How to assert if a std::mutex is locked?

Question

With GCC 4.8.2 (on Linux/Debian/Sid 64 bits) -or GCC 4.9 when available - in C++11- I have some mutex

std::mutex gmtx;

^{actually, it is}

Answer 1

It's not technically an assertion, but I've used a similar approach to prevent unlocked access to shared state: add a reference parameter to the lock guard class on the in the unsafe function (beta in your example). Then the function cannot be called unless the caller has created a lock guard. It solves the problem of accidentally calling the function outside of the lock, and it does so at compile time with no races.

So, using your example:

typedef std::lock_guard LockGuard;
void alpha(void) {
   LockGuard g(gmtx);
   beta(g);
   // some other work
}

void beta(LockGuard&) {
   // some real work
}

void gamma(void) {
   LockGuard g(gmtx);
   beta(g);
   // some other work
}

//works recursively too
void delta(LockGuard& g)
{
   beta(g);
}

Drawbacks:

• Does not validate that the lock actually wraps the correct mutex.
• Requires the dummy parameter. In practice, I usually keep such unsafe functions private so this is not an issue.