The Do-It Yourself Guide to Squeak Primitives

Notes

First off, the entire proceeding discussion is inexcusably C-centric. You can, of course, write primitives in any language that adheres to C's calling convention and can be cross-linked with C on your host platform (so why not use FORTRAN, Pascal, or ADA?).

There's another whole note yet to be written about debugging primitives, but on most platforms you can simply use the debugger to put breakpoints in the C primitive methods and single-step through them (Smalltalk will be frozen all the while, of course).

There is really no net (in terms of memory protection or "safe" primitives) here; it's quite easy to corrupt Smalltalk's heap or other memory with C, and to end up with a system that crashes unpredictable some time after you call your primitive. Be really careful about memory and stack management. Also remember the note above (in all-bold) that objects can be moved by the garbage collector between primitive calls, so if you ever pass a poiner to the VM to hold onto, you have to register it in Squeak as being external.

You can also trigger Smalltalk semaphores from C primitives; see John Maloney's SoundPlayer class or Siren's PrimMIDIPort for examples. This is by far the best way to implement "call-backs" from C to Smalltalk--have the Smalltalk application class pass down a semaphore to the VM and then start a loop process that waits for the semaphore and handles it asynchronously (If you're really clever, you can even create events and post them in Squeak's event input queue.)

For more examples: See the socket primitives for a simple interface to an external API (that passes structures around and coerces between Smalltalk objects and C structs); see the sound player primitives for examples of asynchronous I/O; see the AbstractSound classes for examples of automatically generated primitives.