Thread can't count, giving wrong result

Question

I wrote this piece of code

#include       /* Input/Output */
#include      /* General Utilities */
#include            


        

Answer 1

The increment operator is usually implemented by a read-modify-write, which is non-atomic.

A non-atomic read-modify-write across threads can sometimes do this:

Thread 1:    Thread 2:     count
Read count   ...           1
Add 1        Read count    1
Write count  Add 1         2 
...          Write count   2

Resulting in the count being less than expected.

If you will access a shared resource across multiple threads, you need to protect it with some kind of threading aware locking mechanism, such as a mutex.