source: trunk/binutils/bfd/doc/bfd.info-1@ 3232

This is bfd.info, produced by makeinfo version 4.3 from bfd.texinfo.

START-INFO-DIR-ENTRY
* Bfd: (bfd).                   The Binary File Descriptor library.
END-INFO-DIR-ENTRY

   This file documents the BFD library.

   Copyright (C) 1991, 2000, 2001, 2003 Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
     under the terms of the GNU Free Documentation License, Version 1.1
     or any later version published by the Free Software Foundation;
   with no Invariant Sections, with no Front-Cover Texts, and with no
    Back-Cover Texts.  A copy of the license is included in the
section entitled "GNU Free Documentation License".


File: bfd.info,  Node: Top,  Next: Overview,  Prev: (dir),  Up: (dir)

   This file documents the binary file descriptor library libbfd.

* Menu:

* Overview::                    Overview of BFD
* BFD front end::               BFD front end
* BFD back ends::               BFD back ends
* GNU Free Documentation License::  GNU Free Documentation License
* Index::                       Index


File: bfd.info,  Node: Overview,  Next: BFD front end,  Prev: Top,  Up: Top

Introduction
************

   BFD is a package which allows applications to use the same routines
to operate on object files whatever the object file format.  A new
object file format can be supported simply by creating a new BFD back
end and adding it to the library.

   BFD is split into two parts: the front end, and the back ends (one
for each object file format).
   * The front end of BFD provides the interface to the user. It manages
     memory and various canonical data structures. The front end also
     decides which back end to use and when to call back end routines.

   * The back ends provide BFD its view of the real world. Each back
     end provides a set of calls which the BFD front end can use to
     maintain its canonical form. The back ends also may keep around
     information for their own use, for greater efficiency.

* Menu:

* History::                     History
* How It Works::                How It Works
* What BFD Version 2 Can Do::   What BFD Version 2 Can Do


File: bfd.info,  Node: History,  Next: How It Works,  Prev: Overview,  Up: Overview

History
=======

   One spur behind BFD was the desire, on the part of the GNU 960 team
at Intel Oregon, for interoperability of applications on their COFF and
b.out file formats.  Cygnus was providing GNU support for the team, and
was contracted to provide the required functionality.

   The name came from a conversation David Wallace was having with
Richard Stallman about the library: RMS said that it would be quite
hard--David said "BFD".  Stallman was right, but the name stuck.

   At the same time, Ready Systems wanted much the same thing, but for
different object file formats: IEEE-695, Oasys, Srecords, a.out and 68k
coff.

   BFD was first implemented by members of Cygnus Support; Steve
Chamberlain (`sac@cygnus.com'), John Gilmore (`gnu@cygnus.com'), K.
Richard Pixley (`rich@cygnus.com') and David Henkel-Wallace
(`gumby@cygnus.com').


File: bfd.info,  Node: How It Works,  Next: What BFD Version 2 Can Do,  Prev: History,  Up: Overview

How To Use BFD
==============

   To use the library, include `bfd.h' and link with `libbfd.a'.

   BFD provides a common interface to the parts of an object file for a
calling application.

   When an application sucessfully opens a target file (object,
archive, or whatever), a pointer to an internal structure is returned.
This pointer points to a structure called `bfd', described in `bfd.h'.
Our convention is to call this pointer a BFD, and instances of it
within code `abfd'.  All operations on the target object file are
applied as methods to the BFD.  The mapping is defined within `bfd.h'
in a set of macros, all beginning with `bfd_' to reduce namespace
pollution.

   For example, this sequence does what you would probably expect:
return the number of sections in an object file attached to a BFD
`abfd'.

     #include "bfd.h"
     
     unsigned int number_of_sections (abfd)
     bfd *abfd;
     {
       return bfd_count_sections (abfd);
     }

   The abstraction used within BFD is that an object file has:

   * a header,

   * a number of sections containing raw data (*note Sections::),

   * a set of relocations (*note Relocations::), and

   * some symbol information (*note Symbols::).

Also, BFDs opened for archives have the additional attribute of an index
and contain subordinate BFDs. This approach is fine for a.out and coff,
but loses efficiency when applied to formats such as S-records and
IEEE-695.


