Is it thread safe to change a variable with no calculation of previous data involved?

I heard it's thread unsafe to do "global_variable += individual_thread_data"; for example [without locks].

But is it thread safe if one only does "global_variable = individua开发者_C百科l_thread_data" in each thread?

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Without considering the variable type, it is not thread safe. You should use mutexes/critical sections or atomic variables.

Sometimes it useful to write a simple code which assigns variables of several types and disassemble it to see how it is going to be assigned.

Also, for a cas-supporting architecture you can use assembler code to simulate an atomic variable.

(Another tip: the variable should be declared as volatile if used in threads, to prevent the optimization done by compiler.)

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No, it's just as unsafe (another thread may be using that variable at the same time).