objective c - Is there a way to wrap an ObjectiveC block into function pointer?

Question

I have to provide a C-style callback for a specific C library in an iOS app. The callback has no void *userData or something similar. So I am not able to loop in a context. I'd like to avoid introducing a global context to solve this. An ideal solution would be an Objective-C block.

My question: Is there a way to 'cast' a block into a function pointer or to wrap/cloak it somehow?

Answer 1

Technically, you could get access to a function pointer for the block. But it's totally unsafe to do so, so I certainly don't recommend it. To see how, consider the following example:

#import <Foundation/Foundation.h>

struct Block_layout {
    void *isa;
    int flags;
    int reserved; 
    void (*invoke)(void *, ...);
    struct Block_descriptor *descriptor;
};

int main(int argc, char *argv[]) {
    @autoreleasepool {
        // Block that doesn't take or return anything
        void(^block)() = ^{
            NSLog(@"Howdy %i", argc);
        };

        // Cast to a struct with the same memory layout
        struct Block_layout *blockStr = (struct Block_layout *)(__bridge void *)block;

        // Now do same as `block()':
        blockStr->invoke(blockStr);




        // Block that takes an int and returns an int
        int(^returnBlock)(int) = ^int(int a){
            return a;
        };

        // Cast to a struct with the same memory layout
        struct Block_layout *blockStr2 = (struct Block_layout *)(__bridge void *)returnBlock;

        // Now do same as `returnBlock(argc)':
        int ret = ((int(*)(void*, int a, ...))(blockStr2->invoke))(blockStr2, argc);
        NSLog(@"ret = %i", ret);
    }
}

Running that yields:

Howdy 1
ret = 1

Which is what we'd expect from purely executing those blocks directly with block(). So, you could use invoke as your function pointer.

But as I say, this is totally unsafe. Don't actually use this!

If you want to see a write-up of a way to do what you're asking, then check this out: http://www.mikeash.com/pyblog/friday-qa-2010-02-12-trampolining-blocks-with-mutable-code.html

It's just a great write-up of what you would need to do to get this to work. Sadly, it's never going to work on iOS though (since you need to mark a page as executable which you're not allowed to do within your app's sandbox). But nevertheless, a great article.