EXC_BAD_ACCESS when using recursive block - iphone

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 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 2

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;
}

Answer 3

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.