Hi guys! It's me again, still going on about struct-by-value in chicken-bind<http://wiki.call-cc.org/eggref/4/bind>. This time I think I may have code<https://github.com/kristianlm/chicken-bind> worthy of entering the official repo. The patches add three new features:
1. Struct-by-value in arguments 2. Struct-by-value return types 3. Nested structs (practically same as 2) Functions on the Scheme-side interface all functions using pointers or locatives, regardless of their original signature. You can have a look at my 10 commits that make up the patch on github<https://github.com/kristianlm/chicken-bind/commits/>. I tried to be descriptive in my commit messages. Please let me know of your thoughts and concerns. If nothing pops up, I'll pass it on Felix (chicken-bind maintainer) for review. *Motivation* While most C libraries pass structs by reference, both physics engines I've come across, Chipmunk and Box2D, pass small structs like 2d-vectors around by value everywhere. This patch made my life easier. *Code samples* Let's walk through the new foreign-lambda snippets that it generates. I use the point struct in my examples, pretend it's some 2d/3d vector of floats. First, let's look at passing a struct by reference: *1. Struct arguments* [klm@kth chicken-bind]$ echo "float length(struct point*)" | chicken-bind - -o - (begin (begin (define length (foreign-lambda float "length" (c-pointer (struct "point")))))) * * Nothing's changed there, my patch will kick in when you pass structs by value. The patch checks if any arguments are non-pointer struct arguments, and if there are any, it wraps the call in a foreign-lambda* with all struct-by-val arguments to c-pointer variant which are dereferenced in C: [klm@kth chicken-bind]$ echo "float length(struct point)" | chicken-bind - -o - (begin (begin (define length (foreign-lambda* float (((c-pointer (struct "point")) a0)) "C_return(length(*a0));")))) *2. Struct return-types* Struct return-types are a little trickier and are split into two functions. One will call the original function, storing the result in a additional destination operand. The other will allocate memory to use as this destination and calls the first: [klm@kth chicken-bind]$ echo "struct point intersection(struct line*, struct line)" | chicken-bind - -o - (begin (begin (begin (define intersection/overwrite! (foreign-lambda* void (((c-pointer (struct "point")) dest) ((c-pointer (struct "line")) a0) ((c-pointer (struct "line")) a1)) "*dest=(intersection(a0,*a1));")) (define (intersection a0 a1) (let ((dest (location (make-blob (foreign-value "sizeof(struct point)" int))))) (intersection/overwrite! dest a0 a1) dest))))) As shown above, you can mix and match struct value-passing and pointer-passing in the arguments. *3. Nested structs* Nested structs face the same problem as struct return-types, but unfortunately I haven't looked into uniting the codebase. However, it follows the same destination-method as above: [klm@kth chicken-bind]$ echo "struct circle { struct point origin; float radius ; }" | chicken-bind - -o - (begin (define circle-origin (lambda (s) (let ((blob (location (make-blob (foreign-value "sizeof(struct point)" int)))) (copy-struct! (foreign-lambda* void (((c-pointer (struct "point")) _dest) ((c-pointer (struct "circle")) s)) "*_dest = s->origin;"))) (copy-struct! blob s) blob))) (define circle-radius (foreign-lambda* float (((c-pointer (struct "circle")) s)) "return(s->radius);")) (define make-circle (foreign-lambda* (c-pointer (struct "circle")) (((c-pointer (struct "point")) origin) (float radius)) "struct circle *tmp_ = (struct circle *)C_malloc(sizeof(struct circle));\ntmp_->origin = *origin;\n\ntmp_->radius = radius;\n\nC_return(tmp_);"))) *Caveats* Struct-by-value return types and nested-struct getters return locatives. This is nice because it will be like any other scheme-object and doesn't need to be explicitly freed. Be careful though, locatives will be moved around by the GC and thus pointers to it are not permanent. A also added a small test-suite<https://github.com/kristianlm/chicken-bind/blob/master/tests/struct-passing-tests.scm>for these features. Cheers fellow Chickeners, - Kris
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