cache_entry * const bfd_und_symbol;
    extern const struct symbol_cache_entry * const bfd_ind_symbol;
    #define bfd_get_section_size_before_reloc(section) \
         (section->reloc_done ? (abort(),1): (section)->_raw_size)
    #define bfd_get_section_size_after_reloc(section) \
         ((section->reloc_done) ? (section)->_cooked_size: (abort(),1))

### Section prototypes

These are the functions exported by the section handling part of BFD.

#### `bfd_get_section_by_name`

Synopsis

    asection *bfd_get_section_by_name(bfd *abfd, CONST char *name);

Description
Run through abfd and return the one of the `asection`s whose name matches name, otherwise `NULL`. See section Sections, for more information.

This should only be used in special cases; the normal way to process all sections of a given name is to use `bfd_map_over_sections` and `strcmp` on the name (or better yet, base it on the section flags or something else) for each section.

#### `bfd_make_section_old_way`

Synopsis

    asection *bfd_make_section_old_way(bfd *abfd, CONST char *name);

Description
Create a new empty section called name and attach it to the end of the chain of sections for the BFD abfd. An attempt to create a section with a name which is already in use returns its pointer without changing the section chain.

It has the funny name since this is the way it used to be before it was rewritten....

Possible errors are:

- `bfd_error_invalid_operation` - If output has already started for this BFD.
- `bfd_error_no_memory` - If memory allocation fails.

#### `bfd_make_section_anyway`

Synopsis

    asection *bfd_make_section_anyway(bfd *abfd, CONST char *name);

Description
Create a new empty section called name and attach it to the end of the chain of sections for abfd. Create a new section even if there is already a section with that name.

Return `NULL` and set `bfd_error` on error; possible errors are:

- `bfd_error_invalid_operation` - If output has already started for abfd.
- `bfd_error_no_memory` - If memory allocation fails.

#### `bfd_make_section`

Synopsis

    asection *bfd_make_section(bfd *, CONST char *name);

Description
Like `bfd_make_section_anyway`, but return `NULL` (without calling bfd_set_error ()) without changing the section chain if there is already a section named name. If there is an error, return `NULL` and set `bfd_error`.

#### `bfd_set_section_flags`

Synopsis

    boolean bfd_set_section_flags(bfd *abfd, asection *sec, flagword flags);

Description
Set the attributes of the section sec in the BFD abfd to the value flags. Return `true` on success, `false` on error. Possible error returns are:

- `bfd_error_invalid_operation` - The section cannot have one or more of the attributes requested. For example, a .bss section in `a.out` may not have the `SEC_HAS_CONTENTS` field set.

#### `bfd_map_over_sections`

Synopsis

    void bfd_map_over_sections(bfd *abfd,
        void (*func)(bfd *abfd,
        asection *sect,
        PTR obj),
        PTR obj);

Description
Call the provided function func for each section attached to the BFD abfd, passing obj as an argument. The function will be called as if by

    	func(abfd, the_section, obj);

This is the prefered method for iterating over sections; an alternative would be to use a loop:

    	   section *p;
    	   for (p = abfd->sections; p != NULL; p = p->next)
    	      func(abfd, p, ...)

#### `bfd_set_section_size`

Synopsis

    boolean bfd_set_section_size(bfd *abfd, asection *sec, bfd_size_type val);

Description
Set sec to the size val. If the operation is ok, then `true` is returned, else `false`.

Possible error returns:

- `bfd_error_invalid_operation` - Writing has started to the BFD, so setting the size is invalid.

#### `bfd_set_section_contents`

Synopsis

    boolean bfd_set_section_contents
       (bfd *abfd,
        asection *section,
        PTR data,
        file_ptr offset,
        bfd_size_type count);

Description
Sets the contents of the section section in BFD abfd to the data starting in memory at data. The data is written to the output section starting at offset offset for count bytes.

Normally `true` is returned, else `false`. Possible error returns are:

- `bfd_error_no_contents` - The output section does not have the `SEC_HAS_CONTENTS` attribute, so nothing can be written to it.
- and some more too

This routine is front end to the back end function `_bfd_set_section_contents`.

#### `bfd_get_section_contents`

Synopsis

    boolean bfd_get_section_contents
       (bfd *abfd, asection *section, PTR location,
        file_ptr offset, bfd_size_type count);

Description
Read data from section in BFD abfd into memory starting at location. The data is read at an offset of offset from the start of the input section, and is read for count bytes.

If the contents of a constructor with the `SEC_CONSTRUCTOR` flag set are requested or if the section does not have the `SEC_HAS_CONTENTS` flag set, then the location is filled with zeroes. If no errors occur, `true` is returned, else `false`.

#### `bfd_copy_private_section_data`

Synopsis

    boolean bfd_copy_private_section_data(bfd *ibfd, asection *isec, bfd *obfd, asection *osec);

Description
Copy private section information from isec in the BFD ibfd to the section osec in the BFD obfd. Return `true` on success, `false` on error. Possible error returns are:

- `bfd_error_no_memory` - Not enough memory exists to create private data for osec.

    #define bfd_copy_private_section_data(ibfd, isection, obfd, osection) \
         BFD_SEND (obfd, _bfd_copy_private_section_data, \
    		(ibfd, isection, obfd, osection))

## 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 `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 `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.

- Reading Symbols
- Writing Symbols
- Mini Symbols
- typedef asymbol
- symbol handling functions

### 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:

    	  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 = (asymbol **) 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]);
    	  }

All storage for the symbols themselves is in an objalloc connected to the BFD; it is freed when the BFD is closed.

### 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 `bfd_make_empty_symbol`. Here is 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 = bfd_make_section_old_way(abfd, ".text");
    	  new->flags = 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 `a.out` object format does not allow an arbitary number of sections. A symbol pointing to a section which is not one of `.text`, `.data` or `.bss` cannot be described.

### 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 `bfd_read_minisymbols` function will read the symbols into memory in an internal form. It will return a `void *` pointer to a block of memory, a symbol count, and the size of each symbol. The pointer is allocated using `malloc`, and should be freed by the caller when it is no longer needed.

The function `bfd_minisymbol_to_symbol` will take a pointer to a minisymbol, and a pointer to a structure returned by `bfd_make_empty_symbol`, and return a `asymbol` structure. The return value may or may not be the same as the value from `bfd_make_empty_symbol` which was passed in.

### typedef asymbol

An `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
       	   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; static in C. The value
     	   is the offset into the section of the data. */
    #define BSF_LOCAL	0x01

    	/* The symbol has global scope; initialized data in 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:
    	   BSF_LOCAL, BSF_FORT_COMM,  BSF_UNDEFINED or
    	   BSF_GLOBAL */

    	/* The symbol is a debugging record. The value has an arbitary
    	   meaning. */
    #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 ISFCN symbol
    	   which is also 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.  */
    #define BSF_FILE          0x4000

    	/* Symbol is from dynamic linking information.  */
    #define BSF_DYNAMIC	   0x8000

           /* The symbol denotes a data object.  Used in ELF, and perhaps
              others someday.  */
    #define BSF_OBJECT	   0x10000

      flagword flags;

    	/* A pointer to the section to which this symbol is
    	   relative.  This will always be non NULL, there are special
              sections for undefined and absolute symbols.  */
      struct sec *section;

    	/* Back end special data.  */
      union
        {
          PTR p;
          bfd_vma i;
        } udata;

    } asymbol;

### Symbol handling functions

#### `bfd_get_symtab_upper_bound`

Description
Return the number of bytes required to store a vector of pointers to `asymbols` for all the symbols in the BFD abfd, including a terminal NULL pointer. If there are no symbols in the BFD, then return 0. If an error occurs, return -1.

    #define bfd_get_symtab_upper_bound(abfd) \
         BFD_SEND (abfd, _bfd_get_symtab_upper_bound, (abfd))

#### `bfd_is_local_label`

Synopsis

    boolean bfd_is_local_label(bfd *abfd, asymbol *sym);

Description
Return true if the given symbol sym in the BFD abfd is a compiler generated local label, else return false.

#### `bfd_is_local_label_name`

Synopsis

    boolean bfd_is_local_label_name(bfd *abfd, const char *name);

Description
Return true if a symbol with the name name in the BFD abfd is a compiler generated local label, else return false. This just checks whether the name has the form of a local label.

    #define bfd_is_local_label_name(abfd, name) \
         BFD_SEND (abfd, _bfd_is_local_label_name, (abfd, name))

#### `bfd_canonicalize_symtab`

Description
Read the symbols from the BFD abfd, and fills in the vector location with pointers to the symbols and a trailing NULL. Return the actual number of symbol pointers, not including the NULL.

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

#### `bfd_set_symtab`

Synopsis

    boolean bfd_set_symtab (bfd *abfd, asymbol **location, unsigned int count);

Description
Arrange that when the output BFD abfd is closed, the table location of count pointers to symbols will be written.

#### `bfd_print_symbol_vandf`

Synopsis

    void bfd_print_symbol_vandf(PTR file, asymbol *symbol);

Description
Print the value and flags of the symbol supplied to the stream file.

#### `bfd_make_empty_symbol`

Description
Create a new `asymbol` structure for the BFD abfd and return a pointer to it.

This routine is necessary because each back end has private information surrounding the `asymbol`. Building your own `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))

#### `bfd_make_debug_symbol`

Description
Create a new `asymbol` structure for the BFD abfd, to be used as a debugging symbol. Further details of its use have yet to be worked out.

    #define bfd_make_debug_symbol(abfd,ptr,size) \
            BFD_SEND (abfd, _bfd_make_debug_symbol, (abfd, ptr, size))

#### `bfd_decode_symclass`

Description
Return a character corresponding to the symbol class of symbol, or '?' for an unknown class.
Synopsis

    int bfd_decode_symclass(asymbol *symbol);

#### `bfd_symbol_info`

Description
Fill in the basic info about symbol that nm needs. Additional info may be added by the back-ends after calling this function.
Synopsis

    void bfd_symbol_info(asymbol *symbol, symbol_info *ret);

#### `bfd_copy_private_symbol_data`

Synopsis

    boolean bfd_copy_private_symbol_data(bfd *ibfd, asymbol *isym, bfd *obfd, asymbol *osym);

Description
Copy private symbol information from isym in the BFD ibfd to the symbol osym in the BFD obfd. Return `true` on success, `false` on error. Possible error returns are:

- `bfd_error_no_memory` - Not enough memory exists to create private data for osec.

    #define bfd_copy_private_symbol_data(ibfd, isymbol, obfd, osymbol) \
         BFD_SEND (obfd, _bfd_copy_private_symbol_data, \
    		(ibfd, isymbol, obfd, osymbol))

## Archives

Description
An archive (or library) is just another BFD. It has a symbol table, although there's not much a user program will do with it.

