16.11 Emacs Dynamic Modules

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.

variable module-file-suffix

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 load or 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 signal an error if your program tries 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 (Application Programming Interface) defined and documented in the header file emacs-module.h that is part of the Emacs distribution. See Writing Dynamic Modules, for details of using that API when writing your own modules.

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. See Module Values.

function user-ptrp object

This function returns t if its argument is a user-ptr object.

function module-load file

Emacs calls this low-level primitive to load a module from the specified file and perform the necessary initialization of the module. This is the primitive which makes sure the module exports the plugin_is_GPL_compatible symbol, calls the module’s emacs_module_init function, and signals an error if that function returns an error indication, or if the use typed C-g during the initialization. If the initialization succeeds, module-load returns t. Note that file must already have the proper file-name extension, as this function doesn’t try looking for files with known extensions, unlike load.

Unlike load, module-load doesn’t record the module in load-history, doesn’t print any messages, and doesn’t protect against recursive loads. Most users should therefore use load, load-file, load-library, or require instead of module-load.

Loadable modules in Emacs are enabled by using the --with-modules option at configure time.