A dynamic Emacs module is a shared library that provides additional functionality for use in Emacs Lisp programs, just like a package written in Emacs Lisp would.
Functions that load Emacs Lisp packages can also load dynamic modules. They recognize dynamic modules by looking at their file-name extension, a.k.a. “suffix”. This suffix is platform-dependent.
This variable holds the system-dependent value of the file-name extension of the module files. Its value is .so on POSIX hosts and .dll on MS-Windows.
Every dynamic module should export a C-callable function named
emacs_module_init, which Emacs will call as part of the call to
require which loads the module. It should also
export a symbol named
plugin_is_GPL_compatible to indicate that
its code is released under the GPL or compatible license; Emacs will
refuse to load modules that don't export such a symbol.
If a module needs to call Emacs functions, it should do so through the API defined and documented in the header file emacs-module.h that is part of the Emacs distribution.
Modules can create
user-ptr Lisp objects that embed pointers to
C struct's defined by the module. This is useful for keeping around
complex data structures created by a module, to be passed back to the
module's functions. User-ptr objects can also have associated
finalizers – functions to be run when the object is GC'ed; this
is useful for freeing any resources allocated for the underlying data
structure, such as memory, open file descriptors, etc.
Loadable modules in Emacs are enabled by using the --with-modules option at configure time.
If you write your own dynamic modules, you may wish to verify their conformance to the Emacs dynamic module API. Invoking Emacs with the --module-assertions option will help you in this matter. See Initial Options.