[libdispatch-dev] GCD libdispatch w/Blocks support working on Free (f

[Chuck Swiger]

Hi, Hans--

On Oct 2, 2009, at 5:40 AM, Hans Petter Selasky wrote:
> Can the Apple's "Blocks" C language extension be used when  
> programming in the kernel? Or is this a user-space only feature?

While the main benefit of blocks is in conjunction with libdispatch  
for userland apps, they can be used by themselves, in the kernel or  
elsewhere.

A block is a closure and starts off living on the stack (although, a  
block can outlive the stack frame of the caller by being copied over  
to the heap if needed); the compiler wraps automatic variables which  
were around in the scope of the caller, turns their type into const  
(unless you explicitly declare that you need to change a variable by  
using __block storage keyword, in which case that variable is kept on  
the heap and accessed by reference) in order to preserve the state  
until the block runs.

In effect, blocks are normal function invocations which also have an  
extra argument which is the context or pointer to the saved  
environment state.  They can be used to implement callbacks and  
continuations in a clean way, although you do have some overhead with  
accessing mutable variables via pointer dereference to the struct  
holding your saved context.  However, most uses of callbacks in C are  
implemented as functions which accept a void *, which is used to feed  
the callback function a struct * of some sort, so the end result is  
fairly similar.

Regards,
-- 
-Chuck