binutils-gdb/bfd/bfd.texinfo

\input texinfo
@setfilename bfdinfo
@c $Id$
@synindex ky cp
@ifinfo
This file documents the BFD library.

Copyright (C) 1991 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

@ignore
Permission is granted to process this file through Tex and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, subject to the terms
of the GNU General Public License, which includes the provision that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions.
@end ifinfo
@iftex
@c@finalout
@c@setchapternewpage odd
@settitle LIB BFD, the Binary File Descriptor Library
@titlepage
@title{libbfd}
@subtitle{The Binary File Descriptor Library}
@sp 1
@subtitle First Edition---@code{bfd} version < 2.0
@subtitle April 1991
@author {Steve Chamberlain}
@author {Cygnus Support}
@page

@tex
\def\$#1${{#1}}  % Kluge: collect RCS revision info without $...$
\xdef\manvers{\$Revision$}  % For use in headers, footers too
{\parskip=0pt
\hfill Cygnus Support\par
\hfill steve\@cygnus.com\par
\hfill {\it BFD}, \manvers\par
\hfill \TeX{}info \texinfoversion\par
}
\global\parindent=0pt % Steve likes it this way
@end tex

@vskip 0pt plus 1filll
Copyright @copyright{} 1991 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, subject to the terms
of the GNU General Public License, which includes the provision that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions.
@end titlepage
@end iftex

@node Top, Overview, (dir), (dir)
@ifinfo
This file documents the binary file descriptor library libbfd.
@end ifinfo

@menu
* Overview::			Overview of bfd
* History::			History of bfd
* Backends::			Backends
* Porting::			Porting
* Future::			Future
* Index::			Index

BFD body:
* Memory usage::
* Sections::
* Symbols::
* Archives::
* Formats::
* Relocations::
* Core Files::
* Targets::
* Architecturs::
* Opening and Closing::
* Internal::
* File Caching::

Bfd backends:
* a.out backends::
* coff backends::
@end menu

@node Overview, History, Top, Top
@chapter Introduction
@cindex BFD
@cindex what is it?
Simply put, @code{bfd} is a package which allow applications to use the
same routines to operate on object files whatever the object file
format.  A different 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 many back ends.
@itemize @bullet
@item 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.
@item 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.
@end itemize
@node History, How It Works, Overview,Top
@section History

One spur behind @code{bfd} was the Intel Oregon's GNU 960 team desire for
interoperability of applications on their COFF and b.out file formats.
Cygnus was providing GNU support for the team, and Cygnus were
contracted to provid the required functionality.

The name came from a conversation Gumby Wallace was
having with Richard Stallman about the library, RMS said that it
would be quite hard, Gumby 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 Steve Chamberlain (steve@@cygnus.com),
John Gilmore (gnu@@cygnus.com),  K. Richard Pixley (rich@@cygnus.com) and
Gumby Wallace (gumby@@cygnus.com) at Cygnus Support in Palo Alto,
California.

@node How It Works, History, Porting, Top
@section How It Works

To use the library, include @code{bfd.h} and link with @code{libbfd.a}.	

@code{bfd} provides a common interface to the parts of an object file
to 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 structure described in
@code{include/bfd.h}, called @code{bfd}. Conventionally this pointer is
called a @code{bfd}, and instances of it within code are called
@code{abfd}. All operations on the target object file are applied as
methods to the @code{bfd}, the mapping is defined within @code{bfd.h} in
a set of macros, all beginning @code{bfd}_something. 

For example, this sequence would do what you expect:
@lisp
@cartouche
#include "bfd.h"

unsigned int number_of_sections(abfd)
bfd *abfd;
@{
  return bfd_count_sections(abfd);
@}
@end cartouche
@end lisp

The metaphor used within @code{bfd} is that an object file has a header,
a number of sections containing raw data, a set of relocations, and some
symbol information. Also, @code{bfd}s opened upon archives have the
additional attribute of an index and contained sub bfds. This approach is
find for a.out and coff, but looses efficiency when applied to formats
such as S-records and IEEE-695. 

@section What BFD Version 1 Can't Do
As different information from the 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 the memory pointer to the relevant BFD
back end routine which reads and converts the table into a canonical
form.  The linker then operates upon the common form. When the link is
finished and the linker writes the symbol table of the output file,
another BFD back end routine is called which takes the newly
created symbol table and converts it into the chosen output format.

@node BFD information loss, Mechanism, BFD outline, BFD
@subsection Information Loss
@emph{Some information is lost due to the nature of the file format.} The output targets
supported by BFD do not provide identical facilities, and
information which may be described in one form has nowhere to go in
another format. One example of this is alignment information in
@code{b.out}. There is nowhere in an @code{a.out} format file to store
alignment information on the contained data, so when a file is linked
from @code{b.out} and an @code{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 (eg
@code{a.out}) or has sections without names (eg 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.

