[plt-scheme] FFI callbacks stored in uncooperatively realloc'd C

Hello,

I am trying to make FFI calls into a legacy C library. The C library makes
heavy use of callback functions. The callbacks are stored in arrays which
are, themselves, sized using C's own realloc function. There is nothing
that I can do to rewrite the C code.

When an array of callbacks is realloc'd, and scheme performs a garbage
collection, then the next callback invocation (from C) throws a
Segmentation Fault.

I am using mzscheme (cgc) 4.1 on Solaris 5.10 for i386. However, I would
expect the SEGV to be thrown, since I'm not making any attempt to preserve
the wrapped callback. Principally because I need help in generating the
code to do so.

A minimal C library. Don't ask me to change it, as it typifies the
problem. Built into shard object "tim.so":

/* 
-------------------------------------------------------------------------- */

typedef int (*int_thunk_type)(void);

int_thunk_type  Cb   = 0;
int_thunk_type* p_Cb = 0;

void configure_callback(int_thunk_type cb)
   {
   p_Cb = malloc(sizeof(int_thunk_type));
   *p_Cb = cb;
   }

void call_callback(void)
   {
   int i = 1;
   if(p_Cb && *p_Cb)
     {
     printf("calling: 0x%p\n", *p_Cb);
     printf("*p_Cb returned: %d\n", (*p_Cb)());
     printf("called: 0x%p\n", *p_Cb);
     printf("pre realloc: p_Cb = 0x%p\n", p_Cb);
     p_Cb = realloc(p_Cb, sizeof(int_thunk_type) * 10000*i);
     printf("post realloc: p_Cb = 0x%p\n", p_Cb);
     }
   }
/* 
-------------------------------------------------------------------------- */

The naive, broken, scheme that I wish to use:

(require scheme/foreign)
(unsafe!)

(define int-thunk (_fun -> _int))
(define lib:tim (ffi-lib "tim.so"))

(define configure-callback
  (get-ffi-obj "configure_callback" lib:tim
   (_fun int-thunk -> _void)))

(define call-callback
  (get-ffi-obj "call_callback" lib:tim
   (_fun -> _void)))

(define (meaning-of-life) 42)
(configure-callback meaning-of-life)
(define meaning-of-life #f)
(call-callback)
(collect-garbage)
(call-callback)
(collect-garbage)
(call-callback)


Any assistance is gratefully received.

Thanks and regards,

Tim

-- 
Tim Brown <tim.brown at cityc.co.uk>  | City Computing Limited            |
T: +44 20 8770 2110                | City House, Sutton Park Road      |
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