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  --------|

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;

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 these 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

The base address of the section in the address space of the target.

        bfd_vma vma;

The size of the section in bytes of the loaded section. This contains a value even if the section has no contents (eg, the size of .bss).

        bfd_size_type 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;

Which section is it 0..nth

        int index;

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

        unsigned 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;

     } asection ;

section prototypes
------------------

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

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

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

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

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 the old section by that name instead.

Possible errors are:

invalid_operation
     If output has already started for this BFD.

no_memory
     If obstack alloc fails.

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

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

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.

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

bfd_map_over_sections
.....................

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);

     void bfd_map_over_sections(bfd *abfd, void (*func)(), PTR 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
....................

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

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

bfd_set_section_contents
........................

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.

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

bfd_get_section_contents
........................

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. Possible errors are:

unknown yet

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