Deadlock in C++

A deadlock occurs when all members of a group, including themselves, are waiting for another member to act, typically by sending a message or, more frequently, by releasing a lock. A lock is necessary when a concurrent program wants to modify shared memory, and if there are multiple locks in one situation or the issue is not handled carefully, a deadlock is likely to occur. Deadlock is a phenomenon where the software hangs indefinitely and is frequently exceedingly challenging to fix.

A deadlock occurs when two or more processes are obstructed indefinitely while they wait for one another to release a resource. In the context of a computer program, this usually occurs when multiple threads need to access the same shared resources, but each thread holds a resource that the other threads need, creating a circular wait. In an operating system, a deadlock occurs when many processes that are running in memory are unable to execute because the resources required for their execution are being held by other resources that are waiting for other resources to finish.

In C++, deadlocks can occur when multiple threads use mutexes (short for "mutual exclusion") to synchronize access to shared data. Only one thread at a time can access a shared resource thanks to a feature called a mutex. If two or more threads each hold a mutex on a resource that the others need, and each waits for the others to release their locks, a deadlock can occur.

To avoid deadlocks, it's important to follow a set of rules known as the "four conditions of deadlock":

• Mutual Exclusion: Only one process can access a resource at a time.
• Hold and Wait: A process holds at least one resource and waits for others.
• No Preemption: Resources cannot be taken away from a process.
• Circular Wait: Two or more processes wait for each other in a circular chain.

By following these conditions, you can prevent deadlocks in your code. For example, you can ensure that multiple threads acquire locks in a consistent order, so that they do not wait for each other in a circular pattern. Additionally, you can implement a timeout mechanism, so that a thread that is waiting for a resource for too long can release its lock and allow another thread to proceed.

Deadlocks are a common problem in multithreaded programming, and they occur when two or more threads block each other, preventing any of them from making progress. In C++, deadlocks can happen when two or more threads try to acquire the same set of resources, but each of them is holding onto a resource that the other needs.

Deadlocks can cause significant problems in a system, including degraded performance and complete system failure. To prevent deadlocks, a system can use several methods, such as the following:

• Resource ordering: A fixed global ordering of resources can be used to ensure that processes request resources in a specific order, reducing the likelihood of deadlocks.
• Timeouts: If a process has not finished its task after a predetermined amount of time, it may be forced to surrender its resources.
• Preemption: Resources can be taken away from a process and given to another process that needs them.
• Detection and recovery: Deadlocks can be detected and resolved by aborting one or more processes to break the deadlock cycle.

Here's an example of a deadlock in C++:

#include <iostream>
#include <thread>
#include <mutex>


std::mutex mutex1, mutex2;


void thread1()
{
    mutex1.lock();
std::cout << "Thread 1 acquired mutex 1" << std::endl;
std::this_thread::sleep_for(std::chrono::seconds(1));
    mutex2.lock();
std::cout << "Thread 1 acquired mutex 2" << std::endl;
    mutex2.unlock();
    mutex1.unlock();
}


void thread2()
{
    mutex2.lock();
std::cout << "Thread 2 acquired mutex 2" << std::endl;
std::this_thread::sleep_for(std::chrono::seconds(1));
    mutex1.lock();
std::cout << "Thread 2 acquired mutex 1" << std::endl;
    mutex1.unlock();
    mutex2.unlock();
}


int main ()
{
std::thread t1(thread1);
std::thread t2(thread2);
    t1.join();
    t2.join();
    return 0;
}

Output: This code will produce an output similar to the following:

Thread 1: Holding both mutexes.

Thread 2: Holding both mutexes.

Because both threads are waiting for each other to release the mutexes, it will continue to hang indefinitely.

In this example, both thread1 and thread2 try to acquire the same two mutexes mutex1 and mutex2, but in a different order. As a result, each thread blocks the other from making progress, creating a deadlock. To avoid deadlocks in your code, you need to make sure that all threads follow a consistent ordering when acquiring resources.