EXC_BAD_ACCESS when using recursive block

Question

I'm trying to create recursion using blocks. It works for a while, but eventually it crashes and gives me a bad access exception. This is my code:

BOOL (^Block)(Square *square, NSMutableArray *processedSquares) = ^(Square *square, NSMutableArray *processedSquares) {
    [processedSquares addObject:square];

    if (square.nuked) {
        return YES; // Found a nuked square, immediately return
    }

    for (Square *adjacentSquare in square.adjacentSquares) {
        if ([processedSquares containsObject:adjacentSquare]) {
            continue; // Prevent infinite recursion
        }

        if (Block(adjacentSquare, processedSquares)) {
            return YES;
        }
    }

    return NO;
};

__block NSMutableArray *processedSquares = [NSMutableArray array];
BOOL foundNukedSquare = Block(square, processedSquares);

Explanation: I have a Square class that has a BOOL nuked. It also has an NSArray adjacentSquares containing other Squares.

I want to check whether a square, or one of its 'connected' squares, is nuked or not.

The array processedSquares is to keep track of the squares I have checked to prevent infinite recursion.

When I run this, it's doing a lot of calls of this block (as expected). But at some point, it crashes at the last line with a bad access exception.

I also get this in the console:

Cannot access memory at address 0x1
Cannot access memory at address 0x1
Cannot access memory at address 0x1
Cannot access memory at address 0x1
warning: Cancelling call - objc code on the current thread's stack makes this unsafe.

I'm not that familiar with blocks and recursion. Any ideas?

---

Edit 1

As requested, the backtrace:

#0  0x00000001 in ??
#1  0x000115fb in -[Square connectedToNukedSquare] at   Square.m:105
#2  0x00010059 in __-[Bot makeMove]_block_invoke_1 at Bot.m:94
#3  0x91f3f024 in _dispatch_call_block_and_release
#4  0x91f31a8c in _dispatch_queue_drain
#5  0x91f314e8 in _dispatch_queue_invoke
#6  0x91f312fe in _dispatch_worker_thread2
#7  0x91f30d81 in _pthread_wqthread
#8  0x91f30bc6 in start_wqthread

Answer 1

You seem to be adding squares to the array while traversing the array. I'm speaking of this line:

[processedSquares addObject:square];

Might that have to do with it? You're adding an object while traversing. I'm surprised that this works at all.

Answer 2

You need a __block on Block, change the declaration to:

__block BOOL (^Block)(Square *square, NSMutableArray *processedSquares);
Block = ^(Square *square, NSMutableArray *processedSquares) {

When a variable (Block) is referenced within a block then its current value is copied into the block. In your code Block hasn't yet been given a value, as you are constructing the block in the assignment...

The __block prefix passes the variable by reference - by the time your block makes its recursive call Block has a value, the reference to it is used to obtain that value, and the recursive call is OK.

I don't know why its worked at all for you without the __block - failed straightaway for me. With the modifier however I can recurse to at least a depth of 10,000 - so stack space isn't a problem!

Answer 3

You're liking doing something wrong with the setup -- your Square objects are probably messed up somehow. Here's a complete example that works fine for me, maybe it can help you find your mistake:

#include <stdio.h>
#include <Foundation/Foundation.h>

@interface Square : NSObject
{
  BOOL nuked;
  NSArray *adjacentSquares;
}

@property(nonatomic) BOOL nuked;
@property(nonatomic, retain) NSArray *adjacentSquares;
@end

@implementation Square

@synthesize nuked;
@synthesize adjacentSquares;

@end;

BOOL (^Block)(Square *square, NSMutableArray *processedSquares) = ^(Square *square, NSMutableArray *processedSquares) {
  [processedSquares addObject:square];

  if (square.nuked) {
    return YES; // Found a nuked square, immediately return
  }

  for (Square *adjacentSquare in square.adjacentSquares) {
    if ([processedSquares containsObject:adjacentSquare]) {
      continue; // Prevent infinite recursion
    }

    if (Block(adjacentSquare, processedSquares)) {
      return YES;
    }
  }

  return NO;
};

int main(int argc, char **argv)
{
  NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

  Square *s1, *s2;
  s1 = [[Square alloc] init];
  s2 = [[Square alloc] init];
  s1.adjacentSquares = [NSArray arrayWithObjects:s2, nil];
  s2.adjacentSquares = [NSArray arrayWithObjects:s1, nil];

  __block NSMutableArray *processedSquares = [NSMutableArray array];
  BOOL foundNukedSquare = Block(s1, processedSquares);
  printf("%d\n", foundNukedSquare);

  [s1 release];
  [s2 release];

  [pool release];

  return 0;
}