"Kjetil S. Matheussen" <[EMAIL PROTECTED]> writes: > Ludovic Court?s: >> Hi, >> >> "Maciek Godek" <[EMAIL PROTECTED]> writes: >> >>> is there any portable and recommended way for passing C pointers around >>> in guile environment? >>> I think of something like scm_to_ptr (analogous to scm_to_int etc.). >>> Certainly such a value would be completely useless for the interpreter. >>> One can achieve simillar functionality using the aforementioned >>> scm_to_int (keeping track on the word length of the build target), >>> but it seems rather unnatural. >> >> If the goal is to make a C object pointed to by some pointer available >> to Scheme functions, the recommended (and simplest) way is to use a >> "SMOB" (see the manual for details). In practice, SMOBs incur some >> storage overhead because they hold 4 machines words, while you may only >> need one. >> >> Another possibility is to use a "uo" field in a struct: it allows you to >> have a struct field holding a machine word inaccessible to Scheme >> functions (see the "Vtables" node of the manual). >> > > I haven't heard of the "uo" field before, but at least using a SMOB > is really inconvenient, and using an unsigned long for storing > pointers is really convenient. (BTW. How does swig and gwrap handle > pointers?) > > I agree with Maciek that it would at least be mind-comforting to have > functions like scm_to_ptr/etc, although not strictly necessary, > since using integers works just fine.
Does C guarantee that pointers fit in unsigned long? I suspect SMOBs are the right answer. Part of the point is to do the right thing when the scheme object is garbage collected. When discarding a pointer often one should be freeing an object or decrementing a refcount. I've use SMOBs, and they really weren't that hard once I dug in.
