multithreading - Are incrementers / decrementers (var++, var--) etc thread safe?

Question

Inspired by this question: In Complexity Analysis why is ++ considered to be 2 operations?

Take the following psuedo code:

class test
{
   int _counter;
   void Increment()
   {
     _counter++;
   } 
}

Would this be considered thread safe on an x86 architechure? Further more are the Inc / Dec assembly instructions thread safe?

Answer 1

No, incrementing is not thread-safe. Neither are the INC and DEC instructions. They all require a load and a store, and a thread running on another CPU could do its own load or store on the same memory location interleaved between those operations.

Some languages have built-in support for thread synchronization, but it's usually something you have to ask for, not something you get automatically on every variable. Those that don't have built-in support usually have access to a library that provides similar functionality.