File: bfd.info,  Node: What BFD Version 2 Can Do,  Prev: How It Works,  Up: Overview

What BFD Version 2 Can Do
=========================

   When an object file is opened, BFD subroutines automatically
determine the format of the input object file.  They then build a
descriptor in memory with pointers to routines that will be used to
access elements of the object file's data structures.

   As different information from the object files is required, BFD
reads from different sections of the file and processes them.  For
example, a very common operation for the linker is processing symbol
tables.  Each BFD back end provides a routine for converting between
the object file's representation of symbols and an internal canonical
format. When the linker asks for the symbol table of an object file, it
calls through a memory pointer to the routine from the relevant BFD
back end which reads and converts the table into a canonical form.  The
linker then operates upon the canonical form. When the link is finished
and the linker writes the output file's symbol table, another BFD back
end routine is called to take the newly created symbol table and
convert it into the chosen output format.

* Menu:

* BFD information loss::        Information Loss
* Canonical format::            The BFD canonical object-file format


File: bfd.info,  Node: BFD information loss,  Next: Canonical format,  Up: What BFD Version 2 Can Do

Information Loss
----------------

   _Information can be lost during output._ The output formats
supported by BFD do not provide identical facilities, and information
which can be described in one form has nowhere to go in another format.
One example of this is alignment information in `b.out'. There is
nowhere in an `a.out' format file to store alignment information on the
contained data, so when a file is linked from `b.out' and an `a.out'
image is produced, alignment information will not propagate to the
output file. (The linker will still use the alignment information
internally, so the link is performed correctly).

   Another example is COFF section names. COFF files may contain an
unlimited number of sections, each one with a textual section name. If
the target of the link is a format which does not have many sections
(e.g., `a.out') or has sections without names (e.g., the Oasys format),
the link cannot be done simply. You can circumvent this problem by
describing the desired input-to-output section mapping with the linker
command language.

   _Information can be lost during canonicalization._ The BFD internal
canonical form of the external formats is not exhaustive; there are
structures in input formats for which there is no direct representation
internally.  This means that the BFD back ends cannot maintain all
possible data richness through the transformation between external to
internal and back to external formats.

   This limitation is only a problem when an application reads one
format and writes another.  Each BFD back end is responsible for
maintaining as much data as possible, and the internal BFD canonical
form has structures which are opaque to the BFD core, and exported only
to the back ends. When a file is read in one format, the canonical form
is generated for BFD and the application. At the same time, the back
end saves away any information which may otherwise be lost. If the data
is then written back in the same format, the back end routine will be
able to use the canonical form provided by the BFD core as well as the
information it prepared earlier.  Since there is a great deal of
commonality between back ends, there is no information lost when
linking or copying big endian COFF to little endian COFF, or `a.out' to
`b.out'.  When a mixture of formats is linked, the information is only
lost from the files whose format differs from the destination.


File: bfd.info,  Node: Canonical format,  Prev: BFD information loss,  Up: What BFD Version 2 Can Do

The BFD canonical object-file format
------------------------------------

   The greatest potential for loss of information occurs when there is
the least overlap between the information provided by the source
format, that stored by the canonical format, and that needed by the
destination format. A brief description of the canonical form may help
you understand which kinds of data you can count on preserving across
conversions.

_files_
     Information stored on a per-file basis includes target machine
     architecture, particular implementation format type, a demand
     pageable bit, and a write protected bit.  Information like Unix
     magic numbers is not stored here--only the magic numbers' meaning,
     so a `ZMAGIC' file would have both the demand pageable bit and the
     write protected text bit set.  The byte order of the target is
     stored on a per-file basis, so that big- and little-endian object
     files may be used with one another.

_sections_
     Each section in the input file contains the name of the section,
     the section's original address in the object file, size and
     alignment information, various flags, and pointers into other BFD
     data structures.

_symbols_
     Each symbol contains a pointer to the information for the object
     file which originally defined it, its name, its value, and various
     flag bits.  When a BFD back end reads in a symbol table, it
     relocates all symbols to make them relative to the base of the
     section where they were defined.  Doing this ensures that each
     symbol points to its containing section.  Each symbol also has a
     varying amount of hidden private data for the BFD back end.  Since
     the symbol points to the original file, the private data format
     for that symbol is accessible.  `ld' can operate on a collection
     of symbols of wildly different formats without problems.