The big difference between an archive BFD and an ordinary BFD is that the archive doesn't have sections. Instead it has a chain of BFDs that are considered its contents. These BFDs can be manipulated like any other. The BFDs contained in an archive opened for reading will all be opened for reading. You may put either input or output BFDs into an archive opened for output; they will be handled correctly when the archive is closed.

Use `bfd_openr_next_archived_file` to step through the contents of an archive opened for input. You don't have to read the entire archive if you don't want to! Read it until you find what you want.

Archive contents of output BFDs are chained through the `next` pointer in a BFD. The first one is findable through the `archive_head` slot of the archive. Set it with `bfd_set_archive_head` (q.v.). A given BFD may be in only one open output archive at a time.

As expected, the BFD archive code is more general than the archive code of any given environment. BFD archives may contain files of different formats (e.g., a.out and coff) and even different architectures. You may even place archives recursively into archives!

This can cause unexpected confusion, since some archive formats are more expressive than others. For instance, Intel COFF archives can preserve long filenames; SunOS a.out archives cannot. If you move a file from the first to the second format and back again, the filename may be truncated. Likewise, different a.out environments have different conventions as to how they truncate filenames, whether they preserve directory names in filenames, etc. When interoperating with native tools, be sure your files are homogeneous.

Beware: most of these formats do not react well to the presence of spaces in filenames. We do the best we can, but can't always handle this case due to restrictions in the format of archives. Many Unix utilities are braindead in regards to spaces and such in filenames anyway, so this shouldn't be much of a restriction.

Archives are supported in BFD in `archive.c`.

#### `bfd_get_next_mapent`

Synopsis

    symindex bfd_get_next_mapent(bfd *abfd, symindex previous, carsym **sym);

Description
Step through archive abfd's symbol table (if it has one). Successively update sym with the next symbol's information, returning that symbol's (internal) index into the symbol table.

Supply `BFD_NO_MORE_SYMBOLS` as the previous entry to get the first one; returns `BFD_NO_MORE_SYMBOLS` when you've already got the last one.

A `carsym` is a canonical archive symbol. The only user-visible element is its name, a null-terminated string.

#### `bfd_set_archive_head`

Synopsis

    boolean bfd_set_archive_head(bfd *output, bfd *new_head);

Description
Set the head of the chain of BFDs contained in the archive output to new_head.

#### `bfd_openr_next_archived_file`

Synopsis

    bfd *bfd_openr_next_archived_file(bfd *archive, bfd *previous);

Description
Provided a BFD, archive, containing an archive and NULL, open an input BFD on the first contained element and returns that. Subsequent calls should pass the archive and the previous return value to return a created BFD to the next contained element. NULL is returned when there are no more.

## File formats

A format is a BFD concept of high level file contents type. The formats supported by BFD are:

- `bfd_object`

The BFD may contain data, symbols, relocations and debug info.

- `bfd_archive`

The BFD contains other BFDs and an optional index.

- `bfd_core`

The BFD contains the result of an executable core dump.

#### `bfd_check_format`

Synopsis

    boolean bfd_check_format(bfd *abfd, bfd_format format);

Description
Verify if the file attached to the BFD abfd is compatible with the format format (i.e., one of `bfd_object`, `bfd_archive` or `bfd_core`).

If the BFD has been set to a specific target before the call, only the named target and format combination is checked. If the target has not been set, or has been set to `default`, then all the known target backends is interrogated to determine a match. If the default target matches, it is used. If not, exactly one target must recognize the file, or an error results.

The function returns `true` on success, otherwise `false` with one of the following error codes:

- `bfd_error_invalid_operation` - if `format` is not one of `bfd_object`, `bfd_archive` or `bfd_core`.
- `bfd_error_system_call` - if an error occured during a read - even some file mismatches can cause bfd_error_system_calls.
- `file_not_recognised` - none of the backends recognised the file format.
- `bfd_error_file_ambiguously_recognized` - more than one backend recognised the file format.

#### `bfd_check_format_matches`

Synopsis

    boolean bfd_check_format_matches(bfd *abfd, bfd_format format, char ***matching);

Description
Like `bfd_check_format`, except when it returns false with `bfd_errno` set to `bfd_error_file_ambiguously_recognized`. In that case, if matching is not NULL, it will be filled in with a NULL-terminated list of the names of the formats that matched, allocated with `malloc`. Then the user may choose a format and try again.

When done with the list that matching points to, the caller should free it.

#### `bfd_set_format`

Synopsis

    boolean bfd_set_format(bfd *abfd, bfd_format format);

Description
This function sets the file format of the BFD abfd to the format format. If the target set in the BFD does not support the format requested, the format is invalid, or the BFD is not open for writing, then an error occurs.

#### `bfd_format_string`

Synopsis

    CONST char *bfd_format_string(bfd_format format);

Description
Return a pointer to a const string `invalid`, `object`, `archive`, `core`, or `unknown`, depending upon the value of format.

## Relocations

BFD maintains relocations in much the same way it maintains symbols: they are left alone until required, then read in en-mass and translated into an internal form. A common routine `bfd_perform_relocation` acts upon the canonical form to do the fixup.

Relocations are maintained on a per section basis, while symbols are maintained on a per BFD basis.

All that a back end has to do to fit the BFD interface is to create a `struct reloc_cache_entry` for each relocation in a particular section, and fill in the right bits of the structures.

- typedef arelent
- howto manager

### typedef arelent

This is the structure of a relocation entry:
.

    typedef enum bfd_reloc_status
    {
           /* No errors detected */
      bfd_reloc_ok,

           /* The relocation was performed, but there was an overflow. */
      bfd_reloc_overflow,

           /* The address to relocate was not within the section supplied. */
      bfd_reloc_outofrange,

           /* Used by special functions */
      bfd_reloc_continue,

           /* Unsupported relocation size requested. */
      bfd_reloc_notsupported,

           /* Unused */
      bfd_reloc_other,

           /* The symbol to relocate against was undefined. */
      bfd_reloc_undefined,

           /* The relocation was performed, but may not be ok - presently
              generated only when linking i960 coff files with i960 b.out
              symbols.  If this type is returned, the error_message argument
              to bfd_perform_relocation will be set.  */
      bfd_reloc_dangerous
     }
     bfd_reloc_status_type;

    typedef struct reloc_cache_entry
    {
           /* A pointer into the canonical table of pointers  */
      struct symbol_cache_entry **sym_ptr_ptr;

           /* offset in section */
      bfd_size_type address;

           /* addend for relocation value */
      bfd_vma addend;

           /* Pointer to how to perform the required relocation */
      reloc_howto_type *howto;

    } arelent;

Description
Here is a description of each of the fields within an `arelent`:

- `sym_ptr_ptr`

The symbol table pointer points to a pointer to the symbol associated with the relocation request. It is the pointer into the table returned by the back end's `get_symtab` action. See section Symbols. The symbol is referenced through a pointer to a pointer so that tools like the linker can fix up all the symbols of the same name by modifying only one pointer. The relocation routine looks in the symbol and uses the base of the section the symbol is attached to and the value of the symbol as the initial relocation offset. If the symbol pointer is zero, then the section provided is looked up.

- `address`

The `address` field gives the offset in bytes from the base of the section data which owns the relocation record to the first byte of relocatable information. The actual data relocated will be relative to this point; for example, a relocation type which modifies the bottom two bytes of a four byte word would not touch the first byte pointed to in a big endian world.

- `addend`

The `addend` is a value provided by the back end to be added (!) to the relocation offset. Its interpretation is dependent upon the howto. For example, on the 68k the code:

            char foo[];
            main()
                    {
                    return foo[0x12345678];
                    }

Could be compiled into:

            linkw fp,#-4
            moveb @#12345678,d0
            extbl d0
            unlk fp
            rts

This could create a reloc pointing to `foo`, but leave the offset in the data, something like:

    RELOCATION RECORDS FOR [.text]:
    offset   type      value
    00000006 32        _foo

    00000000 4e56 fffc          ; linkw fp,#-4
    00000004 1039 1234 5678     ; moveb @#12345678,d0
    0000000a 49c0               ; extbl d0
    0000000c 4e5e               ; unlk fp
    0000000e 4e75               ; rts

Using coff and an 88k, some instructions don't have enough space in them to represent the full address range, and pointers have to be loaded in two parts. So you'd get something like:

            or.u     r13,r0,hi16(_foo+0x12345678)
            ld.b     r2,r13,lo16(_foo+0x12345678)
            jmp      r1

This should create two relocs, both pointing to `_foo`, and with 0x12340000 in their addend field. The data would consist of:

    RELOCATION RECORDS FOR [.text]:
    offset   type      value
    00000002 HVRT16    _foo+0x12340000
    00000006 LVRT16    _foo+0x12340000

    00000000 5da05678           ; or.u r13,r0,0x5678
    00000004 1c4d5678           ; ld.b r2,r13,0x5678
    00000008 f400c001           ; jmp r1

The relocation routine digs out the value from the data, adds it to the addend to get the original offset, and then adds the value of `_foo`. Note that all 32 bits have to be kept around somewhere, to cope with carry from bit 15 to bit 16.

One further example is the sparc and the a.out format. The sparc has a similar problem to the 88k, in that some instructions don't have room for an entire offset, but on the sparc the parts are created in odd sized lumps. The designers of the a.out format chose to not use the data within the section for storing part of the offset; all the offset is kept within the reloc. Anything in the data should be ignored.

            save %sp,-112,%sp
            sethi %hi(_foo+0x12345678),%g2
            ldsb [%g2+%lo(_foo+0x12345678)],%i0
            ret
            restore

Both relocs contain a pointer to `foo`, and the offsets contain junk.

    RELOCATION RECORDS FOR [.text]:
    offset   type      value
    00000004 HI22      _foo+0x12345678
    00000008 LO10      _foo+0x12345678

    00000000 9de3bf90     ; save %sp,-112,%sp
    00000004 05000000     ; sethi %hi(_foo+0),%g2
    00000008 f048a000     ; ldsb [%g2+%lo(_foo+0)],%i0
    0000000c 81c7e008     ; ret
    00000010 81e80000     ; restore

- `howto`

The `howto` field can be imagined as a relocation instruction. It is a pointer to a structure which contains information on what to do with all of the other information in the reloc record and data section. A back end would normally have a relocation instruction set and turn relocations into pointers to the correct structure on input - but it would be possible to create each howto field on demand.

