Dynamic function loading is a GNU Emacs Lisp byte-compilation feature that defers reading individual function definitions until they are first called. The term does not have one established meaning across programming languages, and this Emacs feature is different from loading a shared library at runtime.
How dynamic function loading works
When a file is compiled with dynamic loading enabled, loading the compiled file does not immediately read every function body. Instead, a placeholder for each deferred function points to the compiled file. On that function’s first call, Emacs reads its full definition from the file and replaces the placeholder. The GNU Emacs Lisp Reference Manual describes this mechanism in its Dynamic Loading documentation.
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- Compile a file with dynamic loading enabled.
- Load the compiled file; some function bodies remain unread and are represented by placeholders.
- Call one of those functions for the first time.
- Emacs reads that function’s definition from the compiled file and replaces its placeholder.
When it can help
The manual identifies a potential loading-time benefit when a file contains many separately callable functions but a user is likely to use only a subset. For example, a specialized mode may provide numerous commands even though enabling the mode does not mean every command will be used. Deferring unused definitions can avoid reading all of them during the initial file load.
This is a qualitative benefit, not a promise of a particular speedup. The documentation does not provide a benchmark or measured figure, and it does not claim that a function runs faster after its definition has been loaded.
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How it differs from autoloading and shared libraries
| Mechanism | What is deferred | What triggers loading |
|---|---|---|
| Dynamic function loading in Emacs Lisp | An individual function definition within an already loaded compiled file | The first call to that function |
| Autoloading in Emacs Lisp | The file that contains a function’s definition | The first call to the function, which loads its defining file |
| Dynamic or shared-library loading | Code in a library resolved or loaded by a program at runtime | Depends on the language, runtime, and operating system |
Autoloading and dynamic function loading both postpone work, but at different scopes: autoloading defers a defining file, while dynamic function loading defers a function body within a compiled file. Shared-library loading is a broader runtime mechanism and should not be treated as another name for this specific Emacs Lisp feature. The Emacs manual discusses autoloading separately.
Why the compiled file must remain usable
Deferred function definitions still depend on their compiled file for later reads. The manual warns that deleting or moving that file can leave definitions that have not yet been read unavailable. Altering the file—for example, by compiling a newer version over it—can also make later reads produce incorrect results. Its guidance is to reload the new compiled file after recompilation. These cautions appear in the manual’s dynamic loading discussion.
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Version context
A mirror of the GNU Emacs Lisp manual reports that dynamic function loading has been deprecated since Emacs 27.1 and that its benefits are barely measurable in experience. Because this version-specific status is reported by a manual mirror rather than verified here against the current official documentation, check the manual for the Emacs version you use before relying on the feature. The general definition above describes the documented mechanism, not a recommendation to adopt it in current code.
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