Comment by wonrax

This is the exact same problem I encountered back when I was taking the Parallel Computing course, except I was an absolute novice and had no idea about debugging or profiling. I remember it took me several days speculating and searching for an answer to this issue on Google, and I finally found an explanation on Stack Overflow, which also suggested using rand_r. It was one of the first instances where I was introduced to solving and debugging a difficult computer programming problem, and it still sticks with me to this day.