#### `enum complain_overflow`

Indicates what sort of overflow checking should be done when performing a relocation.
.

    enum complain_overflow
    {
    	/* Do not complain on overflow. */
      complain_overflow_dont,

    	/* Complain if the bitfield overflows, whether it is considered
    	   as signed or unsigned. */
      complain_overflow_bitfield,

    	/* Complain if the value overflows when considered as signed
    	   number. */
      complain_overflow_signed,

    	/* Complain if the value overflows when considered as an
    	   unsigned number. */
      complain_overflow_unsigned
    };

#### `reloc_howto_type`

The `reloc_howto_type` is a structure which contains all the information that libbfd needs to know to tie up a back end's data.
.struct symbol_cache_entry; /* Forward declaration */

    struct reloc_howto_struct
    {
           /*  The type field has mainly a documentary use - the back end can
               do what it wants with it, though normally the back end's
               external idea of what a reloc number is stored
               in this field. For example, a PC relative word relocation
               in a coff environment has the type 023 - because that's
               what the outside world calls a R_PCRWORD reloc. */
      unsigned int type;

           /*  The value the final relocation is shifted right by. This drops
               unwanted data from the relocation.  */
      unsigned int rightshift;

    	/*  The size of the item to be relocated.  This is *not* a
    	    power-of-two measure.  To get the number of bytes operated
    	    on by a type of relocation, use bfd_get_reloc_size.  */
      int size;

           /*  The number of bits in the item to be relocated.  This is used
    	    when doing overflow checking.  */
      unsigned int bitsize;

           /*  Notes that the relocation is relative to the location in the
               data section of the addend. The relocation function will
               subtract from the relocation value the address of the location
               being relocated. */
      boolean pc_relative;

    	/*  The bit position of the reloc value in the destination.
    	    The relocated value is left shifted by this amount. */
      unsigned int bitpos;

    	/* What type of overflow error should be checked for when
    	   relocating. */
      enum complain_overflow complain_on_overflow;

           /* If this field is non null, then the supplied function is
              called rather than the normal function. This allows really
              strange relocation methods to be accomodated (e.g., i960 callj
              instructions). */
      bfd_reloc_status_type (*special_function)
    				    PARAMS ((bfd *abfd,
    					     arelent *reloc_entry,
                                                struct symbol_cache_entry *symbol,
                                                PTR data,
                                                asection *input_section,
                                                bfd *output_bfd,
                                                char **error_message));

           /* The textual name of the relocation type. */
      char *name;

           /* When performing a partial link, some formats must modify the
              relocations rather than the data - this flag signals this.*/
      boolean partial_inplace;

           /* The src_mask selects which parts of the read in data
              are to be used in the relocation sum.  E.g., if this was an 8 bit
              bit of data which we read and relocated, this would be
              0x000000ff. When we have relocs which have an addend, such as
              sun4 extended relocs, the value in the offset part of a
              relocating field is garbage so we never use it. In this case
              the mask would be 0x00000000. */
      bfd_vma src_mask;

           /* The dst_mask selects which parts of the instruction are replaced
              into the instruction. In most cases src_mask == dst_mask,
              except in the above special case, where dst_mask would be
              0x000000ff, and src_mask would be 0x00000000.   */
      bfd_vma dst_mask;

           /* When some formats create PC relative instructions, they leave
              the value of the pc of the place being relocated in the offset
              slot of the instruction, so that a PC relative relocation can
              be made just by adding in an ordinary offset (e.g., sun3 a.out).
              Some formats leave the displacement part of an instruction
              empty (e.g., m88k bcs); this flag signals the fact.*/
      boolean pcrel_offset;

    };

#### `The HOWTO Macro`

Description
The HOWTO define is horrible and will go away.

    #define HOWTO(C, R,S,B, P, BI, O, SF, NAME, INPLACE, MASKSRC, MASKDST, PC) \
      {(unsigned)C,R,S,B, P, BI, O,SF,NAME,INPLACE,MASKSRC,MASKDST,PC}

Description
And will be replaced with the totally magic way. But for the moment, we are compatible, so do it this way.

    #define NEWHOWTO( FUNCTION, NAME,SIZE,REL,IN) HOWTO(0,0,SIZE,0,REL,0,complain_overflow_dont,FUNCTION, NAME,false,0,0,IN)

Description
Helper routine to turn a symbol into a relocation value.

    #define HOWTO_PREPARE(relocation, symbol)      \
      {                                            \
      if (symbol != (asymbol *)NULL) {             \
        if (bfd_is_com_section (symbol->section)) { \
          relocation = 0;                          \
        }                                          \
        else {                                     \
          relocation = symbol->value;              \
        }                                          \
      }                                            \
    }

#### `bfd_get_reloc_size`

Synopsis

    int bfd_get_reloc_size (reloc_howto_type *);

Description
For a reloc_howto_type that operates on a fixed number of bytes, this returns the number of bytes operated on.

#### `arelent_chain`

Description
How relocs are tied together in an `asection`:

    typedef struct relent_chain {
      arelent relent;
      struct   relent_chain *next;
    } arelent_chain;

#### `bfd_perform_relocation`

Synopsis

    bfd_reloc_status_type
    bfd_perform_relocation
       (bfd *abfd,
        arelent *reloc_entry,
        PTR data,
        asection *input_section,
        bfd *output_bfd,
        char **error_message);

Description
If output_bfd is supplied to this function, the generated image will be relocatable; the relocations are copied to the output file after they have been changed to reflect the new state of the world. There are two ways of reflecting the results of partial linkage in an output file: by modifying the output data in place, and by modifying the relocation record. Some native formats (e.g., basic a.out and basic coff) have no way of specifying an addend in the relocation type, so the addend has to go in the output data. This is no big deal since in these formats the output data slot will always be big enough for the addend. Complex reloc types with addends were invented to solve just this problem. The error_message argument is set to an error message if this return `bfd_reloc_dangerous`.

#### `bfd_install_relocation`

Synopsis

    bfd_reloc_status_type
    bfd_install_relocation
       (bfd *abfd,
        arelent *reloc_entry,
        PTR data, bfd_vma data_start,
        asection *input_section,
        char **error_message);

Description
This looks remarkably like `bfd_perform_relocation`, except it does not expect that the section contents have been filled in. I.e., it's suitable for use when creating, rather than applying a relocation.

For now, this function should be considered reserved for the assembler.

## The howto manager

When an application wants to create a relocation, but doesn't know what the target machine might call it, it can find out by using this bit of code.

#### `bfd_reloc_code_type`

Description
The insides of a reloc code. The idea is that, eventually, there will be one enumerator for every type of relocation we ever do. Pass one of these values to `bfd_reloc_type_lookup`, and it'll return a howto pointer.

This does mean that the application must determine the correct enumerator value; you can't get a howto pointer from a random set of attributes.
Here are the possible values for `enum bfd_reloc_code_real`:

: BFD_RELOC_64
: BFD_RELOC_32
: BFD_RELOC_26
: BFD_RELOC_24
: BFD_RELOC_16
: BFD_RELOC_14
: BFD_RELOC_8
  Basic absolute relocations of N bits.

: BFD_RELOC_64_PCREL
: BFD_RELOC_32_PCREL
: BFD_RELOC_24_PCREL
: BFD_RELOC_16_PCREL
: BFD_RELOC_12_PCREL
: BFD_RELOC_8_PCREL
  PC-relative relocations. Sometimes these are relative to the address of the relocation itself; sometimes they are relative to the start of the section containing the relocation. It depends on the specific target.

  The 24-bit relocation is used in some Intel 960 configurations.

: BFD_RELOC_32_GOT_PCREL
: BFD_RELOC_16_GOT_PCREL
: BFD_RELOC_8_GOT_PCREL
: BFD_RELOC_32_GOTOFF
: BFD_RELOC_16_GOTOFF
: BFD_RELOC_LO16_GOTOFF
: BFD_RELOC_HI16_GOTOFF
: BFD_RELOC_HI16_S_GOTOFF
: BFD_RELOC_8_GOTOFF
: BFD_RELOC_32_PLT_PCREL
: BFD_RELOC_24_PLT_PCREL
: BFD_RELOC_16_PLT_PCREL
: BFD_RELOC_8_PLT_PCREL
: BFD_RELOC_32_PLTOFF
: BFD_RELOC_16_PLTOFF
: BFD_RELOC_LO16_PLTOFF
: BFD_RELOC_HI16_PLTOFF
: BFD_RELOC_HI16_S_PLTOFF
: BFD_RELOC_8_PLTOFF
  For ELF.

: BFD_RELOC_68K_GLOB_DAT
: BFD_RELOC_68K_JMP_SLOT
: BFD_RELOC_68K_RELATIVE
  Relocations used by 68K ELF.

: BFD_RELOC_32_BASEREL
: BFD_RELOC_16_BASEREL
: BFD_RELOC_LO16_BASEREL
: BFD_RELOC_HI16_BASEREL
: BFD_RELOC_HI16_S_BASEREL
: BFD_RELOC_8_BASEREL
: BFD_RELOC_RVA
  Linkage-table relative.

: BFD_RELOC_8_FFnn
  Absolute 8-bit relocation, but used to form an address like 0xFFnn.

: BFD_RELOC_32_PCREL_S2
: BFD_RELOC_16_PCREL_S2
: BFD_RELOC_23_PCREL_S2
  These PC-relative relocations are stored as word displacements -- i.e., byte displacements shifted right two bits. The 30-bit word displacement (<<32_PCREL_S2>> -- 32 bits, shifted 2) is used on the SPARC. (SPARC tools generally refer to this as <<WDISP30>>.) The signed 16-bit displacement is used on the MIPS, and the 23-bit displacement is used on the Alpha.

: BFD_RELOC_HI22
: BFD_RELOC_LO10
  High 22 bits and low 10 bits of 32-bit value, placed into lower bits of the target word. These are used on the SPARC.

: BFD_RELOC_GPREL16
: BFD_RELOC_GPREL32
  For systems that allocate a Global Pointer register, these are displacements off that register. These relocation types are handled specially, because the value the register will have is decided relatively late.

: BFD_RELOC_I960_CALLJ
  Reloc types used for i960/b.out.

: BFD_RELOC_NONE
: BFD_RELOC_SPARC_WDISP22
: BFD_RELOC_SPARC22
: BFD_RELOC_SPARC13
: BFD_RELOC_SPARC_GOT10
: BFD_RELOC_SPARC_GOT13
: BFD_RELOC_SPARC_GOT22
: BFD_RELOC_SPARC_PC10
: BFD_RELOC_SPARC_PC22
: BFD_RELOC_SPARC_WPLT30
: BFD_RELOC_SPARC_COPY
: BFD_RELOC_SPARC_GLOB_DAT
: BFD_RELOC_SPARC_JMP_SLOT
: BFD_RELOC_SPARC_RELATIVE
: BFD_RELOC_SPARC_UA32
  SPARC ELF relocations. There is probably some overlap with other relocation types already defined.

