What exactly is a critical section?

Question

Just want a little clarity on the this. Imagine I use the windows api of EnterCriticalSection. I call all of them with EnterCriticalSection(&criticalsection);

Answer 1

See this:

Consider a variable

int k

two threads are operating on k with this statement

k+=100;

Now assume k equals to 0. The first thread starts to read k, find k=0, then add k by 100. Then the second thread starts to read k before the 1st thread write k=100 back. Then the second thread will assume k=0 and add it by 100 and finally after two threads join k=100 not expected 200. This is the reason we set k+=100 a critical section.