The Smalltalk callin mechanism can be used effectively to construct bindings to C libraries that require callbacks into Smalltalk. However, it is a “static” mechanism, as the callback functions passed to the libraries have to be written in C and their type signatures are fixed.
If the signatures of the callbacks are not known in advance,
and the only way to define callbacks is via C function pointers (as
opposed to reflective mechanisms such as the ones in GTK+), then
VMProxy functions for Smalltalk callin are not enough.
gnu Smalltalk provides a more dynamic way to convert Smalltalk blocks into
C function pointers through the
This class has a constructor method that is similar to the
cCall: annotation used for callouts. The method is
for:returning:withArgs: and its parameters are:
The array passed as the third parameter represents values that
are passed from C to Smalltalk and, as such, should be
filled with the same rules that are used by the return
type of a C callout. In particular, if the C callback
int * it is possible (and indeed useful)
to specify the type of the argument as
so that the block will receive a
Here is an example of creating a callback which is passed to
glutReshapeFunc1. The desired
signature in C is
void (*) (int, int).
| glut | ... glut glutReshapeFunc: (CCallbackDescriptor for: [ :x :y | self reshape: x@y ] returning: #void withArgs: #(#int #int))
It is important to note that this kind of callback does not survive
across an image load (this restriction may be lifted in a future version).
When the image is loaded, it has to be reset by sending it the
message before it is passed to any C function. Sending the
message to an already valid callback is harmless and cheap.
 The GLUT bindings use a different scheme for setting up callbacks.