     Normal global and simple local symbols are maintained on output,
     so an output file (no matter its format) will retain symbols
     pointing to functions and to global, static, and common variables.
     Some symbol information is not worth retaining; in `a.out', type
     information is stored in the symbol table as long symbol names.
     This information would be useless to most COFF debuggers; the
     linker has command line switches to allow users to throw it away.

     There is one word of type information within the symbol, so if the
     format supports symbol type information within symbols (for
     example, COFF, IEEE, Oasys) and the type is simple enough to fit
     within one word (nearly everything but aggregates), the
     information will be preserved.

_relocation level_
     Each canonical BFD relocation record contains a pointer to the
     symbol to relocate to, the offset of the data to relocate, the
     section the data is in, and a pointer to a relocation type
     descriptor. Relocation is performed by passing messages through
     the relocation type descriptor and the symbol pointer. Therefore,
     relocations can be performed on output data using a relocation
     method that is only available in one of the input formats. For
     instance, Oasys provides a byte relocation format.  A relocation
     record requesting this relocation type would point indirectly to a
     routine to perform this, so the relocation may be performed on a
     byte being written to a 68k COFF file, even though 68k COFF has no
     such relocation type.

_line numbers_
     Object formats can contain, for debugging purposes, some form of
     mapping between symbols, source line numbers, and addresses in the
     output file.  These addresses have to be relocated along with the
     symbol information.  Each symbol with an associated list of line
     number records points to the first record of the list.  The head
     of a line number list consists of a pointer to the symbol, which
     allows finding out the address of the function whose line number
     is being described. The rest of the list is made up of pairs:
     offsets into the section and line numbers. Any format which can
     simply derive this information can pass it successfully between
     formats (COFF, IEEE and Oasys).


File: bfd.info,  Node: BFD front end,  Next: BFD back ends,  Prev: Overview,  Up: Top

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
     {
       /* A unique identifier of the BFD  */
       unsigned int id;
     
       /* 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?  */
       bfd_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.  */
       bfd_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...  */
       ufile_ptr where;
     
       /* ... and here: (``once'' means at least once).  */
       bfd_boolean opened_once;
     
       /* Set if we have a locally maintained mtime value, rather than
          getting it from the file each time.  */
       bfd_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 with which the BFD was opened.  */
       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.  */
       ufile_ptr origin;
     
       /* Remember when output has begun, to stop strange things
          from happening.  */
       bfd_boolean output_has_begun;
     
       /* A hash table for section names.  */
       struct bfd_hash_table section_htab;
     
       /* Pointer to linked list of sections.  */
       struct sec *sections;
     
       /* The place where we add to the section list.  */
       struct sec **section_tail;
     
       /* 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;
     
       /* Used for slurped dynamic symbol tables.  */
       unsigned int dynsymcount;
     
       /* 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.  */
       bfd_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 mmo_data_struct *mmo_data;
           struct sun_core_struct *sun_core_data;
           struct sco5_core_struct *sco5_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;
           struct mach_o_data_struct *mach_o_data;
           struct mach_o_fat_data_struct *mach_o_fat_data;
           struct bfd_pef_data_struct *pef_data;
           struct bfd_pef_xlib_data_struct *pef_xlib_data;
           struct bfd_sym_data_struct *sym_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_wrong_object_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.

`bfd_get_error_handler'
.......................

   *Synopsis*
     bfd_error_handler_type bfd_get_error_handler (void);
   *Description*
Return the BFD error handler function.

`bfd_archive_filename'
......................

   *Synopsis*
     const char *bfd_archive_filename (bfd *);
   *Description*
For a BFD that is a component of an archive, returns a string with both
the archive name and file name.  For other BFDs, just returns the file
name.

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*
     bfd_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_get_arch_size'
...................

   *Synopsis*
     int bfd_get_arch_size (bfd *abfd);
   *Description*
Returns the architecture address size, in bits, as determined by the
object file's format.  For ELF, this information is included in the
header.

   *Returns*
Returns the arch size in bits if known, `-1' otherwise.

`bfd_get_sign_extend_vma'
.........................

   *Synopsis*
     int bfd_get_sign_extend_vma (bfd *abfd);
   *Description*
Indicates if the target architecture "naturally" sign extends an
address.  Some architectures implicitly sign extend address values when
they are converted to types larger than the size of an address.  For
instance, bfd_get_start_address() will return an address sign extended
to fill a bfd_vma when this is the case.

