Do I need to use locking with integers in c++ threads

Question

If I am accessing a single integer type (e.g. long, int, bool, etc...) in multiple threads, do I need to use a synchronisation mechanism such as a mutex to lock them. My un

Answer 1

You are never locking a value - you are locking an operation ON a value.

C & C++ do not explicitly mention threads or atomic operations - so operations that look like they could or should be atomic - are not guaranteed by the language specification to be atomic.

It would admittedly be a pretty deviant compiler that managed a non atomic read on an int: If you have an operation that reads a value - theres probably no need to guard it. However- it might be non atomic if it spans a machine word boundary.

Operations as simple as m_counter++ involves a fetch, increment, and store operation - a race condition: another thread can change the value after the fetch but before the store - and hence needs to be protected by a mutex - OR find your compilers support for interlocked operations. MSVC has functions like _InterlockedIncrement() that will safely increment a memory location as long as all other writes are similarly using interlocked apis to update the memory location - which is orders of magnitude more lightweight than invoking a even a critical section.

GCC has intrinsic functions like __sync_add_and_fetch which also can be used to perform interlocked operations on machine word values.