Type Alias dispatch_block_t 

pub type dispatch_block_t = *mut DynBlock<dyn Fn()>;

The type of blocks submitted to dispatch queues, which take no arguments and have no return value.

When not building with Objective-C ARC, a block object allocated on or copied to the heap must be released with a -[release] message or the Block_release() function.

The declaration of a block literal allocates storage on the stack. Therefore, this is an invalid construct: dispatch_block_t block; if (x) { block = ^{ printf(“true\n”); }; } else { block = ^{ printf(“false\n”); }; } block(); // unsafe!!!

What is happening behind the scenes: if (x) { struct Block __tmp_1 = …; // setup details block = & __tmp_1; } else { struct Block __tmp_2 = …; // setup details block = & __tmp_2; }

As the example demonstrates, the address of a stack variable is escaping the scope in which it is allocated. That is a classic C bug.

Instead, the block literal must be copied to the heap with the Block_copy() function or by sending it a -[copy] message.

See also Apple’s documentation