Is Block_copy recursive?

Question

I have some code that essentially boils down to this:

-(void)doSomethingWithBlock:(BlockTypedef)block
{
    [Foo doSomethingElseWithBlock:^() {
        block         


        

Answer 1

I quote the Blocks Programming Topics guide on Apple's developer documentation site:

When you copy a block, any references to other blocks from within that block are copied if necessary—an entire tree may be copied (from the top). If you have block variables and you reference a block from within the block, that block will be copied.

When you copy a stack-based block, you get a new block. If you copy a heap-based block, however, you simply increment the retain count of that block and get it back as the returned value of the copy function or method.

Answer 2

Yes, this is safe. You don't need to make a copy. At the time that -[Foo doSomethingElseWithBlock:] makes a copy of your literal block, it will copy the inner block to the heap.

I wrote some test code to prove to myself that this happens; see how printer (used only in block1) is copied from the stack to the heap at the time that Block_copy(block2) is called.

#include <Block.h>
#include <dispatch/dispatch.h>
#include <stdio.h>

typedef void (^void_block)();

class ScopedPrinter {
  public:
    ScopedPrinter() {
        printf("construct %p\n", this);
    }
    ScopedPrinter(const ScopedPrinter& other) {
        printf("copy %p <- %p\n", this, &other);
    }
    ~ScopedPrinter() {
        printf("destroy %p\n", this);
    }
};

void_block invoke(void_block other) {
    printf("other %p\n", (void*)other);
    void_block block2 = ^{
        printf("other %p\n", (void*)other);
        other();
    };
    printf("block2 created\n");
    block2 = Block_copy(block2);
    printf("block2 copied\n");
    return block2;
}

void_block make_block() {
    ScopedPrinter printer;
    printf("printer created\n");
    void_block block1 = ^{
        printf("block1 %p\n", &printer);
    };
    printf("block1 created\n");
    return invoke(block1);
}

int main() {
    void_block block = make_block();
    block();
    Block_release(block);
    return 0;
}

Transcript:

construct 0x7fff6a23fa70
printer created
copy 0x7fff6a23fa50 <- 0x7fff6a23fa70
block1 created
other 0x7fff6a23fa30
block2 created
copy 0x10a700970 <- 0x7fff6a23fa50
block2 copied
destroy 0x7fff6a23fa50
destroy 0x7fff6a23fa70
other 0x10a700950
block1 0x10a700970
destroy 0x10a700970

Answer 3

The inner Block_copy() isn't really relevant here. What you want to keep track of is whether a given block lives on the stack or on the heap. Consider this code based on your example:

@interface Foo : NSObject
@end

@implementation Foo

typedef void(^BlockTypedef)(void);

+(void)doSomethingElseWithBlock:(BlockTypedef)block
{
  NSLog(@"block=%@", block);
  BlockTypedef myBlock = Block_copy(block);
  NSLog(@"myBlock=%@", myBlock);
  myBlock();
  Block_release(myBlock);
}

+(void)doSomethingWithBlock:(BlockTypedef)block
{
  [Foo doSomethingElseWithBlock:^() {
    block();
  }];
}
@end

int main (int argc, const char * argv[])
{
  NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
  int i = 3;
  BlockTypedef block = ^{ printf("i=%d\n", i); };
  NSLog(@"block=%@", block);
  [Foo doSomethingWithBlock:block];
  block();
  NSLog(@"block=%@", block);
  [pool drain];
  return 0;
}

This should be ok, but block and myblock are different kinds of blocks. block is a stack block and has the scope of the calling stack. It will exist until main() exits. myblock is a malloc (heap) block, and will exist until it is released. You need to make sure that you don't try to take a non-copied reference to block and use it after the stack is done. You can't stick block in an ivar without copying it.

Joachim Bengtsson has the best write-up of this that I know of. @bbum has also written about it. (If bbum wanders in here and says I'm an idiot about this, then listen to him, but I think I'm right here.)