1.3.2 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.
- 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
ZMAGICfile 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.
- 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
- 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.
ldcan 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).