mirror of
https://github.com/autc04/Retro68.git
synced 2024-09-07 13:54:33 +00:00
151 lines
7.6 KiB
Plaintext
151 lines
7.6 KiB
Plaintext
@c This summary of BFD is shared by the BFD and LD docs.
|
|
@c Copyright (C) 2012-2018 Free Software Foundation, Inc.
|
|
|
|
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
|
|
@end menu
|
|
|
|
@node BFD information loss
|
|
@subsection Information Loss
|
|
|
|
@emph{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
|
|
@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 (e.g.,
|
|
@code{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.
|
|
|
|
@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 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 @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 Canonical format
|
|
@subsection 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.
|
|
@cindex BFD canonical format
|
|
@cindex internal object-file format
|
|
|
|
@table @emph
|
|
@item 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 @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 used with one
|
|
another.
|
|
|
|
@item 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.
|
|
|
|
@item 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. @code{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 @code{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,
|
|
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 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.
|
|
|
|
@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 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.
|
|
@end table
|