: BFD_RELOC_SPARC_BASE13
: BFD_RELOC_SPARC_BASE22
  I think these are specific to SPARC a.out (e.g., Sun 4).

: BFD_RELOC_SPARC_64
: BFD_RELOC_SPARC_10
: BFD_RELOC_SPARC_11
: BFD_RELOC_SPARC_OLO10
: BFD_RELOC_SPARC_HH22
: BFD_RELOC_SPARC_HM10
: BFD_RELOC_SPARC_LM22
: BFD_RELOC_SPARC_PC_HH22
: BFD_RELOC_SPARC_PC_HM10
: BFD_RELOC_SPARC_PC_LM22
: BFD_RELOC_SPARC_WDISP16
: BFD_RELOC_SPARC_WDISP19
: BFD_RELOC_SPARC_GLOB_JMP
: BFD_RELOC_SPARC_7
: BFD_RELOC_SPARC_6
: BFD_RELOC_SPARC_5
  Some relocations we're using for SPARC V9 -- subject to change.

: BFD_RELOC_ALPHA_GPDISP_HI16
  Alpha ECOFF and ELF relocations. Some of these treat the symbol or "addend" in some special way. For GPDISP_HI16 ("gpdisp") relocations, the symbol is ignored when writing; when reading, it will be the absolute section symbol. The addend is the displacement in bytes of the "lda" instruction from the "ldah" instruction (which is at the address of this reloc).

: BFD_RELOC_ALPHA_GPDISP_LO16
  For GPDISP_LO16 ("ignore") relocations, the symbol is handled as with GPDISP_HI16 relocs. The addend is ignored when writing the relocations out, and is filled in with the file's GP value on reading, for convenience.

: BFD_RELOC_ALPHA_GPDISP
  The ELF GPDISP relocation is exactly the same as the GPDISP_HI16 relocation except that there is no accompanying GPDISP_LO16 relocation.

: BFD_RELOC_ALPHA_LITERAL
: BFD_RELOC_ALPHA_ELF_LITERAL
: BFD_RELOC_ALPHA_LITUSE
  The Alpha LITERAL/LITUSE relocs are produced by a symbol reference; the assembler turns it into a LDQ instruction to load the address of the symbol, and then fills in a register in the real instruction.

  The LITERAL reloc, at the LDQ instruction, refers to the .lita section symbol. The addend is ignored when writing, but is filled in with the file's GP value on reading, for convenience, as with the GPDISP_LO16 reloc.

  The ELF_LITERAL reloc is somewhere between 16_GOTOFF and GPDISP_LO16. It should refer to the symbol to be referenced, as with 16_GOTOFF, but it generates output not based on the position within the .got section, but relative to the GP value chosen for the file during the final link stage.

  The LITUSE reloc, on the instruction using the loaded address, gives information to the linker that it might be able to use to optimize away some literal section references. The symbol is ignored (read as the absolute section symbol), and the "addend" indicates the type of instruction using the register: 1 - "memory" fmt insn 2 - byte-manipulation (byte offset reg) 3 - jsr (target of branch)

  The GNU linker currently doesn't do any of this optimizing.

: BFD_RELOC_ALPHA_HINT
  The HINT relocation indicates a value that should be filled into the "hint" field of a jmp/jsr/ret instruction, for possible branch- prediction logic which may be provided on some processors.

: BFD_RELOC_ALPHA_LINKAGE
  The LINKAGE relocation outputs a linkage pair in the object file, which is filled by the linker.

: BFD_RELOC_ALPHA_CODEADDR
  The CODEADDR relocation outputs a STO_CA in the object file, which is filled by the linker.

: BFD_RELOC_MIPS_JMP
  Bits 27..2 of the relocation address shifted right 2 bits; simple reloc otherwise.

: BFD_RELOC_MIPS16_JMP
  The MIPS16 jump instruction.

: BFD_RELOC_MIPS16_GPREL
  MIPS16 GP relative reloc.

: BFD_RELOC_HI16
  High 16 bits of 32-bit value; simple reloc.

: BFD_RELOC_HI16_S
  High 16 bits of 32-bit value but the low 16 bits will be sign extended and added to form the final result. If the low 16 bits form a negative number, we need to add one to the high value to compensate for the borrow when the low bits are added.

: BFD_RELOC_LO16
  Low 16 bits.

: BFD_RELOC_PCREL_HI16_S
  Like BFD_RELOC_HI16_S, but PC relative.

: BFD_RELOC_PCREL_LO16
  Like BFD_RELOC_LO16, but PC relative.

: BFD_RELOC_MIPS_GPREL
  Relocation relative to the global pointer.

: BFD_RELOC_MIPS_LITERAL
  Relocation against a MIPS literal section.

: BFD_RELOC_MIPS_GOT16
: BFD_RELOC_MIPS_CALL16
: BFD_RELOC_MIPS_GPREL32
: BFD_RELOC_MIPS_GOT_HI16
: BFD_RELOC_MIPS_GOT_LO16
: BFD_RELOC_MIPS_CALL_HI16
: BFD_RELOC_MIPS_CALL_LO16
  MIPS ELF relocations.

: BFD_RELOC_386_GOT32
: BFD_RELOC_386_PLT32
: BFD_RELOC_386_COPY
: BFD_RELOC_386_GLOB_DAT
: BFD_RELOC_386_JUMP_SLOT
: BFD_RELOC_386_RELATIVE
: BFD_RELOC_386_GOTOFF
: BFD_RELOC_386_GOTPC
  i386/elf relocations

: BFD_RELOC_NS32K_IMM_8
: BFD_RELOC_NS32K_IMM_16
: BFD_RELOC_NS32K_IMM_32
: BFD_RELOC_NS32K_IMM_8_PCREL
: BFD_RELOC_NS32K_IMM_16_PCREL
: BFD_RELOC_NS32K_IMM_32_PCREL
: BFD_RELOC_NS32K_DISP_8
: BFD_RELOC_NS32K_DISP_16
: BFD_RELOC_NS32K_DISP_32
: BFD_RELOC_NS32K_DISP_8_PCREL
: BFD_RELOC_NS32K_DISP_16_PCREL
: BFD_RELOC_NS32K_DISP_32_PCREL
  ns32k relocations

: BFD_RELOC_PPC_B26
: BFD_RELOC_PPC_BA26
: BFD_RELOC_PPC_TOC16
: BFD_RELOC_PPC_B16
: BFD_RELOC_PPC_B16_BRTAKEN
: BFD_RELOC_PPC_B16_BRNTAKEN
: BFD_RELOC_PPC_BA16
: BFD_RELOC_PPC_BA16_BRTAKEN
: BFD_RELOC_PPC_BA16_BRNTAKEN
: BFD_RELOC_PPC_COPY
: BFD_RELOC_PPC_GLOB_DAT
: BFD_RELOC_PPC_JMP_SLOT
: BFD_RELOC_PPC_RELATIVE
: BFD_RELOC_PPC_LOCAL24PC
: BFD_RELOC_PPC_EMB_NADDR32
: BFD_RELOC_PPC_EMB_NADDR16
: BFD_RELOC_PPC_EMB_NADDR16_LO
: BFD_RELOC_PPC_EMB_NADDR16_HI
: BFD_RELOC_PPC_EMB_NADDR16_HA
: BFD_RELOC_PPC_EMB_SDAI16
: BFD_RELOC_PPC_EMB_SDA2I16
: BFD_RELOC_PPC_EMB_SDA2REL
: BFD_RELOC_PPC_EMB_SDA21
: BFD_RELOC_PPC_EMB_MRKREF
: BFD_RELOC_PPC_EMB_RELSEC16
: BFD_RELOC_PPC_EMB_RELST_LO
: BFD_RELOC_PPC_EMB_RELST_HI
: BFD_RELOC_PPC_EMB_RELST_HA
: BFD_RELOC_PPC_EMB_BIT_FLD
: BFD_RELOC_PPC_EMB_RELSDA
  Power(rs6000) and PowerPC relocations.

: BFD_RELOC_CTOR
  The type of reloc used to build a contructor table - at the moment probably a 32 bit wide absolute relocation, but the target can choose. It generally does map to one of the other relocation types.

: BFD_RELOC_ARM_PCREL_BRANCH
  ARM 26 bit pc-relative branch. The lowest two bits must be zero and are not stored in the instruction.

: BFD_RELOC_ARM_IMMEDIATE
: BFD_RELOC_ARM_OFFSET_IMM
: BFD_RELOC_ARM_SHIFT_IMM
: BFD_RELOC_ARM_SWI
: BFD_RELOC_ARM_MULTI
: BFD_RELOC_ARM_CP_OFF_IMM
: BFD_RELOC_ARM_ADR_IMM
: BFD_RELOC_ARM_LDR_IMM
: BFD_RELOC_ARM_LITERAL
: BFD_RELOC_ARM_IN_POOL
: BFD_RELOC_ARM_OFFSET_IMM8
: BFD_RELOC_ARM_HWLITERAL
: BFD_RELOC_ARM_THUMB_ADD
: BFD_RELOC_ARM_THUMB_IMM
: BFD_RELOC_ARM_THUMB_SHIFT
: BFD_RELOC_ARM_THUMB_OFFSET
  These relocs are only used within the ARM assembler. They are not (at present) written to any object files.

: BFD_RELOC_SH_PCDISP8BY2
: BFD_RELOC_SH_PCDISP12BY2
: BFD_RELOC_SH_IMM4
: BFD_RELOC_SH_IMM4BY2
: BFD_RELOC_SH_IMM4BY4
: BFD_RELOC_SH_IMM8
: BFD_RELOC_SH_IMM8BY2
: BFD_RELOC_SH_IMM8BY4
: BFD_RELOC_SH_PCRELIMM8BY2
: BFD_RELOC_SH_PCRELIMM8BY4
: BFD_RELOC_SH_SWITCH16
: BFD_RELOC_SH_SWITCH32
: BFD_RELOC_SH_USES
: BFD_RELOC_SH_COUNT
: BFD_RELOC_SH_ALIGN
: BFD_RELOC_SH_CODE
: BFD_RELOC_SH_DATA
: BFD_RELOC_SH_LABEL
  Hitachi SH relocs. Not all of these appear in object files.

: BFD_RELOC_D10V_10_PCREL_R
  Mitsubishi D10V relocs. This is a 10-bit reloc with the right 2 bits assumed to be 0.

: BFD_RELOC_D10V_10_PCREL_L
  Mitsubishi D10V relocs. This is a 10-bit reloc with the right 2 bits assumed to be 0. This is the same as the previous reloc except it is in the left container, i.e., shifted left 15 bits.

