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2.17.2.1 Differing file formats

Normally all the files involved in a link will be of the same
format, but it is also possible to link together different
format object files, and the back end must support that. The
_bfd_link_add_symbols entry point is called via the target
vector of the file to be added. This has an important
consequence: the function may not assume that the hash table
is the type created by the corresponding
_bfd_link_hash_table_create vector. All the
_bfd_link_add_symbols function can assume about the hash
table is that it is derived from struct
bfd_link_hash_table.

Sometimes the _bfd_link_add_symbols function must store
some information in the hash table entry to be used by the
_bfd_final_link function. In such a case the output bfd
xvec must be checked to make sure that the hash table was
created by an object file of the same format.

The _bfd_final_link routine must be prepared to handle a
hash entry without any extra information added by the
_bfd_link_add_symbols function. A hash entry without
extra information will also occur when the linker script
directs the linker to create a symbol. Note that, regardless
of how a hash table entry is added, all the fields will be
initialized to some sort of null value by the hash table entry
initialization function.

See ecoff_link_add_externals for an example of how to
check the output bfd before saving information (in this
case, the ECOFF external symbol debugging information) in a
hash table entry.