Deadlocks

A deadlock is a state in which at least one thread is blocked forever because it waits for the release of a resource, it does never get.

There are two main reasons for deadlocks:

1. A mutex has not been unlocked.
2. You lock your mutexes in a different order.

For overcoming the second issue, techniques such as lock hierachies are used in classical C++.

For the details about deadlocks and how to overcome them with modern C++, read the subsection issues of mutexes and locks.

Locking a non-recursive mutex more than once

Locking a non-recursive mutex more than once is undefined behaviour.

Locking more than once

 1 // lockTwice.cpp
 2 
 3 #include <iostream>
 4 #include <mutex>
 5 
 6 int main(){
 7 
 8    std::mutex mut;
 9 
10    std::cout << std::endl;
11    
12    std::cout << "first lock call" << std::endl;
13 
14    mut.lock();
15 
16    std::cout << "second lock call" << std::endl;
17 
18...