: BFD_RELOC_D10V_18
  This is an 18-bit reloc with the right 2 bits assumed to be 0.

: BFD_RELOC_D10V_18_PCREL
  This is an 18-bit reloc with the right 2 bits assumed to be 0.

: BFD_RELOC_M32R_24
  Mitsubishi M32R relocs. This is a 24 bit absolute address.

: BFD_RELOC_M32R_10_PCREL
  This is a 10-bit pc-relative reloc with the right 2 bits assumed to be 0.

: BFD_RELOC_M32R_18_PCREL
  This is an 18-bit reloc with the right 2 bits assumed to be 0.

: BFD_RELOC_M32R_26_PCREL
  This is a 26-bit reloc with the right 2 bits assumed to be 0.

: BFD_RELOC_M32R_HI16_ULO
  This is a 16-bit reloc containing the high 16 bits of an address used when the lower 16 bits are treated as unsigned.

: BFD_RELOC_M32R_HI16_SLO
  This is a 16-bit reloc containing the high 16 bits of an address used when the lower 16 bits are treated as signed.

: BFD_RELOC_M32R_LO16
  This is a 16-bit reloc containing the lower 16 bits of an address.

: BFD_RELOC_M32R_SDA16
  This is a 16-bit reloc containing the small data area offset for use in add3, load, and store instructions.

: BFD_RELOC_MN10300_32_PCREL
  This is a 32bit pcrel reloc for the mn10300, offset by two bytes in the instruction.

: BFD_RELOC_MN10300_16_PCREL
  This is a 16bit pcrel reloc for the mn10300, offset by two bytes in the instruction.

.

    typedef enum bfd_reloc_code_real bfd_reloc_code_real_type;

#### `bfd_reloc_type_lookup`

Synopsis

    reloc_howto_type *
    bfd_reloc_type_lookup (bfd *abfd, bfd_reloc_code_real_type code);

Description
Return a pointer to a howto structure which, when invoked, will perform the relocation code on data from the architecture noted.

#### `bfd_default_reloc_type_lookup`

Synopsis

    reloc_howto_type *bfd_default_reloc_type_lookup
       (bfd *abfd, bfd_reloc_code_real_type  code);

Description
Provides a default relocation lookup routine for any architecture.

#### `bfd_get_reloc_code_name`

Synopsis

    const char *bfd_get_reloc_code_name (bfd_reloc_code_real_type code);

Description
Provides a printable name for the supplied relocation code. Useful mainly for printing error messages.

#### `bfd_generic_relax_section`

Synopsis

    boolean bfd_generic_relax_section
       (bfd *abfd,
        asection *section,
        struct bfd_link_info *,
        boolean *);

Description
Provides default handling for relaxing for back ends which don't do relaxing -- i.e., does nothing.

#### `bfd_generic_get_relocated_section_contents`

Synopsis

    bfd_byte *
    bfd_generic_get_relocated_section_contents (bfd *abfd,
        struct bfd_link_info *link_info,
        struct bfd_link_order *link_order,
        bfd_byte *data,
        boolean relocateable,
        asymbol **symbols);

Description
Provides default handling of relocation effort for back ends which can't be bothered to do it efficiently.

## Core files

Description
These are functions pertaining to core files.

#### `bfd_core_file_failing_command`

Synopsis

    CONST char *bfd_core_file_failing_command(bfd *abfd);

Description
Return a read-only string explaining which program was running when it failed and produced the core file abfd.

#### `bfd_core_file_failing_signal`

Synopsis

    int bfd_core_file_failing_signal(bfd *abfd);

Description
Returns the signal number which caused the core dump which generated the file the BFD abfd is attached to.

#### `core_file_matches_executable_p`

Synopsis

    boolean core_file_matches_executable_p
       (bfd *core_bfd, bfd *exec_bfd);

Description
Return `true` if the core file attached to core_bfd was generated by a run of the executable file attached to exec_bfd, `false` otherwise.

## 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 section bfd_target. See section File 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

### 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_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.

       /* 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)
      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 *));

      /* 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 **));

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.

## Architectures

BFD keeps one atom in a BFD describing the architecture of the data attached to the BFD: a pointer to a `bfd_arch_info_type`.

Pointers to structures can be requested independently of a BFD so that an architecture's information can be interrogated without access to an open BFD.

The architecture information is provided by each architecture package. The set of default architectures is selected by the macro `SELECT_ARCHITECTURES`. This is normally set up in the ``config/target.mt'` file of your choice. If the name is not defined, then all the architectures supported are included.

When BFD starts up, all the architectures are called with an initialize method. It is up to the architecture back end to insert as many items into the list of architectures as it wants to; generally this would be one for each machine and one for the default case (an item with a machine field of 0).

BFD's idea of an architecture is implemented in ``archures.c'`.

### bfd_architecture

Description
This enum gives the object file's CPU architecture, in a global sense--i.e., what processor family does it belong to? Another field indicates which processor within the family is in use. The machine gives a number which distinguishes different versions of the architecture, containing, for example, 2 and 3 for Intel i960 KA and i960 KB, and 68020 and 68030 for Motorola 68020 and 68030.

    enum bfd_architecture
    {
      bfd_arch_unknown,    /* File arch not known */
      bfd_arch_obscure,    /* Arch known, not one of these */
      bfd_arch_m68k,       /* Motorola 68xxx */
      bfd_arch_vax,        /* DEC Vax */
      bfd_arch_i960,       /* Intel 960 */
         /* The order of the following is important.
           lower number indicates a machine type that
           only accepts a subset of the instructions
           available to machines with higher numbers.
           The exception is the "ca", which is
           incompatible with all other machines except
           "core". */

    #define bfd_mach_i960_core      1
    #define bfd_mach_i960_ka_sa     2
    #define bfd_mach_i960_kb_sb     3
    #define bfd_mach_i960_mc        4
    #define bfd_mach_i960_xa        5
    #define bfd_mach_i960_ca        6
    #define bfd_mach_i960_jx	 7
    #define bfd_mach_i960_hx        8

      bfd_arch_a29k,       /* AMD 29000 */
      bfd_arch_sparc,      /* SPARC */
    #define bfd_mach_sparc			1
     /* The difference between v8plus and v9 is that v9 is a true 64 bit env.  */
    #define bfd_mach_sparc_sparclet	2
    #define bfd_mach_sparc_sparclite	3
    #define bfd_mach_sparc_v8plus		4
    #define bfd_mach_sparc_v8plusa		5  /* with ultrasparc add'ns */
    #define bfd_mach_sparc_v9		6
    #define bfd_mach_sparc_v9a		7  /* with ultrasparc add'ns */
     /* Nonzero if MACH has the v9 instruction set.  */
    #define bfd_mach_sparc_v9_p(mach) \
      ((mach) >= bfd_mach_sparc_v8plus && (mach) <= bfd_mach_sparc_v9a)
      bfd_arch_mips,       /* MIPS Rxxxx */
      bfd_arch_i386,       /* Intel 386 */
    #define bfd_mach_i386_i386 0
    #define bfd_mach_i386_i8086 1
      bfd_arch_we32k,      /* AT&T WE32xxx */
      bfd_arch_tahoe,      /* CCI/Harris Tahoe */
      bfd_arch_i860,       /* Intel 860 */
      bfd_arch_romp,       /* IBM ROMP PC/RT */
      bfd_arch_alliant,    /* Alliant */
      bfd_arch_convex,     /* Convex */
      bfd_arch_m88k,       /* Motorola 88xxx */
      bfd_arch_pyramid,    /* Pyramid Technology */
      bfd_arch_h8300,      /* Hitachi H8/300 */
    #define bfd_mach_h8300   1
    #define bfd_mach_h8300h  2
    #define bfd_mach_h8300s  3
      bfd_arch_powerpc,    /* PowerPC */
      bfd_arch_rs6000,     /* IBM RS/6000 */
      bfd_arch_hppa,       /* HP PA RISC */
      bfd_arch_d10v,       /* Mitsubishi D10V */
      bfd_arch_z8k,        /* Zilog Z8000 */
    #define bfd_mach_z8001		1
    #define bfd_mach_z8002		2
      bfd_arch_h8500,      /* Hitachi H8/500 */
      bfd_arch_sh,         /* Hitachi SH */
      bfd_arch_alpha,      /* Dec Alpha */
      bfd_arch_arm,        /* Advanced Risc Machines ARM */
      bfd_arch_ns32k,      /* National Semiconductors ns32000 */
      bfd_arch_w65,        /* WDC 65816 */
      bfd_arch_m32r,       /* Mitsubishi M32R/D */
      bfd_arch_mn10200,    /* Matsushita MN10200 */
      bfd_arch_mn10300,    /* Matsushita MN10300 */
      bfd_arch_last
      };

### bfd_arch_info

Description
This structure contains information on architectures for use within BFD.

    typedef struct bfd_arch_info
    {
      int bits_per_word;
      int bits_per_address;
      int bits_per_byte;
      enum bfd_architecture arch;
      unsigned long mach;
      const char *arch_name;
      const char *printable_name;
      unsigned int section_align_power;
      /* true if this is the default machine for the architecture */
      boolean the_default;
      const struct bfd_arch_info * (*compatible)
    	PARAMS ((const struct bfd_arch_info *a,
    	         const struct bfd_arch_info *b));

      boolean (*scan) PARAMS ((const struct bfd_arch_info *, const char *));

      const struct bfd_arch_info *next;
    } bfd_arch_info_type;

#### `bfd_printable_name`

Synopsis

    const char *bfd_printable_name(bfd *abfd);

Description
Return a printable string representing the architecture and machine from the pointer to the architecture info structure.

#### `bfd_scan_arch`

Synopsis

    const bfd_arch_info_type *bfd_scan_arch(const char *string);

Description
Figure out if BFD supports any cpu which could be described with the name string. Return a pointer to an `arch_info` structure if a machine is found, otherwise NULL.

#### `bfd_arch_get_compatible`

Synopsis

    const bfd_arch_info_type *bfd_arch_get_compatible(
        const bfd *abfd,
        const bfd *bbfd);

Description
Determine whether two BFDs' architectures and machine types are compatible. Calculates the lowest common denominator between the two architectures and machine types implied by the BFDs and returns a pointer to an `arch_info` structure describing the compatible machine.

#### `bfd_default_arch_struct`

Description
The `bfd_default_arch_struct` is an item of `bfd_arch_info_type` which has been initialized to a fairly generic state. A BFD starts life by pointing to this structure, until the correct back end has determined the real architecture of the file.

    extern const bfd_arch_info_type bfd_default_arch_struct;

#### `bfd_set_arch_info`

Synopsis

    void bfd_set_arch_info(bfd *abfd, const bfd_arch_info_type *arg);

Description
Set the architecture info of abfd to arg.

