The recursive mutex is another subtype of mutex. Even though it has exactly the same functions as a regular mutex, it allows the calling thread, which initially locked the mutex, to lock the same mutex repeatedly. By doing this, the mutex doesn't become available for other threads until the owning thread has unlocked the mutex as many times as it has locked it.

One good reason to use a recursive mutex is for example when using recursive functions. With a regular mutex one would need to invent some kind of entry point which would lock the mutex before entering the recursive function.

With a recursive mutex, each iteration of the recursive function would lock the recursive mutex again, and upon finishing one iteration, it would unlock the mutex. As a result the mutex would be unlocked and unlocked the same number of times.

A potential complication hereby is that the maximum number of times that a recursive mutex can be locked is not defined in the standard. When the implementation's limit has been reached, a std::system_error will be thrown if one tries to lock it, or false is returned when using the non-blocking try_lock function.