section.texi

gdb-6.0 linux 下的调试工具

@section 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 @code{section.c}.

@menu
* Section Input::
* Section Output::
* typedef asection::
* section prototypes::
@end menu

@node Section Input, Section Output, Sections, Sections
@subsection 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, @code{a.out} would contain at least
three sections, called @code{.text}, @code{.data} and @code{.bss}.

Names need not be unique; for example a COFF file may have several
sections named @code{.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
@code{bfd_make_section}) to the sections attached to an already open
BFD. For example, the linker creates an extra section
@code{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 @code{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.

@node Section Output, typedef asection, Section Input, Sections
@subsection 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 @code{bfd_set_section_contents}.

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

The data to be written comes from input sections attached
(via @code{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 @code{asection}
structures would look like:

@example
   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  --------|
@end example

@subsection Link orders
The data within a section is stored in a @dfn{link_order}.
These are much like the fixups in @code{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.


@node typedef asection, section prototypes, Section Output, Sections
@subsection typedef asection
Here is the section structure:


@example

/* This structure is used for a comdat section, as in PE.  A comdat
   section is associated with a particular symbol.  When the linker
   sees a comdat section, it keeps only one of the sections with a
   given name and associated with a given symbol.  */

struct bfd_comdat_info
@{
  /* The name of the symbol associated with a comdat section.  */
  const char *name;

  /* The local symbol table index of the symbol associated with a
     comdat section.  This is only meaningful to the object file format
     specific code; it is not an index into the list returned by
     bfd_canonicalize_symtab.  */
  long symbol;
@};

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;

  /* A unique sequence number.  */
  int id;

  /* Which section in the bfd; 0..n-1 as sections are created in a bfd.  */
  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

  /* ELF reserves 4 processor specific bits and 8 operating system
     specific bits in sh_flags; at present we can get away with just
     one in communicating between the assembler and BFD, but this
     isn't a good long-term solution.  */
#define SEC_ARCH_BIT_0 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 @code{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., @code{__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 @code{__CTOR_LIST__} and relocate the data
     contained within - exactly the operations it would peform on
     standard data.  */
#define SEC_CONSTRUCTOR 0x100

  /* The section has contents - a data section could be
     @code{SEC_ALLOC} | @code{SEC_HAS_CONTENTS}; a debug section could be
     @code{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 thread local data.  */
#define SEC_THREAD_LOCAL 0x1000

  /* The section has GOT references.  This flag is only for the
     linker, and is currently only used by the elf32-hppa back end.
     It will be set if global offset table references were detected
     in this section, which indicate to the linker that the section
     contains PIC code, and must be handled specially when doing a
     static link.  */
#define SEC_HAS_GOT_REF 0x4000

  /* 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 based on the sum of
     the symbol and addend values specified by the associated relocation
     entries.  Entries without associated relocation entries will be
     appended to the end of the section in an unspecified order.  */
#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

  /* This section should not be subject to garbage collection.  */
#define SEC_KEEP 0x1000000

  /* This section contains "short" data, and should be placed
     "near" the GP.  */
#define SEC_SMALL_DATA 0x2000000

  /* This section contains data which may be shared with other
     executables or shared objects.  */
#define SEC_SHARED 0x4000000

  /* When a section with this flag is being linked, then if the size of
     the input section is less than a page, it should not cross a page
     boundary.  If the size of the input section is one page or more, it
     should be aligned on a page boundary.  */
#define SEC_BLOCK 0x8000000

  /* Conditionally link this section; do not link if there are no
     references found to any symbol in the section.  */
#define SEC_CLINK 0x10000000

  /* Attempt to merge identical entities in the section.
     Entity size is given in the entsize field.  */
#define SEC_MERGE 0x20000000

  /* If given with SEC_MERGE, entities to merge are zero terminated
     strings where entsize specifies character size instead of fixed
     size entries.  */
#define SEC_STRINGS 0x40000000

  /* This section contains data about section groups.  */
#define SEC_GROUP 0x80000000

  /*  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;

  /* Another mark flag used by some of the linker backends.  Set for
     output sections that have an input section.  */
  unsigned int linker_has_input : 1;

  /* A mark flag used by some linker backends for garbage collection.  */
  unsigned int gc_mark : 1;

  /* The following flags are used by the ELF linker. */

  /* Mark sections which have been allocated to segments.  */
  unsigned int segment_mark : 1;

  /* Type of sec_info information.  */
  unsigned int sec_info_type:3;
#define ELF_INFO_TYPE_NONE      0
#define ELF_INFO_TYPE_STABS     1
#define ELF_INFO_TYPE_MERGE     2
#define ELF_INFO_TYPE_EH_FRAME  3
#define ELF_INFO_TYPE_JUST_SYMS 4

  /* Nonzero if this section uses RELA relocations, rather than REL.  */
  unsigned int use_rela_p:1;

  /* Bits used by various backends.  */
  unsigned int has_tls_reloc:1;

  /* Nonzero if this section needs the relax finalize pass.  */
  unsigned int need_finalize_relax:1;

  /* Nonzero if this section has a gp reloc.  */
  unsigned int has_gp_reloc:1;

  /* Usused bits.  */
  unsigned int flag13:1;
  unsigned int flag14:1;
  unsigned int flag15:1;
  unsigned int flag16:4;
  unsigned int flag20:4;
  unsigned int flag24:8;

  /* 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 @code{a.out}, where
      the default address for @code{.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 octets, as it will be output.
     Contains a value even if the section has no contents (e.g., the
     size of @code{.bss}).  This will be filled in after relocation.  */
  bfd_size_type _cooked_size;

  /* The original size on disk of the section, in octets.  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 in *bytes* into the output section of the first byte in the
     input section (byte ==> smallest addressable unit on the
     target).  In most cases, if this was going to start at the
     100th octet (8-bit quantity) in the output section, this value
     would be 100.  However, if the target byte size is 16 bits
     (bfd_octets_per_byte is "2"), this value would be 50.  */
  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;