Re: Temporarily protect array from garbage collection
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!
Temporarily protect array from garbage collection
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
Re: Struct size
On Saturday, 19 April 2014 at 12:26:16 UTC, Andrej Mitrovic via Digitalmars-d-learn wrote: On 4/19/14, Lars T. Kyllingstad via Digitalmars-d-learn wrote: Say I have two structs, defined like this: struct A { /* could contain whatever */ } struct B { A a; } My question is, is it now guaranteed that A.sizeof==B.sizeof? The best thing to do is add a static assert and then you can relax: That's what I've done, but it would be nice to know the code won't break due to some combination of platform and/or compiler switches I didn't think to test. Anyway, I've played around a bit, and found that a combination of struct and field alignment *can* break my assumption: align(1) struct A { char c; align(1) int i; } struct B { A a; } Now, A.sizeof is 5, while B.sizeof is 8. I'd have to add align(1) to the declaration of B to fix it.
Struct size
Say I have two structs, defined like this: struct A { /* could contain whatever */ } struct B { A a; } My question is, is it now guaranteed that A.sizeof==B.sizeof, regardless of how A is defined (member variable types, alignment, etc.)? More to the point, say I have a function foo() which looks like this: extern(C) void foo(A* ptr, size_t len); Is it now guaranteed that I can safely pass it a pointer to an array of Bs? That is, auto arr = new B[10]; foo(cast(A*) arr.ptr, arr.length); Thanks, Lars