In operating systems and concurrent programming, what is a mutex (mutual exclusion object)?

Introduction / Context:
When multiple threads or processes access shared data, there is a risk of race conditions and inconsistent results. To prevent this, operating systems and programming languages provide synchronization mechanisms. One of the most fundamental mechanisms is the mutex, short for mutual exclusion. Exam questions frequently test whether you can identify a mutex and explain how it protects critical sections.

Given Data / Assumptions:

We are dealing with concurrent execution of multiple threads or processes.
A shared resource or critical section must not be accessed by more than one thread at a time.
The goal is to enforce mutual exclusion and preserve data consistency.
The name mutex is an abbreviation for mutual exclusion.

Concept / Approach:
A mutex is a lock like synchronization object. Only the thread or process that successfully locks the mutex is allowed to enter the associated critical section. Other threads attempting to lock the same mutex will block or wait until the mutex is released. This ensures that shared data is accessed in a controlled, exclusive way, preventing overlapping operations that could corrupt the data. Mutexes are widely used in operating system kernels and in user level libraries such as pthreads and Java synchronized constructs.

Step-by-Step Solution:
Step 1: Identify that the problem is controlling access to shared resources in a concurrent program. Step 2: Recognize that the essential property required is mutual exclusion, meaning that only one thread at a time executes the critical section code. Step 3: Understand that a mutex provides operations to lock and unlock. Locking succeeds only if the mutex is currently unlocked. Step 4: When a thread holds the mutex, other threads that attempt to lock it must wait until the holder unlocks it. Step 5: Conclude that any correct definition of a mutex must mention exclusive access to a critical section or shared resource.

Verification / Alternative check:
Programming references for POSIX threads, Java and C sharp describe mutexes or similar constructs as mutual exclusion locks that guard critical sections. None of these sources describe mutex as a hardware backup device, memory type or scheduling algorithm, which confirms that the synchronization primitive definition is correct.

Why Other Options Are Wrong:
Backup devices like tape drives are unrelated to thread synchronization.
Dynamic main memory is DRAM, not a synchronization primitive.
Scheduling algorithms choose which process runs on the CPU and are not used to lock critical sections directly.

Common Pitfalls:
Some learners confuse mutexes with semaphores. While both can enforce mutual exclusion, semaphores are more general and may allow multiple units, whereas a binary mutex is dedicated to exclusive locking and often tracks ownership. Another pitfall is forgetting to release a mutex, which can cause other threads to block indefinitely.

Final Answer:
A mutex is a mutual exclusion synchronization object that allows only one thread or process at a time to enter a critical section and access a shared resource, thereby preventing race conditions.