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  • Thanks for answering both of my questions......It is my bad that I did not initialize "sum"! This is just a simple testing program to learn pthread so that's why I have to use it. After initializing sum, sometimes I got "38 23" and sometimes I got "23 38". Why this happens? Is it because the time cost on each function is different? Commented Dec 7, 2015 at 21:57
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    It's because the order in which your threads are executing is not defined. Sometimes the first thread will be executed first (and grab the mutex first, making another thread to wait), sometimes the second will be the first. Commented Dec 7, 2015 at 21:59
  • Why is there an "order" of execution? Aren't they synchronized and supposed to start running at the same time? (Sorry I am new with pthread so I am really confused) Commented Dec 8, 2015 at 17:05
  • What you have written right now is two threads, starting at (approximately!) same time (they can start exactly at the same time, as they are started one after another, but the interval between those starts is very small), which than begin executing and after delay based on multituted of factors try to grab a mutex. This delay is unpredictable, it is based on general computer state (to simplify things). The first thread to grab the mutex wins, the second has to wait. Commented Dec 8, 2015 at 17:08
  • I see! So is there any way that I can make two or more thread functions end at the same time? Commented Dec 8, 2015 at 17:24