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What is Reentrancy and Thread-Safety?

Depending on definition Reentrancy (= Interrupt Safety) and Thread Safety are two different things.

Reentrant: Reentrant functions can be called recursively and can be called simultaneously by two or more processes. Reentrant functions are often required in real-time applications or in situations where interrupt code and non-interrupt code must share a function (see Keil).

Thread Safe: Thread-safe code only manipulates shared data structures in a manner that ensures that all threads behave properly and fulfill their design specifications without unintended interaction (see Wikipedia). Thread-Safety is required when using an operating system and functions are called by different threads.

Which Functions are reentrant/ Thread-Safe?

Reentrant: Reentrant library function usually end with "_r". Reentrant newlib library functions can be found here.

Thread-Safe: When thread safety is required, the developer needs to use thread safe functions. In many cases, thread safety of non thread safe functions can be achieved using mutex Mutex functionalities.