Concurrency in C++: Race Conditions, Locks, and Deadlocks Quiz Quiz

Explore essential concepts such as race conditions, locking mechanisms, and deadlock prevention in C++ multithreaded programming. This quiz is designed for anyone looking to strengthen their understanding of safe and efficient concurrency in C++ applications.

1. Understanding Race Conditions

   Which of the following best describes a race condition in a C++ multithreaded program?

   1. When threads run in order without any synchronization problems
   2. When threads always acquire separate locks for each resource
   3. When a program has a syntax error in the threading code
   4. When two threads access shared data simultaneously and at least one modifies it

   Explanation: A race condition occurs when two threads access shared data at the same time, and at least one of them writes to the data, leading to unpredictable results. Threads running in order without synchronization issues do not face race conditions. Separate locks prevent such problems and do not define race conditions themselves. Syntax errors are unrelated to the occurrence of race conditions in logic.

2. Critical Sections and Their Role

   What is a critical section in the context of C++ concurrency?

   1. A template function that must be used in threading programs
   2. A segment where no thread is allowed to enter at any time
   3. A piece of code that must not be interrupted by any signal
   4. A section of code that accesses shared resources and must be executed by only one thread at a time

   Explanation: A critical section refers to code sections accessing shared resources where only one thread should execute at a time to prevent data corruption. Being uninterrupted by signals is not necessarily related to concurrency control. A section that no thread can enter does not exist in convergence; code templates are unrelated.

3. Identifying Mutex Locks

   Which synchronization tool is most commonly used in C++ to protect shared data from concurrent access by multiple threads?

   1. Stream
   2. Alias
   3. Mutex
   4. Vector

   Explanation: A mutex is a mutual exclusion object that provides a locking mechanism, ensuring that only one thread can access certain data at a time. Streams are related to input/output, not concurrency protection. Vectors are containers and do not manage thread safety alone. Alias does not refer to a concurrency control mechanism.

4. Deadlock Detection

   What is a deadlock in a C++ program using multiple locks for resources?

   1. When a for-loop never terminates
   2. When threads execute code in the wrong order
   3. When an object is deleted while still in use
   4. When threads never finish because each waits for resources locked by others indefinitely

   Explanation: A deadlock occurs when two or more threads are blocked forever, each waiting for the other to release a resource. Execution order problems may cause errors but not deadlocks. Infinite loops relate to looping errors, not locking. Deleting an object in use can cause undefined behavior but is distinct from deadlock.

5. Avoiding Race Conditions

   In a simple bank account transfer example with two threads, which technique can prevent race conditions when both threads update the account balances?

   1. Declaring the balances as static variables
   2. Increasing the number of threads
   3. Using a mutex to lock the transfer operation
   4. Using a global variable to track thread progress

   Explanation: A mutex ensures that only one thread can perform the transfer at a time, preventing simultaneous operations that cause race conditions. Global variables do not provide synchronization. Making variables static does not solve concurrency issues. More threads can increase the problem rather than fix it.

6. Lock Guard Convenience

   What is the main advantage of using a lock_guard in C++ compared to manually locking and unlocking a mutex?

   1. It prevents compilation errors in non-threaded code
   2. It makes the code run faster regardless of other factors
   3. It allows multiple threads to access the critical section simultaneously
   4. It automatically releases the lock when leaving scope

   Explanation: A lock_guard releases the lock when its scope ends, reducing the chance of forgetting to unlock and causing a deadlock. Allowing multiple threads in a critical section would defeat the purpose. Compilation errors are not directly affected, and while lock_guard can avoid programmer errors, it does not impact execution speed on its own.

7. Recognizing a Live Lock

   What best describes a live lock scenario in C++ multithreaded applications?

   1. Threads keep changing state without making progress due to repeated conflict resolution
   2. The program has a memory leak in the thread pool
   3. All threads run to completion without interference
   4. One thread is permanently blocked waiting for a mutex

   Explanation: A live lock occurs when threads are active and not blocked, but they continually respond to each other without progressing. Permanent blocking is a deadlock, not a live lock. Proper thread completion excludes both liveness and safety failures. Memory leaks are resource issues and not directly related to live lock behavior.

8. Potential for Deadlock

   Which situation is likely to cause a deadlock when using two mutexes in C++?

   1. Threads never share data or resources
   2. Two threads acquire mutexes in different orders and try to lock one while holding the other
   3. A mutex is never locked by any thread
   4. Both threads use only one shared mutex

   Explanation: Acquiring mutexes in different orders can lead to each thread holding one lock and waiting for the other, creating a deadlock. If only one mutex is used or if threads don't share resources, deadlocks cannot occur. Not using a mutex at all does not cause locking issues, though it may cause data races.

9. Atomic Variables Usage

   Why are atomic variables useful in C++ concurrency?

   1. They prevent all types of program bugs
   2. They make all variable updates faster automatically
   3. They ensure operations on their value are performed as a single, indivisible step
   4. They allow direct synchronous communication between threads

   Explanation: Atomic variables guarantee that operations are atomic, preventing data races in concurrent environments. Atomic types do not enable or enforce synchronous thread communication. They may not always increase speed and cannot solve every kind of programming bug, only those related to data races.

10. Best Practice for Lock Ordering

    What is a recommended practice to avoid deadlocks when acquiring multiple locks in a C++ application?

    1. Avoid using any locks at all
    2. Wait for a fixed time before attempting to lock the next mutex
    3. Randomly choose which lock to acquire first in each thread
    4. Always acquire locks in a consistent global order across all threads

    Explanation: Acquiring locks consistently in the same order across threads prevents circular wait conditions, a major cause of deadlocks. Random lock ordering or fixed waiting times do not eliminate the risk and may introduce delays or race conditions. Avoiding locks removes the protective benefits of synchronization and may cause data corruption.