#### `bfd_default_set_arch_mach`

Synopsis

    boolean bfd_default_set_arch_mach(bfd *abfd,
        enum bfd_architecture arch,
        unsigned long mach);

Description
Set the architecture and machine type in BFD abfd to arch and mach. Find the correct pointer to a structure and insert it into the `arch_info` pointer.

#### `bfd_get_arch`

Synopsis

    enum bfd_architecture bfd_get_arch(bfd *abfd);

Description
Return the enumerated type which describes the BFD abfd's architecture.

#### `bfd_get_mach`

Synopsis

    unsigned long bfd_get_mach(bfd *abfd);

Description
Return the long type which describes the BFD abfd's machine.

#### `bfd_arch_bits_per_byte`

Synopsis

    unsigned int bfd_arch_bits_per_byte(bfd *abfd);

Description
Return the number of bits in one of the BFD abfd's architecture's bytes.

#### `bfd_arch_bits_per_address`

Synopsis

    unsigned int bfd_arch_bits_per_address(bfd *abfd);

Description
Return the number of bits in one of the BFD abfd's architecture's addresses.

#### `bfd_default_compatible`

Synopsis

    const bfd_arch_info_type *bfd_default_compatible
       (const bfd_arch_info_type *a,
        const bfd_arch_info_type *b);

Description
The default function for testing for compatibility.

#### `bfd_default_scan`

Synopsis

    boolean bfd_default_scan(const struct bfd_arch_info *info, const char *string);

Description
The default function for working out whether this is an architecture hit and a machine hit.

#### `bfd_get_arch_info`

Synopsis

    const bfd_arch_info_type * bfd_get_arch_info(bfd *abfd);

Description
Return the architecture info struct in abfd.

#### `bfd_lookup_arch`

Synopsis

    const bfd_arch_info_type *bfd_lookup_arch
       (enum bfd_architecture
        arch,
        unsigned long machine);

Description
Look for the architecure info structure which matches the arguments arch and machine. A machine of 0 matches the machine/architecture structure which marks itself as the default.

#### `bfd_printable_arch_mach`

Synopsis

    const char *bfd_printable_arch_mach
       (enum bfd_architecture arch, unsigned long machine);

Description
Return a printable string representing the architecture and machine type.

This routine is depreciated.

## Opening and closing BFDs

#### `bfd_openr`

Synopsis

    bfd *bfd_openr(CONST char *filename, CONST char *target);

Description
Open the file filename (using `fopen`) with the target target. Return a pointer to the created BFD.

Calls `bfd_find_target`, so target is interpreted as by that function.

If `NULL` is returned then an error has occured. Possible errors are `bfd_error_no_memory`, `bfd_error_invalid_target` or `system_call` error.

#### `bfd_fdopenr`

Synopsis

    bfd *bfd_fdopenr(CONST char *filename, CONST char *target, int fd);

Description
`bfd_fdopenr` is to `bfd_fopenr` much like `fdopen` is to `fopen`. It opens a BFD on a file already described by the fd supplied.

When the file is later `bfd_close`d, the file descriptor will be closed.

If the caller desires that this file descriptor be cached by BFD (opened as needed, closed as needed to free descriptors for other opens), with the supplied fd used as an initial file descriptor (but subject to closure at any time), call bfd_set_cacheable(bfd, 1) on the returned BFD. The default is to assume no cacheing; the file descriptor will remain open until `bfd_close`, and will not be affected by BFD operations on other files.

Possible errors are `bfd_error_no_memory`, `bfd_error_invalid_target` and `bfd_error_system_call`.

#### `bfd_openstreamr`

Synopsis

    bfd *bfd_openstreamr(const char *, const char *, PTR);

Description
Open a BFD for read access on an existing stdio stream. When the BFD is passed to `bfd_close`, the stream will be closed.

#### `bfd_openw`

Synopsis

    bfd *bfd_openw(CONST char *filename, CONST char *target);

Description
Create a BFD, associated with file filename, using the file format target, and return a pointer to it.

Possible errors are `bfd_error_system_call`, `bfd_error_no_memory`, `bfd_error_invalid_target`.

#### `bfd_close`

Synopsis

    boolean bfd_close(bfd *abfd);

Description
Close a BFD. If the BFD was open for writing, then pending operations are completed and the file written out and closed. If the created file is executable, then `chmod` is called to mark it as such.

All memory attached to the BFD is released.

The file descriptor associated with the BFD is closed (even if it was passed in to BFD by `bfd_fdopenr`).
Returns
`true` is returned if all is ok, otherwise `false`.

#### `bfd_close_all_done`

Synopsis

    boolean bfd_close_all_done(bfd *);

Description
Close a BFD. Differs from `bfd_close` since it does not complete any pending operations. This routine would be used if the application had just used BFD for swapping and didn't want to use any of the writing code.

If the created file is executable, then `chmod` is called to mark it as such.

All memory attached to the BFD is released.
Returns
`true` is returned if all is ok, otherwise `false`.

#### `bfd_create`

Synopsis

    bfd *bfd_create(CONST char *filename, bfd *templ);

Description
Create a new BFD in the manner of `bfd_openw`, but without opening a file. The new BFD takes the target from the target used by template. The format is always set to `bfd_object`.

#### `bfd_alloc`

Synopsis

    PTR bfd_alloc (bfd *abfd, size_t wanted);

Description
Allocate a block of wanted bytes of memory attached to `abfd` and return a pointer to it.

## Internal functions

Description
These routines are used within BFD. They are not intended for export, but are documented here for completeness.

#### `bfd_write_bigendian_4byte_int`

Synopsis

    void bfd_write_bigendian_4byte_int(bfd *abfd,  int i);

Description
Write a 4 byte integer i to the output BFD abfd, in big endian order regardless of what else is going on. This is useful in archives.

#### `bfd_put_size`

#### `bfd_get_size`

Description
These macros as used for reading and writing raw data in sections; each access (except for bytes) is vectored through the target format of the BFD and mangled accordingly. The mangling performs any necessary endian translations and removes alignment restrictions. Note that types accepted and returned by these macros are identical so they can be swapped around in macros--for example, ``libaout.h'` defines `GET_WORD` to either `bfd_get_32` or `bfd_get_64`.

In the put routines, val must be a `bfd_vma`. If we are on a system without prototypes, the caller is responsible for making sure that is true, with a cast if necessary. We don't cast them in the macro definitions because that would prevent `lint` or `gcc -Wall` from detecting sins such as passing a pointer. To detect calling these with less than a `bfd_vma`, use `gcc -Wconversion` on a host with 64 bit `bfd_vma`'s.

     /* Byte swapping macros for user section data.  */

    #define bfd_put_8(abfd, val, ptr) \
                    (*((unsigned char *)(ptr)) = (unsigned char)(val))
    #define bfd_put_signed_8 \
    		bfd_put_8
    #define bfd_get_8(abfd, ptr) \
                    (*(unsigned char *)(ptr))
    #define bfd_get_signed_8(abfd, ptr) \
    		((*(unsigned char *)(ptr) ^ 0x80) - 0x80)

    #define bfd_put_16(abfd, val, ptr) \
                    BFD_SEND(abfd, bfd_putx16, ((val),(ptr)))
    #define bfd_put_signed_16 \
    		 bfd_put_16
    #define bfd_get_16(abfd, ptr) \
                    BFD_SEND(abfd, bfd_getx16, (ptr))
    #define bfd_get_signed_16(abfd, ptr) \
             	 BFD_SEND (abfd, bfd_getx_signed_16, (ptr))

    #define bfd_put_32(abfd, val, ptr) \
                    BFD_SEND(abfd, bfd_putx32, ((val),(ptr)))
    #define bfd_put_signed_32 \
    		 bfd_put_32
    #define bfd_get_32(abfd, ptr) \
                    BFD_SEND(abfd, bfd_getx32, (ptr))
    #define bfd_get_signed_32(abfd, ptr) \
    		 BFD_SEND(abfd, bfd_getx_signed_32, (ptr))

    #define bfd_put_64(abfd, val, ptr) \
                    BFD_SEND(abfd, bfd_putx64, ((val), (ptr)))
    #define bfd_put_signed_64 \
    		 bfd_put_64
    #define bfd_get_64(abfd, ptr) \
                    BFD_SEND(abfd, bfd_getx64, (ptr))
    #define bfd_get_signed_64(abfd, ptr) \
    		 BFD_SEND(abfd, bfd_getx_signed_64, (ptr))

#### `bfd_h_put_size`

Description
These macros have the same function as their `bfd_get_x` bretheren, except that they are used for removing information for the header records of object files. Believe it or not, some object files keep their header records in big endian order and their data in little endian order.

     /* Byte swapping macros for file header data.  */

    #define bfd_h_put_8(abfd, val, ptr) \
    		bfd_put_8 (abfd, val, ptr)
    #define bfd_h_put_signed_8(abfd, val, ptr) \
    		bfd_put_8 (abfd, val, ptr)
    #define bfd_h_get_8(abfd, ptr) \
    		bfd_get_8 (abfd, ptr)
    #define bfd_h_get_signed_8(abfd, ptr) \
    		bfd_get_signed_8 (abfd, ptr)

    #define bfd_h_put_16(abfd, val, ptr) \
                    BFD_SEND(abfd, bfd_h_putx16,(val,ptr))
    #define bfd_h_put_signed_16 \
    		 bfd_h_put_16
    #define bfd_h_get_16(abfd, ptr) \
                    BFD_SEND(abfd, bfd_h_getx16,(ptr))
    #define bfd_h_get_signed_16(abfd, ptr) \
    		 BFD_SEND(abfd, bfd_h_getx_signed_16, (ptr))

    #define bfd_h_put_32(abfd, val, ptr) \
                    BFD_SEND(abfd, bfd_h_putx32,(val,ptr))
    #define bfd_h_put_signed_32 \
    		 bfd_h_put_32
    #define bfd_h_get_32(abfd, ptr) \
                    BFD_SEND(abfd, bfd_h_getx32,(ptr))
    #define bfd_h_get_signed_32(abfd, ptr) \
    		 BFD_SEND(abfd, bfd_h_getx_signed_32, (ptr))

    #define bfd_h_put_64(abfd, val, ptr) \
                    BFD_SEND(abfd, bfd_h_putx64,(val, ptr))
    #define bfd_h_put_signed_64 \
    		 bfd_h_put_64
    #define bfd_h_get_64(abfd, ptr) \
                    BFD_SEND(abfd, bfd_h_getx64,(ptr))
    #define bfd_h_get_signed_64(abfd, ptr) \
    		 BFD_SEND(abfd, bfd_h_getx_signed_64, (ptr))

#### `bfd_log2`

Synopsis

    unsigned int bfd_log2(bfd_vma x);

Description
Return the log base 2 of the value supplied, rounded up. E.g., an x of 1025 returns 11.