   *Returns*
Returns `1' if the target architecture is known to sign extend
addresses, `0' if the target architecture is known to not sign extend
addresses, and `-1' otherwise.

`bfd_set_start_address'
.......................

   *Synopsis*
     bfd_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_gp_size'
.................

   *Synopsis*
     unsigned 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, unsigned 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.

   If the value would overflow, the maximum `bfd_vma' value is returned.

`bfd_copy_private_bfd_data'
...........................

   *Synopsis*
     bfd_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*
     bfd_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*
     bfd_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))

`Other functions'
.................

   *Description*
The following functions exist but have not yet been 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))
     
     #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_gc_sections(abfd, link_info) \
            BFD_SEND (abfd, _bfd_gc_sections, (abfd, link_info))
     
     #define bfd_merge_sections(abfd, link_info) \
            BFD_SEND (abfd, _bfd_merge_sections, (abfd, link_info))
     
     #define bfd_discard_group(abfd, sec) \
            BFD_SEND (abfd, _bfd_discard_group, (abfd, sec))
     
     #define bfd_link_hash_table_create(abfd) \
            BFD_SEND (abfd, _bfd_link_hash_table_create, (abfd))
     
     #define bfd_link_hash_table_free(abfd, hash) \
            BFD_SEND (abfd, _bfd_link_hash_table_free, (hash))
     
     #define bfd_link_add_symbols(abfd, info) \
            BFD_SEND (abfd, _bfd_link_add_symbols, (abfd, info))
     
     #define bfd_link_just_syms(sec, info) \
            BFD_SEND (abfd, _bfd_link_just_syms, (sec, 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 *,
                      bfd_boolean, asymbol **));

`bfd_alt_mach_code'
...................

   *Synopsis*
     bfd_boolean bfd_alt_mach_code (bfd *abfd, int alternative);
   *Description*
When more than one machine code number is available for the same
machine type, this function can be used to switch between the preferred
one (alternative == 0) and any others.  Currently, only ELF supports
this feature, with up to two alternate machine codes.

     struct bfd_preserve
     {
       PTR marker;
       PTR tdata;
       flagword flags;
       const struct bfd_arch_info *arch_info;
       struct sec *sections;
       struct sec **section_tail;
       unsigned int section_count;
       struct bfd_hash_table section_htab;
     };

`bfd_preserve_save'
...................

   *Synopsis*
     bfd_boolean bfd_preserve_save (bfd *, struct bfd_preserve *);
   *Description*
When testing an object for compatibility with a particular target
back-end, the back-end object_p function needs to set up certain fields
in the bfd on successfully recognizing the object.  This typically
happens in a piecemeal fashion, with failures possible at many points.
On failure, the bfd is supposed to be restored to its initial state,
which is virtually impossible.  However, restoring a subset of the bfd
state works in practice.  This function stores the subset and
reinitializes the bfd.

`bfd_preserve_restore'
......................

   *Synopsis*
     void bfd_preserve_restore (bfd *, struct bfd_preserve *);
   *Description*
This function restores bfd state saved by bfd_preserve_save.  If MARKER
is non-NULL in struct bfd_preserve then that block and all subsequently
bfd_alloc'd memory is freed.

`bfd_preserve_finish'
.....................

   *Synopsis*
     void bfd_preserve_finish (bfd *, struct bfd_preserve *);
   *Description*
This function should be called when the bfd state saved by
bfd_preserve_save is no longer needed.  ie. when the back-end object_p
function returns with success.

`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?".

* Menu:

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


File: bfd.info,  Node: Memory Usage,  Next: Initialization,  Prev: BFD front end,  Up: BFD front end

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.


File: bfd.info,  Node: Initialization,  Next: Sections,  Prev: Memory Usage,  Up: BFD front end

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.


File: bfd.info,  Node: Sections,  Next: Symbols,  Prev: Initialization,  Up: BFD front end

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'.

* Menu:

* Section Input::
* Section Output::
* typedef asection::
* section prototypes::


File: bfd.info,  Node: Section Input,  Next: Section Output,  Prev: Sections,  Up: Sections

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.


File: bfd.info,  Node: Section Output,  Next: typedef asection,  Prev: Section Input,  Up: Sections

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.