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Re: [Help-smalltalk] (long) DLD help / SDL bindings for GNU Smalltalk

From: Paolo Bonzini
Subject: Re: [Help-smalltalk] (long) DLD help / SDL bindings for GNU Smalltalk
Date: Wed, 02 May 2001 18:39:47 +0200

> What SDL has an audio library which fires off a
> callback whenever its audio out buffer is empty.

This *will* work as long as the (C) thread executing the
call-in is the same that executes the code.  You can
process events from C in Smalltalk, like Blox does: it
preempts whatever is happening on the current Smalltalk
Process, then returns control to the C program
when the Smalltalk code completed its evaluation.  This
is a synchronous operation; you can trigger asynchronous
operations by using Semaphores and Processes (possibly
running at higher priorities than the userSchedulingPriority);
asyncSignal is exported through the VMProxy as well.

However, it seems like you want a fast response from the
interpreter.  Be careful!

> tried passing it as a (ptr
> byte), but that made it through to the C as null).

Hmmm... making it a #{CObject} should do as well as
a (ptr byte).  I guess you are using something like

CStruct subclass: #SDLAudio
  declaration: #( ... (cCallback #{CObject}) ... )

and then doing

   anSDLAudio cCallback

In that case, you will obtain a pointer to the field,
not the value of the field!  You will need

   anSDLAudio cCallback value: (DLD getFunc: ...)

to set what in C would be anSDLAudio->cCallback.

I fail to remember exactly the semantics of CStructs, and
it was indeed a quest to understand them when I brought
GDBM back to life (I did not write CStructs), but the basic
rule is that you always need an extra #value or #value:
method send; and remember that CStructs have no setters!


ps: write to help-smalltalk, not to me!

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