@emph{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 using the linker to read one
 format and write 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 linker. 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, this mechanism
is very useful. There is no information lost for this reason when
linking big endian COFF to little endian COFF, or from @code{a.out} to
@code{b.out}.  When a mixture of formats is linked, the information is
only lost from the files whose format differs from the destination.

@node Mechanism,  , BFD information loss, BFD
@subsection Mechanism 
The greatest potential for loss of information is when there is least
overlap between the information provided by the source format, that
stored by the canonical format, and the information needed by the
destination format. A brief description of the canonical form may help
you appreciate what kinds of data you can count on preserving across
conversions.
@cindex BFD canonical format
@cindex internal object-file format

@table @emph
@item files
Information on target machine architecture, particular implementation
and format type are stored on a per-file basis. Other information
includes a demand pageable bit and a write protected bit.  Note that
information like Unix magic numbers is not stored here---only the magic
numbers' meaning, so a @code{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 linked with one another.

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

@item symbols
Each symbol contains a pointer to 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, the back end relocates all
symbols to make them relative to the base of the section where they were
defined.  This ensures that each symbol points to its containing
section.  Each symbol also has a varying amount of hidden data to contain
private data for the BFD back end.  Since the symbol points to the
original file, the private data format for that symbol is accessible.
@code{gld} 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 @code{a.out} type information is
stored in the symbol table as long symbol names. This information would
be useless to most COFF debuggers and may be thrown away with
appropriate command line switches. (The GNU debugger @code{gdb} does
support @code{a.out} style debugging information in COFF).

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.

@item 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 effectively by message passing through the relocation type
descriptor and symbol pointer. It allows relocations to be performed
on output data using a relocation method 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 COFF file, even though 68k COFF
has no such relocation type.

@item 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 divination of 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).
@end table


What is a backend
@node BFD front end, BFD back end, Mechanism, Top
@page
@chapter BFD front end
@include  bfd.texi
@page
@node Memory Usage, Sections, bfd, Top
@section Memory Usage
BFD keeps all 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 libbfd for the closing file will be thrown away.

BFD will not free anything created by an application, but pointers into
bfd structures will be invalidated on a @code{bfd_close}; for example,
after a @code{bfd_close} the vector passed to
@code{bfd_canonicalize_symtab} will still be around, since it has been
allocated by the application, but the data that it pointed to will be
lost.

The general rule is not to 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
(@code{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.

@node Sections,Symbols ,Memory Usage, Top
@include  section.texi
@page
@node Symbols, Archives ,Sections, To
@include  syms.texi
@page
@node Archives, Formats, Symbols, Top
@include  archive.texi
@page
@node Formats, Relocations, Archives, Top
@include  format.texi
@page
@node Relocations, Core Files,Formats, Top
@include  reloc.texi
@page
@node Core Files, Targets,  Relocations, Top
@include  core.texi
@page
@node Targets, Architectures, Core Files, Top
@include  targets.texi
@page
@node Architectures, Opening and Closing, Targets, Top
@include  archures.texi
@page
@node Opening and Closing, Internal, Architectures, Top
@include  opncls.texi
@page
@node Internal, File Caching, Opening and Closing, Top
@include  libbfd.texi
@page
@node File Caching, Top, Internal, Top
@include  cache.texi
@page
@chapter BFD back end
@node BFD back end, ,BFD front end, Top
@menu
* What to put where
* a.out backends::
* coff backends::
* oasys backend::
* ieee backend::
* srecord backend::
@end menu
@node What to Put Where, aout backends, BFD back end, BFD back end
All of bfd lives in one directory.
@page
@node aout backends, coff backends, What to Put Where, BFD back end
@include  aoutx.texi
@page
@node coff backends, oasys backends, aout backends, BFD back end
@include  coffcode.texi
@page
@node Index,  , BFD, Top
@unnumbered Function Index
@printindex  fn
@setchapternewpage on
@unnumbered Index
@printindex cp

@tex
% I think something like @colophon should be in texinfo.  In the
% meantime:
\long\def\colophon{\hbox to0pt{}\vfill
\centerline{The body of this manual is set in}
\centerline{\fontname\tenrm,}
\centerline{with headings in {\bf\fontname\tenbf}}
\centerline{and examples in {\tt\fontname\tentt}.}
\centerline{{\it\fontname\tenit\/} and}
\centerline{{\sl\fontname\tensl\/}}
\centerline{are used for emphasis.}\vfill}
\page\colophon
% Blame: pesch@cygnus.com, 28mar91.
@end tex


@contents
@bye