Relocatable Object Module Format (OMF) Specification
Relocatable Object Module Format (OMF) Specification
Relocatable Object Module Format (OMF) Specification
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<strong>Relocatable</strong> <strong>Object</strong> <strong>Module</strong> <strong>Format</strong><br />
8CH EXTDEF—External Names Definition Record<br />
Description<br />
The EXTDEF record contains a list of symbolic external references—that is, references to symbols defined in<br />
other object modules. The linker resolves external references by matching the symbols declared in EXTDEF<br />
records with symbols declared in PUBDEF records.<br />
History<br />
In the Intel specification and older linkers, the Type Index field was used as an index into TYPDEF records. This<br />
is no longer true; the field now encodes Microsoft symbol and type information (see Appendix 1 for details.) Many<br />
linkers ignore the old style TYPDEF.<br />
Record <strong>Format</strong><br />
1 2 1 1 or 2 1<br />
8C Record String External Type Index Checksum<br />
Length Length Name String<br />
This record provides a list of unresolved references, identified by name and with optional associated type<br />
information. The external names are ordered by occurrence jointly with the COMDEF and LEXTDEF records, and<br />
referenced by an index in other records (FIXUPP records); the name may not be null. Indexes start from 1.<br />
String Length is a 1-byte field containing the length of the name field that follows it.<br />
The Type Index field is encoded as an index field and contains debug information.<br />
Notes<br />
For Microsoft compilers, all referenced functions of global scope and all referenced variables explicitly<br />
declared "extern" will generate an EXTDEF record.<br />
Many linkers impose a limit of 1023 external names and restrict the name length to a value between 1<br />
and 7FH.<br />
Any EXTDEF records in an object module must appear before the FIXUPP records that reference<br />
them.<br />
Resolution of an external reference is by name match (case sensitive) and symbol type match. The<br />
search looks for a matching name in the following sequence:<br />
1. Searches PUBDEF and COMDEF records.<br />
2. If linking a segmented executable, searches imported names (IMPDEF).<br />
3. If linking a segmented executable and not a DLL, searches for an exported name (EXPDEF) with the same<br />
name—a self-imported alias.<br />
4. Searches for the symbol name among undefined symbols. If the reference is to a weak extern, the default<br />
resolution is used. If the reference is to a strong extern, it's an undefined external, and the linker generates<br />
an error.<br />
28 <strong>OMF</strong> <strong>Specification</strong>, Version 1.1 Tool Interface Standards (TIS)
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