Block life cycle in function? - objective-c

Question

Block variable in objective-c is a reference and I learned from somewhere infer that following code may be transformed by compiler to another form.
Original code:
typedef int (^block_type)();
block_type create_k(int i)
{
block_type block = ^(){
return i;
};
//[block copy];
return block;
}
Generated Code:
typedef void (*generic_invoke_funcptr)(void *, ...);
struct __block_literal {
void *isa;
int flags;
int reserved;
generic_invoke_funcptr invoke;
struct __block_descriptor_tmp *descriptor;
const int captured_i;
};
static const struct __block_descriptor_tmp {
unsigned long reserved;
unsigned long literal_size;
/* no copy/dispose helpers needed */
} __block_descriptor_tmp = {
0UL, sizeof(struct __block_literal)
};
// ^int (void) { return i; }
int __create_k_block_invoke_(struct __block_literal *bp) {
return bp->captured_i;
}
typedef int (*iv_funcptr)(struct __block_literal *);
typedef int (^block_type)();
block_type create_k(int i)
{
//block_type block = ^(){
// return i;
//};
struct __block_literal __b = {
.isa = &_NSConcreteStackBlock,
.flags = BLOCK_HAS_DESCRIPTOR,
.reserved = 0,
.invoke = (generic_invoke_funcptr)__f_block_invoke_,
.descriptor = &__block_descriptor_tmp,
.captured_i = i
};
struct __block_literal *block = &__b;
return block;
}
So |_b| in stack and block is only a reference to |_b|. If |create_k| return |block|, the receiver only get a invalid address.
But
int main(int argc, const char *argv[])
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
printf("%d\n", create_k(40)());
[pool drain];
return 0;
}
By exec it, print |40| and |block| is a valid block. What's the matter?

Answer 1

My guess would be that the memory for that stack frame hasn't been zeroed yet. Try calling another function between create_k() and printf() to get some other random data in there.