Subversion Repositories openrisc_2011-10-31

@c This summary of BFD is shared by the BFD and LD docs.

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,

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 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 (COFF, IEEE and Oasys).

@end table