BFD tries to maintain as much symbol information as it can when
it moves information from file to file. BFD passes information
to applications though the asymbol structure. When the
application requests the symbol table, BFD reads the table in
the native form and translates parts of it into the internal
format. To maintain more than the information passed to
applications, some targets keep some information “behind the
scenes” in a structure only the particular back end knows
about. For example, the coff back end keeps the original
symbol table structure as well as the canonical structure when
a BFD is read in. On output, the coff back end can reconstruct
the output symbol table so that no information is lost, even
information unique to coff which BFD doesn't know or
understand. If a coff symbol table were read, but were written
through an a.out back end, all the coff specific information
would be lost. The symbol table of a BFD
is not necessarily read in until a canonicalize request is
made. Then the BFD back end fills in a table provided by the
application with pointers to the canonical information. To
output symbols, the application provides BFD with a table of
pointers to pointers to asymbols. This allows applications
like the linker to output a symbol as it was read, since the “behind
the scenes” information will be still available.