Reentrancy and Thread-Safety

Throughout the documentation, the terms reentrant and thread-safe are used to mark classes and functions to indicate how they can be used in multithread applications:

• A thread-safe function can be called simultaneously from multiple threads, even when the invocations use shared data, because all references to the shared data are serialized.
• A reentrant function can also be called simultaneously from multiple threads, but only if each invocation uses its own data.

Hence, a thread-safe function is always reentrant, but a reentrant function is not always thread-safe.

By extension, a class is said to be reentrant if its member functions can be called safely from multiple threads, as long as each thread uses a different instance of the class. The class is thread-safe if its member functions can be called safely from multiple threads, even if all the threads use the same instance of the class.

Note: Qt classes are only documented as thread-safe if they are intended to be used by multiple threads. If a function is not marked as thread-safe or reentrant, it should not be used from different threads. If a class is not marked as thread-safe or reentrant then a specific instance of that class should not be accessed from different threads.

 class Counter
 {
 public:
     Counter() { n = 0; }

     void increment() { ++n; }
     void decrement() { --n; }
     int value() const { return n; }

 private:
     int n;
 };

 class Counter
 {
 public:
     Counter() { n = 0; }

     void increment() { QMutexLocker locker(&mutex); ++n; }
     void decrement() { QMutexLocker locker(&mutex); --n; }
     int value() const { QMutexLocker locker(&mutex); return n; }

 private:
     mutable QMutex mutex;
     int n;
 };