Thread Block Operation

Question

I asked two questions(1,2) about reading a file using using a thread so that the main thread doesn't get blocked. My problem wasn't so much writing and starting a thread, my problem was I didn't understand what operations were blocking. I've been told before that reading from a file is a blocking, using the for loop in my second example, is blocking. I don't really understand why or even spot it when looking at a piece of code.

So my question obviously is, how do you spot or determine when an operation is blocking a thread, and how do you fix it?

Answer 1

So my question obviously is, how do you spot or determine when an operation is blocking a thread

There's no magic way to do it; in general you have to read the documentation for whatever functions you call in order to get an idea about whether they are guaranteed to return quickly or whether they might block for an extended period of time.

If you're looking at a running program and want to know what its threads are currently doing, you can either watch them using a debugger, or insert print statements at various locations so that you can tell (by seeing what text gets printed to stdout and what text doesn't) roughly where the thread is at and what it is doing.

, and how do you fix it?

Blocking is not "broken", so there's nothing to fix. Blocking is intentional behavior, so that e.g. when you call a function that reads from disk, it can provide you back some useful data when it returns. (Consider an alternative non-blocking read, which would always return immediately, but in most cases wouldn't be able to provide you with any data, since the hard drive had not had time to load in any data yet -- not terribly useful).

That said, for network operations you can set your socket(s) to non-blocking mode so that calls to send(), recv(), etc are guaranteed never to block (they will return an error code instead). That only works for networking though; most OS's don't support non-blocking I/O for disk access.