Sections
========

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

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. 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 a supernumary section COMMON for each input file's BFD to hold information about common storage. The raw data is not necessarily read in at the same time as 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. The linker uses the fields output_section and output_offset to create an output file. The data to be written comes from input sections attached 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. Eg to create a section "O", starting at 0x100, 0x123 long, containing two subsections, "A" at offset 0x0 (ie at vma 0x100) and "B" at offset 0x20 (ie at vma 0x120) the 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  --------|

Seglets
-------

The data within a section is stored in a seglet.  These are much like the fixups in gas.  The seglet abstraction allows the a section to grow and shrink within itself. A seglet knows how big it is, and which is the next seglet and where the raw data for it is, and also points to a list of relocations which apply to it. The seglet is used by the linker to perform relaxing on final code.  The application 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 seglet by seglet basis.

typedef asection
----------------

The shape of a section struct: .

     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 of
                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 loaded.
                This would 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 would be clear for a .bss section */

     #define SEC_LOAD       0x002
             	/* The section contains data still to be relocated, so there will
                be some relocation information too. */

     #define SEC_RELOC      0x004

             	/* Obsolete ? */

     #define SEC_BALIGN     0x008

             	/* 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 (eg <<__CTOR_LIST__>>), attaches
                the symbol to it and builds a relocation. To build the lists
                of constructors, all the linker has to to is catenate all the
                sections called <<__CTOR_LIST__>> and relocte 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 . */

     #define SEC_CONSTRUCTOR_TEXT 0x1100

     #define SEC_CONSTRUCTOR_DATA 0x2100

     #define SEC_CONSTRUCTOR_BSS  0x3100

             	/* The section has contents - a bss 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 not to output sections
               containing this flag even if they have information which
               would normally be written. */

     #define SEC_NEVER_LOAD 0x400

        bfd_vma vma;

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

        bfd_size_type _cooked_size;

             	/* The size on disk of the section in bytes originally.  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. Eg, 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 - eg
                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;

        struct lang_output_section *otheruserdata;

             	/* 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;

        boolean reloc_done;
     	 	/* A symbol which points at this section only */
        struct symbol_cache_entry *symbol;
        struct symbol_cache_entry **symbol_ptr_ptr;
        struct bfd_seclet_struct *seclets_head;
        struct bfd_seclet_struct *seclets_tail;
     } asection ;

     #define BFD_ABS_SECTION_NAME "*ABS*"
     #define BFD_UND_SECTION_NAME "*UND*"
     #define BFD_COM_SECTION_NAME "*COM*"

         	/* the absolute section */
      extern   asection bfd_abs_section;
         	/* Pointer to the undefined section */
      extern   asection bfd_und_section;
         	/* Pointer to the common section */
      extern asection bfd_com_section;

      extern struct symbol_cache_entry *bfd_abs_symbol;
      extern struct symbol_cache_entry *bfd_com_symbol;
      extern struct symbol_cache_entry *bfd_und_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 libbfd

bfd_get_section_by_name
.......................

Synopsis

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

Description

Runs through the provided ABFD and returns the asection who's name matches that provided, otherwise NULL. See Sections, for more information.

bfd_make_section_old_way
........................

Synopsis

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

Description

This function creates a new empty section called NAME and attaches it to the end of the chain of sections for the BFD supplied. 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 is was rewritten... Possible errors are:

  * invalid_operation If output has already started for this BFD.no_memory If obstack alloc fails.

bfd_make_section
................

Synopsis

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

Description

This function creates a new empty section called NAME and attaches it to the end of the chain of sections for the BFD supplied. An attempt to create a section with a name which is already in use, returns NULL without changing the section chain. Possible errors are:

  * invalid_operation - If output has already started for this BFD.no_memory - If obstack alloc fails.

bfd_set_section_flags
.....................

Synopsis

     boolean bfd_set_section_flags(bfd *, asection *, flagword);

Description

Attempts to set the attributes of the section named in the BFD supplied to the value. Returns true on success, false on error. Possible error returns are:

  * 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

Calls 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 *, asection *, bfd_size_type val);

Description

Sets SECTION to the size VAL. If the operation is ok, then true is returned, else false. Possible error returns:

  * 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:

  * 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

This function reads 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 constuctor with the SEC_CONSTUCTOR flag set are requested, then the LOCATION is filled with zeroes. If no errors occur, true is returned, else false.