Is it possible to temporarily prevent the garbage collector from collecting a memory block even if there are no references to it?

The use case is as follows:  I want to call a C library function which expects to take ownership of a buffer.  It looks something like this:

    alias FreeFunc = extern(C) void function(void*, void*) nothrow;

    extern(C) void foo(void* buf, size_t len,
                       FreeFunc free, void* ctx) nothrow;

Here, 'buf' is a pointer to the buffer, 'len' is the length of the buffer, 'free' is a function to deallocate the buffer when the library is done with it, and 'ctx' is a user-supplied context pointer.  Upon deallocation, 'free' receives two parameters; the pointer to the buffer and the context pointer.  The latter can be anything, even null, as it is just passed to 'free' and not used for anything else.

Here is the problem:  I want to be able to use a garbage-collected dynamic array with this function, but I don't want to have to retain a reference to it in my program.  (I don't know when the C library will call the free function.)  In other words, I want something like this:

    extern(C) void myFree(void* ptr, void* ctx)
    {
        enableGCFor(ptr);
    }

    auto arr = new int[123];
    disableGCFor(arr);
    foo(arr.ptr, arr.length, &myFree, null);
    arr = null;

Is this at all possible?

Thanks,
Lars

On Thu, 24 Apr 2014 19:55:37 +0000, Lars T. Kyllingstad wrote:

> Is it possible to temporarily prevent the garbage collector from collecting a memory block even if there are no references to it?
> 
> The use case is as follows:  I want to call a C library function which expects to take ownership of a buffer.  It looks something like this:
> 
>      alias FreeFunc = extern(C) void function(void*, void*)
> nothrow;
> 
>      extern(C) void foo(void* buf, size_t len,
>                         FreeFunc free, void* ctx) nothrow;
> 
> Here, 'buf' is a pointer to the buffer, 'len' is the length of the buffer, 'free' is a function to deallocate the buffer when the library is done with it, and 'ctx' is a user-supplied context pointer.  Upon deallocation, 'free' receives two parameters; the pointer to the buffer and the context pointer.  The latter can be anything, even null, as it is just passed to 'free' and not used for anything else.
> 
> Here is the problem:  I want to be able to use a garbage-collected dynamic array with this function, but I don't want to have to retain a reference to it in my program.  (I don't know when the C library will call the free function.)  In other words, I want something like this:
> 
>      extern(C) void myFree(void* ptr, void* ctx)
>      {
>          enableGCFor(ptr);
>      }
> 
>      auto arr = new int[123];
>      disableGCFor(arr);
>      foo(arr.ptr, arr.length, &myFree, null);
>      arr = null;
> 
> Is this at all possible?
> 
> Thanks,
> Lars

You can use GC.addRoot() from core.memory before passing the pointer to the C function, then use GC.removeRoot in your myFree function.

On Thursday, 24 April 2014 at 20:09:38 UTC, Justin Whear wrote:
> You can use GC.addRoot() from core.memory before passing the pointer to
> the C function, then use GC.removeRoot in your myFree function.

Perfect, thanks!