What about you presume that ___result_voidstar is already allocated, and you just access/mutate it? I.e.
(define make-point (c-lambda (int int) struct-point "___result_voidstar->.x = ___arg1; ___result_voidstar->y = ___arg2;"))
?
2013/8/22 Chris Mueller ruunsmail@gmail.com
I'm opening a new subject for the ffi struct issue to sum up the current discussion:
In 19.1 "The Mapping of types between C and Scheme" of the gambit documentation is mentioned:
"In the case of the struct, union and type types, the default function reclaims the copy on the C heap referenced by the internal pointer (when using a C++ compiler this is done using ‘delete (type*)internal-pointer’, which calls the destructor of type if it is a class) and returns ‘___FIX(___NO_ERR)’. In many situations the default release-function will perform the appropriate cleanup for the foreign type."
If i interpret this correctly, a declared struct type in a ffi provides a default release function that automatically calls a destructor (used with g++) and frees the corresponding memory.
Unfortanetly, i'm little lost to use it correctly and experiment currently heavily to find the right way.
Assume a type:
(c-declare "struct point { int x; int y; };") (c-define-type struct-point (struct "point"))
Assume a test programm that allocates a point 1_000_000_000 times.
(define (alloc-point times) (let loop ((i 0)) (if (< i times) (let ((a (make-point 0 0))) (loop (+ i 1)))))))
(alloc-point 1000000000)
How do we implement (make-point x y) to use a ffi struct type with a default release function in Gambit?
- With a manually allocated pointer it will leak memory.
(define make-point (c-lambda (int int) struct-point "point* a = new Point; a->x = ___arg1; a->y = ___arg2; ___result_voidstar = a;"))
- With a stack-allocated value it will segfault immediately.
(define make-point (c-lambda (int int) struct-point "point a; a.x = ___arg1; a.y = ___arg2; ___result = a;"))
- If we give the address of a stack-allocated value to
___result_voidstar the allocation loop is running with constant memory space but the programm will segfault after quiting with CTRL + X.
(define make-point (c-lambda (int int) struct-point "point a; a.x = ___arg1; a.y = ___arg2; ___result_voidstar = &a;"))
For more details, see discussion:
https://mercure.iro.umontreal.ca/pipermail/gambit-list/2013-August/006950.ht...
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