The extra-gc library adds custom garbage collection functionality to Chez Scheme. Specifically, it provides a centralized mechanism for releasing resources not covered by Chez's default behavior.

The library exports the following functions:

• add-collector!

• custom-collect

• make-custom-collector

(add-collector! collector-function)

add-collector! accepts a function argument and adds it to the internal list of custom collectors. The argument should be a function that accepts no arguments. make-custom-collect returns a function valid for use by add-collector!.

(custom-collect)

custom-collect is a function that accepts no arguments. It runs Chez Scheme's collect function and then calls all the registered collectors. This is exported for convenience and application users are not required to use it. The library sets custom-collect as the collect-request-handler as a side-effect of loading the library.

(make-custom-collector free guardian)

(make-custom-collector free guardian count)

make-custom-collector accepts either two or three arguments. The first argument should be a function that accepts a single argument and releases whatever resources are associated with that object. The second argument should be function which is a guardian. The third argumemt, if provided, should be a positive exact integer. It determines how many objects returned by the guardian procedure are processed for any single collect. If not provided, count defaults to 100

make-custom-collector returns a nullary function that passes objects returned from the given guardian procedure to the given free procedure at most count number of times.

This library is currently only written for Chez Scheme. This will work on either threaded or non-threaded version of Chez Scheme. If Chez Scheme is threaded, the add-collector! and custom-collect functions execute inside a mutex.

If the free or guardian procedures invoke a continuation, you're on your own.

Guardians in Chez Scheme will accept the same object multiple times and return that object the same number of times. If necessary, the free procedure should handle this case.