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Next: <a href="Adding-symbols-from-an-object-file.html#Adding-symbols-from-an-object-file" accesskey="n" rel="next">Adding symbols from an object file</a>, Previous: <a href="Adding-Symbols-to-the-Hash-Table.html#Adding-Symbols-to-the-Hash-Table" accesskey="p" rel="prev">Adding Symbols to the Hash Table</a>, Up: <a href="Adding-Symbols-to-the-Hash-Table.html#Adding-Symbols-to-the-Hash-Table" accesskey="u" rel="up">Adding Symbols to the Hash Table</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="BFD-Index.html#BFD-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Differing-file-formats-1"></a>
<h4 class="subsubsection">2.17.2.1 Differing file formats</h4>
<p>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
<code>_bfd_link_add_symbols</code> 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
<code>_bfd_link_hash_table_create</code> vector.  All the
<code>_bfd_link_add_symbols</code> function can assume about the hash
table is that it is derived from <code>struct
bfd_link_hash_table</code>.
</p>
<p>Sometimes the <code>_bfd_link_add_symbols</code> function must store
some information in the hash table entry to be used by the
<code>_bfd_final_link</code> 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.
</p>
<p>The <code>_bfd_final_link</code> routine must be prepared to handle a
hash entry without any extra information added by the
<code>_bfd_link_add_symbols</code> 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.
</p>
<p>See <code>ecoff_link_add_externals</code> 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.
</p>


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