CODEWITHSUNDEEP
cmutex
Droidum
Droidum

Reputation: 434

Is mutex locked by me already?

A function is doing things on variables that must be protected against concurrent threads by a mutex.
The caller of this function locks a mutex, performs other things and calls this function.

Now a situation arised that requires this function to be called from another piece of code.
Of course this code could lock the mutex too but I thought perhaps the function could see if it must lock the mutex or not.

What I would need is a function that tells me if the current thread has locked the mutex. Then the function could go on without locking. Otherwise it would lock.

int need_to_lock = !my_thread_has_locked_the_mutex(&mutex);

if (need_to_lock)
    mutex_lock(&mutex);

access variables;

if (need_to_lock)
    mutex_unlock(&mutex);

Does my_thread_has_locked_the_mutex exist? I couldn't find anything.

Edit to comments

I didn't know about recursive mutexes. They seem to do what I thought of.
What I use now is a not recursive mutex that blocks if locked twice from the same thread.

Upvotes: 2

Views: 205

Answers (1)

nneonneo
nneonneo

Reputation: 179392

Why not just use a recursive mutex instead? A recursive mutex is substantially similar to a regular mutex, but can be locked multiple times by a single thread (i.e. recursively).

In some libraries, mutexes are recursive by default. With C pthreads, you need to opt in to recursive mutexes by initializing your mutexes appropriately:

pthread_mutexattr_t attr;
pthread_mutexattr_init(&attr);
pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
pthread_mutex_init(mutex, &attr);

Another approach is to split your function into two variants: one that should be called with the mutex held, and one that should be called without the mutex held. The usual naming convention is to append _locked to the former, or _unlocked to the latter. The unlocked function simply locks the mutex and then calls the locked function, so there won’t be any real duplication of code here.

Upvotes: 3

Related Questions