This page applies to Harlequin v13.1r0 and later; and to Harlequin MultiRIP but not Harlequin Core
The .def file contains linker control information. (The same information could be specified equivalently on the linker command line.) A typical .def file looks like this (see the Microsoft online documentation for details; spaces and tabs are not significant, but newlines are):
LIBRARY name
EXPORTS
PLUGIN
The lib operation is used to produce the .exp file for the linker. (It also produces a .lib file, which is irrelevant to plugins.) You should cd to the directory containing the def file.
lib -machine:i386 -def:whatever .def -out:whatever .lib
Note: -machine:i386 should be replaced with -machine:AMD64 to build 64‐bit plugins.