## File caching

The file caching mechanism is embedded within BFD and allows the application to open as many BFDs as it wants without regard to the underlying operating system's file descriptor limit (often as low as 20 open files). The module in `cache.c` maintains a least recently used list of `BFD_CACHE_MAX_OPEN` files, and exports the name `bfd_cache_lookup`, which runs around and makes sure that the required BFD is open. If not, then it chooses a file to close, closes it and opens the one wanted, returning its file handle.

#### `BFD_CACHE_MAX_OPEN macro`

Description
The maximum number of files which the cache will keep open at one time.

    #define BFD_CACHE_MAX_OPEN 10

#### `bfd_last_cache`

Synopsis

    extern bfd *bfd_last_cache;

Description
Zero, or a pointer to the topmost BFD on the chain. This is used by the `bfd_cache_lookup` macro in ``libbfd.h'` to determine when it can avoid a function call.

#### `bfd_cache_lookup`

Description
Check to see if the required BFD is the same as the last one looked up. If so, then it can use the stream in the BFD with impunity, since it can't have changed since the last lookup; otherwise, it has to perform the complicated lookup function.

    #define bfd_cache_lookup(x) \
        ((x)==bfd_last_cache? \
          (FILE*)(bfd_last_cache->iostream): \
           bfd_cache_lookup_worker(x))

#### `bfd_cache_init`

Synopsis

    boolean bfd_cache_init (bfd *abfd);

Description
Add a newly opened BFD to the cache.

#### `bfd_cache_close`

Synopsis

    boolean bfd_cache_close (bfd *abfd);

Description
Remove the BFD abfd from the cache. If the attached file is open, then close it too.
Returns
`false` is returned if closing the file fails, `true` is returned if all is well.

#### `bfd_open_file`

Synopsis

    FILE* bfd_open_file(bfd *abfd);

Description
Call the OS to open a file for abfd. Return the `FILE *` (possibly `NULL`) that results from this operation. Set up the BFD so that future accesses know the file is open. If the `FILE *` returned is `NULL`, then it won't have been put in the cache, so it won't have to be removed from it.

#### `bfd_cache_lookup_worker`

Synopsis

    FILE *bfd_cache_lookup_worker(bfd *abfd);

Description
Called when the macro `bfd_cache_lookup` fails to find a quick answer. Find a file descriptor for abfd. If necessary, it open it. If there are already more than `BFD_CACHE_MAX_OPEN` files open, it tries to close one first, to avoid running out of file descriptors.

## Linker Functions

The linker uses three special entry points in the BFD target vector. It is not necessary to write special routines for these entry points when creating a new BFD back end, since generic versions are provided. However, writing them can speed up linking and make it use significantly less runtime memory.

The first routine creates a hash table used by the other routines. The second routine adds the symbols from an object file to the hash table. The third routine takes all the object files and links them together to create the output file. These routines are designed so that the linker proper does not need to know anything about the symbols in the object files that it is linking. The linker merely arranges the sections as directed by the linker script and lets BFD handle the details of symbols and relocs.

The second routine and third routines are passed a pointer to a `struct bfd_link_info` structure (defined in `bfdlink.h`) which holds information relevant to the link, including the linker hash table (which was created by the first routine) and a set of callback functions to the linker proper.

The generic linker routines are in `linker.c`, and use the header file `genlink.h`. As of this writing, the only back ends which have implemented versions of these routines are a.out (in `aoutx.h`) and ECOFF (in `ecoff.c`). The a.out routines are used as examples throughout this section.

- Creating a Linker Hash Table
- Adding Symbols to the Hash Table
- Performing the Final Link

### Creating a linker hash table

The linker routines must create a hash table, which must be derived from `struct bfd_link_hash_table` described in `bfdlink.c`. See section Hash Tables for information on how to create a derived hash table. This entry point is called using the target vector of the linker output file.

The `_bfd_link_hash_table_create` entry point must allocate and initialize an instance of the desired hash table. If the back end does not require any additional information to be stored with the entries in the hash table, the entry point may simply create a `struct bfd_link_hash_table`. Most likely, however, some additional information will be needed.

For example, with each entry in the hash table the a.out linker keeps the index the symbol has in the final output file (this index number is used so that when doing a relocateable link the symbol index used in the output file can be quickly filled in when copying over a reloc). The a.out linker code defines the required structures and functions for a hash table derived from `struct bfd_link_hash_table`. The a.out linker hash table is created by the function `NAME(aout,link_hash_table_create)`; it simply allocates space for the hash table, initializes it, and returns a pointer to it.

When writing the linker routines for a new back end, you will generally not know exactly which fields will be required until you have finished. You should simply create a new hash table which defines no additional fields, and then simply add fields as they become necessary.

### Adding symbols to the hash table

The linker proper will call the `_bfd_link_add_symbols` entry point for each object file or archive which is to be linked (typically these are the files named on the command line, but some may also come from the linker script). The entry point is responsible for examining the file. For an object file, BFD must add any relevant symbol information to the hash table. For an archive, BFD must determine which elements of the archive should be used and adding them to the link.

The a.out version of this entry point is `NAME(aout,link_add_symbols)`.

- Differing file formats
- Adding symbols from an object file
- Adding symbols from an archive

#### Differing file formats

Normally all the files involved in a link will be of the same format, but it is also possible to link together different format object files, and the back end must support that. The `_bfd_link_add_symbols` entry point is called via the target vector of the file to be added. This has an important consequence: the function may not assume that the hash table is the type created by the corresponding `_bfd_link_hash_table_create` vector. All the `_bfd_link_add_symbols` function can assume about the hash table is that it is derived from `struct bfd_link_hash_table`.

Sometimes the `_bfd_link_add_symbols` function must store some information in the hash table entry to be used by the `_bfd_final_link` function. In such a case the `creator` field of the hash table must be checked to make sure that the hash table was created by an object file of the same format.

The `_bfd_final_link` routine must be prepared to handle a hash entry without any extra information added by the `_bfd_link_add_symbols` function. A hash entry without extra information will also occur when the linker script directs the linker to create a symbol. Note that, regardless of how a hash table entry is added, all the fields will be initialized to some sort of null value by the hash table entry initialization function.

See `ecoff_link_add_externals` for an example of how to check the `creator` field before saving information (in this case, the ECOFF external symbol debugging information) in a hash table entry.

#### Adding symbols from an object file

When the `_bfd_link_add_symbols` routine is passed an object file, it must add all externally visible symbols in that object file to the hash table. The actual work of adding the symbol to the hash table is normally handled by the function `_bfd_generic_link_add_one_symbol`. The `_bfd_link_add_symbols` routine is responsible for reading all the symbols from the object file and passing the correct information to `_bfd_generic_link_add_one_symbol`.

The `_bfd_link_add_symbols` routine should not use `bfd_canonicalize_symtab` to read the symbols. The point of providing this routine is to avoid the overhead of converting the symbols into generic `asymbol` structures.

`_bfd_generic_link_add_one_symbol` handles the details of combining common symbols, warning about multiple definitions, and so forth. It takes arguments which describe the symbol to add, notably symbol flags, a section, and an offset. The symbol flags include such things as `BSF_WEAK` or `BSF_INDIRECT`. The section is a section in the object file, or something like `bfd_und_section_ptr` for an undefined symbol or `bfd_com_section_ptr` for a common symbol.

If the `_bfd_final_link` routine is also going to need to read the symbol information, the `_bfd_link_add_symbols` routine should save it somewhere attached to the object file BFD. However, the information should only be saved if the `keep_memory` field of the `info` argument is true, so that the `-no-keep-memory` linker switch is effective.

The a.out function which adds symbols from an object file is `aout_link_add_object_symbols`, and most of the interesting work is in `aout_link_add_symbols`. The latter saves pointers to the hash tables entries created by `_bfd_generic_link_add_one_symbol` indexed by symbol number, so that the `_bfd_final_link` routine does not have to call the hash table lookup routine to locate the entry.

#### Adding symbols from an archive

When the `_bfd_link_add_symbols` routine is passed an archive, it must look through the symbols defined by the archive and decide which elements of the archive should be included in the link. For each such element it must call the `add_archive_element` linker callback, and it must add the symbols from the object file to the linker hash table.

In most cases the work of looking through the symbols in the archive should be done by the `_bfd_generic_link_add_archive_symbols` function. This function builds a hash table from the archive symbol table and looks through the list of undefined symbols to see which elements should be included. `_bfd_generic_link_add_archive_symbols` is passed a function to call to make the final decision about adding an archive element to the link and to do the actual work of adding the symbols to the linker hash table.

The function passed to `_bfd_generic_link_add_archive_symbols` must read the symbols of the archive element and decide whether the archive element should be included in the link. If the element is to be included, the `add_archive_element` linker callback routine must be called with the element as an argument, and the elements symbols must be added to the linker hash table just as though the element had itself been passed to the `_bfd_link_add_symbols` function.

When the a.out `_bfd_link_add_symbols` function receives an archive, it calls `_bfd_generic_link_add_archive_symbols` passing `aout_link_check_archive_element` as the function argument. `aout_link_check_archive_element` calls `aout_link_check_ar_symbols`. If the latter decides to add the element (an element is only added if it provides a real, non-common, definition for a previously undefined or common symbol) it calls the `add_archive_element` callback and then `aout_link_check_archive_element` calls `aout_link_add_symbols` to actually add the symbols to the linker hash table.

The ECOFF back end is unusual in that it does not normally call `_bfd_generic_link_add_archive_symbols`, because ECOFF archives already contain a hash table of symbols. The ECOFF back end searches the archive itself to avoid the overhead of creating a new hash table.

### Performing the final link

When all the input files have been processed, the linker calls the `_bfd_final_link` entry point of the output BFD. This routine is responsible for producing the final output file, which has several aspects. It must relocate the contents of the input sections and copy the data into the output sections. It must build an output symbol table including any local symbols from the input files and the global symbols from the hash table. When producing relocateable output, it must modify the input relocs and write them into the output file. There may also be object format dependent work to be done.

The linker will also call the `write_object_contents` entry point when the BFD is closed. The two entry points must work together in order to produce the correct output file.

The details of how this works are inevitably dependent upon the specific object file format. The a.out `_bfd_final_link` routine is `NAME(aout,final_link)`.

- Information provided by the linker
- Relocating the section contents
- Writing the symbol table

#### Information provided by the linker

Before the linker calls the `_bfd_final_link` entry point, it sets up some data structures for the function to use.

