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Node:<a name="Canonical%20format">Canonical format</a>,

Previous:<a rel="previous" accesskey="p" href="BFD-information-loss.html#BFD%20information%20loss">BFD information loss</a>,

Up:<a rel="up" accesskey="u" href="BFD-outline.html#BFD%20outline">BFD outline</a>

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<h4 class="subsection">The BFD canonical object-file format</h4>



   <p>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.



     <dl>

<dt><em>files</em>

     <dd>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</code>

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.



     <br><dt><em>sections</em>

     <dd>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.



     <br><dt><em>symbols</em>

     <dd>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</code> can

operate on a collection of symbols of wildly different formats without

problems.



     <p>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</code>, 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.



     <p>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.



     <br><dt><em>relocation level</em>

     <dd>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.



     <br><dt><em>line numbers</em>

     <dd>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). 

</dl>



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