[gambit-list] Proper usage of ffi structs
Chris Mueller
ruunsmail at gmail.com
Wed Aug 21 17:15:01 EDT 2013
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?
1) 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;"))
2) 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;"))
3) 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.html
Chris
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