The `input_bfds` field of the `bfd_link_info` structure will point to a list of all the input files included in the link. These files are linked through the `link_next` field of the `bfd` structure.

Each section in the output file will have a list of `link_order` structures attached to the `link_order_head` field (the `link_order` structure is defined in `bfdlink.h`). These structures describe how to create the contents of the output section in terms of the contents of various input sections, fill constants, and, eventually, other types of information. They also describe relocs that must be created by the BFD backend, but do not correspond to any input file; this is used to support -Ur, which builds constructors while generating a relocateable object file.

#### Relocating the section contents

The `_bfd_final_link` function should look through the `link_order` structures attached to each section of the output file. Each `link_order` structure should either be handled specially, or it should be passed to the function `_bfd_default_link_order` which will do the right thing (`_bfd_default_link_order` is defined in `linker.c`).

For efficiency, a `link_order` of type `bfd_indirect_link_order` whose associated section belongs to a BFD of the same format as the output BFD must be handled specially. This type of `link_order` describes part of an output section in terms of a section belonging to one of the input files. The `_bfd_final_link` function should read the contents of the section and any associated relocs, apply the relocs to the section contents, and write out the modified section contents. If performing a relocateable link, the relocs themselves must also be modified and written out.

The functions `_bfd_relocate_contents` and `_bfd_final_link_relocate` provide some general support for performing the actual relocations, notably overflow checking. Their arguments include information about the symbol the relocation is against and a `reloc_howto_type` argument which describes the relocation to perform. These functions are defined in `reloc.c`.

The a.out function which handles reading, relocating, and writing section contents is `aout_link_input_section`. The actual relocation is done in `aout_link_input_section_std` and `aout_link_input_section_ext`.

#### Writing the symbol table

The `_bfd_final_link` function must gather all the symbols in the input files and write them out. It must also write out all the symbols in the global hash table. This must be controlled by the `strip` and `discard` fields of the `bfd_link_info` structure.

The local symbols of the input files will not have been entered into the linker hash table. The `_bfd_final_link` routine must consider each input file and include the symbols in the output file. It may be convenient to do this when looking through the `link_order` structures, or it may be done by stepping through the `input_bfds` list.

The `_bfd_final_link` routine must also traverse the global hash table to gather all the externally visible symbols. It is possible that most of the externally visible symbols may be written out when considering the symbols of each input file, but it is still necessary to traverse the hash table since the linker script may have defined some symbols that are not in any of the input files.

The `strip` field of the `bfd_link_info` structure controls which symbols are written out. The possible values are listed in `bfdlink.h`. If the value is `strip_some`, then the `keep_hash` field of the `bfd_link_info` structure is a hash table of symbols to keep; each symbol should be looked up in this hash table, and only symbols which are present should be included in the output file.

If the `strip` field of the `bfd_link_info` structure permits local symbols to be written out, the `discard` field is used to further controls which local symbols are included in the output file. If the value is `discard_l`, then all local symbols which begin with a certain prefix are discarded; this is controlled by the `bfd_is_local_label_name` entry point.

The a.out backend handles symbols by calling `aout_link_write_symbols` on each input BFD and then traversing the global hash table with the function `aout_link_write_other_symbol`. It builds a string table while writing out the symbols, which is written to the output file at the end of `NAME(aout,final_link)`.

#### `bfd_link_split_section`

Synopsis

    boolean bfd_link_split_section(bfd *abfd, asection *sec);

Description
Return nonzero if sec should be split during a reloceatable or final link.

    #define bfd_link_split_section(abfd, sec) \
           BFD_SEND (abfd, _bfd_link_split_section, (abfd, sec))

## Hash Tables

BFD provides a simple set of hash table functions. Routines are provided to initialize a hash table, to free a hash table, to look up a string in a hash table and optionally create an entry for it, and to traverse a hash table. There is currently no routine to delete an string from a hash table.

The basic hash table does not permit any data to be stored with a string. However, a hash table is designed to present a base class from which other types of hash tables may be derived. These derived types may store additional information with the string. Hash tables were implemented in this way, rather than simply providing a data pointer in a hash table entry, because they were designed for use by the linker back ends. The linker may create thousands of hash table entries, and the overhead of allocating private data and storing and following pointers becomes noticeable.

The basic hash table code is in `hash.c`.

- Creating and Freeing a Hash Table
- Looking Up or Entering a String
- Traversing a Hash Table
- Deriving a New Hash Table Type

### Creating and freeing a hash table

To create a hash table, create an instance of a `struct bfd_hash_table` (defined in `bfd.h`) and call `bfd_hash_table_init` (if you know approximately how many entries you will need, the function `bfd_hash_table_init_n`, which takes a size argument, may be used). `bfd_hash_table_init` returns `false` if some sort of error occurs.

The function `bfd_hash_table_init` take as an argument a function to use to create new entries. For a basic hash table, use the function `bfd_hash_newfunc`. See section Deriving a new hash table type for why you would want to use a different value for this argument.

`bfd_hash_table_init` will create an objalloc which will be used to allocate new entries. You may allocate memory on this objalloc using `bfd_hash_allocate`.

Use `bfd_hash_table_free` to free up all the memory that has been allocated for a hash table. This will not free up the `struct bfd_hash_table` itself, which you must provide.

### Looking up or entering a string

The function `bfd_hash_lookup` is used both to look up a string in the hash table and to create a new entry.

If the create argument is `false`, `bfd_hash_lookup` will look up a string. If the string is found, it will returns a pointer to a `struct bfd_hash_entry`. If the string is not found in the table `bfd_hash_lookup` will return `NULL`. You should not modify any of the fields in the returns `struct bfd_hash_entry`.

If the create argument is `true`, the string will be entered into the hash table if it is not already there. Either way a pointer to a `struct bfd_hash_entry` will be returned, either to the existing structure or to a newly created one. In this case, a `NULL` return means that an error occurred.

If the create argument is `true`, and a new entry is created, the copy argument is used to decide whether to copy the string onto the hash table objalloc or not. If copy is passed as `false`, you must be careful not to deallocate or modify the string as long as the hash table exists.

### Traversing a hash table

The function `bfd_hash_traverse` may be used to traverse a hash table, calling a function on each element. The traversal is done in a random order.

`bfd_hash_traverse` takes as arguments a function and a generic `void *` pointer. The function is called with a hash table entry (a `struct bfd_hash_entry *`) and the generic pointer passed to `bfd_hash_traverse`. The function must return a `boolean` value, which indicates whether to continue traversing the hash table. If the function returns `false`, `bfd_hash_traverse` will stop the traversal and return immediately.

### Deriving a new hash table type

Many uses of hash tables want to store additional information which each entry in the hash table. Some also find it convenient to store additional information with the hash table itself. This may be done using a derived hash table.

Since C is not an object oriented language, creating a derived hash table requires sticking together some boilerplate routines with a few differences specific to the type of hash table you want to create.

An example of a derived hash table is the linker hash table. The structures for this are defined in `bfdlink.h`. The functions are in `linker.c`.

You may also derive a hash table from an already derived hash table. For example, the a.out linker backend code uses a hash table derived from the linker hash table.

- Define the Derived Structures
- Write the Derived Creation Routine
- Write Other Derived Routines

#### Define the derived structures

You must define a structure for an entry in the hash table, and a structure for the hash table itself.

The first field in the structure for an entry in the hash table must be of the type used for an entry in the hash table you are deriving from. If you are deriving from a basic hash table this is `struct bfd_hash_entry`, which is defined in `bfd.h`. The first field in the structure for the hash table itself must be of the type of the hash table you are deriving from itself. If you are deriving from a basic hash table, this is `struct bfd_hash_table`.

For example, the linker hash table defines `struct bfd_link_hash_entry` (in `bfdlink.h`). The first field, `root`, is of type `struct bfd_hash_entry`. Similarly, the first field in `struct bfd_link_hash_table`, `table`, is of type `struct bfd_hash_table`.

#### Write the derived creation routine

You must write a routine which will create and initialize an entry in the hash table. This routine is passed as the function argument to `bfd_hash_table_init`.

In order to permit other hash tables to be derived from the hash table you are creating, this routine must be written in a standard way.

The first argument to the creation routine is a pointer to a hash table entry. This may be `NULL`, in which case the routine should allocate the right amount of space. Otherwise the space has already been allocated by a hash table type derived from this one.

After allocating space, the creation routine must call the creation routine of the hash table type it is derived from, passing in a pointer to the space it just allocated. This will initialize any fields used by the base hash table.

Finally the creation routine must initialize any local fields for the new hash table type.

Here is a boilerplate example of a creation routine. function_name is the name of the routine. entry_type is the type of an entry in the hash table you are creating. base_newfunc is the name of the creation routine of the hash table type your hash table is derived from.
.struct bfd_hash_entry *

    function_name (entry, table, string)
         struct bfd_hash_entry *entry;
         struct bfd_hash_table *table;
         const char *string;
    {
      struct entry_type *ret = (entry_type *) entry;

     /* Allocate the structure if it has not already been allocated by a
        derived class.  */
      if (ret == (entry_type *) NULL)
        {
          ret = ((entry_type *)
    	      bfd_hash_allocate (table, sizeof (entry_type)));
          if (ret == (entry_type *) NULL)
            return NULL;
        }

     /* Call the allocation method of the base class.  */
      ret = ((entry_type *)
    	 base_newfunc ((struct bfd_hash_entry *) ret, table, string));

     /* Initialize the local fields here.  */

      return (struct bfd_hash_entry *) ret;
    }

Description
The creation routine for the linker hash table, which is in `linker.c`, looks just like this example. function_name is `_bfd_link_hash_newfunc`. entry_type is `struct bfd_link_hash_entry`. base_newfunc is `bfd_hash_newfunc`, the creation routine for a basic hash table.

`_bfd_link_hash_newfunc` also initializes the local fields in a linker hash table entry: `type`, `written` and `next`.

#### Write other derived routines

You will want to write other routines for your new hash table, as well.

You will want an initialization routine which calls the initialization routine of the hash table you are deriving from and initializes any other local fields. For the linker hash table, this is `_bfd_link_hash_table_init` in `linker.c`.

You will want a lookup routine which calls the lookup routine of the hash table you are deriving from and casts the result. The linker hash table uses `bfd_link_hash_lookup` in `linker.c` (this actually takes an additional argument which it uses to decide how to return the looked up value).

You may want a traversal routine. This should just call the traversal routine of the hash table you are deriving from with appropriate casts. The linker hash table uses `bfd_link_hash_traverse` in `linker.c`.

These routines may simply be defined as macros. For example, the a.out backend linker hash table, which is derived from the linker hash table, uses macros for the lookup and traversal routines. These are `aout_link_hash_lookup` and `aout_link_hash_traverse` in aoutx.h.

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