What is a deadlock?

Deadlock is a situation in multithreaded or multiprocess systems where two or more threads (or processes) permanently block each other, waiting for resources held by each other.

Example: Thread A has acquired resource 1 and is waiting for resource 2, while Thread B has acquired resource 2 and is waiting for resource 1. Neither can continue working.

In C/C++, deadlock often occurs due to incorrect use of mutexes:

std::mutex m1, m2;

void thread1() {
    std::lock_guard<std::mutex> lock1(m1);
    // ...
    std::lock_guard<std::mutex> lock2(m2);
    // ...
}

void thread2() {
    std::lock_guard<std::mutex> lock1(m2);
    // ...
    std::lock_guard<std::mutex> lock2(m1);
    // ...
}

If thread1 acquires m1 and thread2 acquires m2, both will wait for each other, leading to deadlock.

To prevent deadlock, the following are used:

• fixed order of resource acquisition,
• timeouts,
• functions like std::lock for simultaneous acquisition of multiple mutexes.