Skip to content

Instantly share code, notes, and snippets.

Last active Aug 29, 2015
What would you like to do?
Blocks on stack and on the heap and pointers to it
#import <Foundation/Foundation.h>
int main(int argc, char *argv[]) {
@autoreleasepool {
typedef int(^Block)(void);
void *p0, *p1;
Block blockOnStack[3];
Block blockOnHeap [3];
for (int i=0; i<3; i++) {
int int1 = 5;
int int2 = 10;
Block closure = ^{ return i; };
blockOnStack[i] = closure;
blockOnHeap [i] = Block_copy( closure );
printf("closure pointer: %p\n", blockOnStack[i] );
printf("Block_copy pointer: %p\n", blockOnHeap [i] );
printf("\nExecuting blocks from the stack\n");
for (int i = 0; i < 3; i++) {
printf("pointer b[%d]: %p\n", i, blockOnStack[i] );
printf("Execute b[%d]: %d\n", i, blockOnStack[i]() );
printf("\nExecuting blocks from the heap\n");
for (int i = 0; i < 3; i++) {
printf("pointer b[%d]: %p\n", i, blockOnHeap[i] );
printf("Execute b[%d]: %d\n", i, blockOnHeap[i]() );
printf("\nC blocks\n");
// "normal" C block scope 1
int blockInt0 = 5;
p0 = &blockInt0;
// "normal" block scope 2
int blockInt1 = 10;
p1 = &blockInt1;
printf("p0 = %p\np1 = %p\n", p0, p1);
return 0;
Commented output:
Every closure that is created inside the
for loop is created at the exact same
memory location on the stack.
The copied closures on the heap all have
different pointer addresses
closure pointer: 0x7fff54850bf0
Block_copy pointer: 0x7fe5eac07c70
closure pointer: 0x7fff54850bf0
Block_copy pointer: 0x7fe5eac07ca0
closure pointer: 0x7fff54850bf0
Block_copy pointer: 0x7fe5eac07cd0
Stack pointers, all same ^^^^^^^^^^
Heap pointers, all distinct ^^^^^^^^^^^^^^
Executing blocks from the stack
pointer b[0]: 0x7fff54850bf0
Execute b[0]: 2
pointer b[1]: 0x7fff54850bf0
Execute b[1]: 2
pointer b[2]: 0x7fff54850bf0
Execute b[2]: 2
Executing blocks from the heap
pointer b[0]: 0x7fe5eac07c70
Execute b[0]: 0
pointer b[1]: 0x7fe5eac07ca0
Execute b[1]: 1
pointer b[2]: 0x7fe5eac07cd0
Execute b[2]: 2
C blocks
Unlike the for loop C block scopes create a new stack
area for every visibility scope.
Unless compiled with e.g. gcc -O2
Remember: This is a implementation detail!!!
p0 = 0x7fff54850be4
p1 = 0x7fff54850be0
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment