Non-Overlapped Serial - Do ReadFile() calls from separate threads block each other?

Question

I've inherited a large code base that contains multiple serial interface classes to various hardware components. Each of these serial models use non-overlapped serial for their communication. I have an issue where I get random CPU spikes to 100% which causes the threads to stall briefly, and then the CPU goes back to normal usage after ~10-20 seconds.

My theory is that due to the blocking nature of non-overlapped serial that there are times when multiple threads are calling readFile() and blocking each other.

My question is if multiple threads are calling readFile() (or writeFile()) at the same time will they block each other? Based on my research I believe that's true but would like confirmation.

The platform is Windows XP running C++03 so I don't have many modern tools available

Answer 1

"if multiple threads are calling readFile() (or writeFile()) at the same time will they block each other?"

As far as I'm concerned, they will block each other.

I suggest you could refer to the Doc:Synchronization and Overlapped Input and Output

When a function is executed synchronously, it does not return until the operation has been completed. This means that the execution of the calling thread can be blocked for an indefinite period while it waits for a time-consuming operation to finish. Functions called for overlapped operation can return immediately, even though the operation has not been completed. This enables a time-consuming I/O operation to be executed in the background while the calling thread is free to perform other tasks.

Using the same event on multiple threads can lead to a race condition in which the event is signaled correctly for the thread whose operation completes first and prematurely for other threads using that event.

And the operating system is in charge of the CPU. Your code only gets to run when the operating system calls it. The OS will not bother running threads that are blocked.Blocking will not occupy the CPU. I suggest you could try to use Windows Performance Toolkik to check cpu utilization.