Are Objective-C blocks always executed in a separate thread?

It executes it on whatever operation queue you specify as the queue argument:

Loads the data for a URL request and executes a handler block on an operation queue when the request completes or fails.

The queue parameter is documented as:

The operation queue to which the handler block is dispatched when the request completes or failed.

So it's really up to the NSOperationQueue exactly how many threads are used. I'd expect pooling behaviour - so while there can be multiple threads, I wouldn't expect a different thread per handler, necessarily.

A block is just a closure, like you have them in python or functional languages. They don't "run on a thread" they run where they are called.

int main(void){    void (^f)(void) { printf("hello world!\n"); }    f();    return 0;}

Does just what you think it does, no dispatch queues, no threads, no nothing.

Though, once you have blocks with all their nice capture semantics, it's very tempting to have APIs to schedule their execution everywhere. But basically, a block, is just the same as a function pointer and an ad-hoc struct containing all the variable captured, passed as an argument to the callback (it's even how it's implemented in the compiler).

As others have said, it will run on whatever queue you have specified. If this is a background queue, and you want to execute some code on the main thread, you can iclude a GCD block accessing the main queue. Your completion block would look something like this:

^(NSURLResponse *response, NSData *data, NSError*error){   // do whatever in the background   dispatch_async(dispatch_get_main_queue(), ^{   // this block will run on the main thread   });}