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21.5 Dynamic Loading of Individual Functions

When you compile a file, you can optionally enable the dynamic function loading feature (also known as lazy loading). With dynamic function loading, loading the file doesn’t fully read the function definitions in the file. Instead, each function definition contains a place-holder which refers to the file. The first time each function is called, it reads the full definition from the file, to replace the place-holder.

The advantage of dynamic function loading is that loading the file becomes much faster. This is a good thing for a file which contains many separate commands, provided that using one of them does not imply you will soon (or ever) use the rest. A specialized mode which provides many keyboard commands often has that usage pattern: a user may invoke the mode, but use only a few of the commands it provides.

The dynamic loading feature has certain disadvantages:

If you compile a new version of the file, the best thing to do is immediately load the new compiled file. That will prevent any future problems.

The byte compiler uses the dynamic function loading feature if the variable byte-compile-dynamic is non-nil at compilation time. Do not set this variable globally, since dynamic loading is desirable only for certain files. Instead, enable the feature for specific source files with file-local variable bindings, like this:

-*-byte-compile-dynamic: t;-*-
Variable: byte-compile-dynamic

If this is non-nil, the byte compiler generates compiled files that are set up for dynamic function loading. Default: nil.

Function: fetch-bytecode function

This immediately finishes loading the definition of function from its byte-compiled file, if it is not fully loaded already. The argument function may be a compiled-function object or a function name.

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