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
-------------

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 asections whose name matches NAME, otherwise NULL. See 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))