Thread (un)safety

4 types of thread-unsafe functions

Access unprotected shared data

Easy to fix, add lock

Function that keeps state across multiple invocation.

Such as rand. solution: stop using state via static variable. Only solution is to change the function such that it doesn’t use static and provide that value via variable.

function that return pointer to static

function that use thread-unsafe function

Thread reentrant

Thread-reentrant is thread function that doesn’t rely on any shared data.
Not the same as thread safe. A function can be thread-safe but not reentrant.

Race

Race condition occurs when the correctness of the program depends on the order of execution.

Deadlock

A pair (or multiple) threads that are blocked because they need resources from other threads.
For mutex, we can prevent it by locking it in one order and unlocking it in reverse order.