Use the deffile field to specify a module-definition file. A module-definition file is required for an overlaid DOS program or a DLL. It is optional for a Windows application. If you specify a base name with no extension, LINK assumes a .DEF extension. If the filename has no extension, put a period (.) at the end of the name.
By default, LINK assumes that a deffile needs to be specified. If you are linking without a .DEF file, use a semicolon to terminate the command line before the deffile field (or accept the default NUL.DEF at the Definitions File prompt).
How LINK Searches for Module-Definition Files
LINK searches for the module-definition file in the following order:
1.The directory specified for the file (if a path is included). If the file is not in that directory, the search terminates.
2.The current directory.
For information on module-definition files, see Chapter 16.