Critical Region

TL;DR

• A critical region is the part of a concurrent program that accesses or modifies shared resources and therefore requires controlled access.
• If multiple threads or processes enter it simultaneously, the result can be data corruption or other errors.
• Synchronization mechanisms (locks, semaphores, monitors) are used to ensure only one thread or process executes the critical region at a time.

Definition

A critical region, also known as a critical section, is a part of a concurrent program where access to shared resources must be strictly controlled. This control is necessary to prevent concurrent access to shared resources from causing errors or inconsistencies in the program’s output.

Explanation

When multiple threads or processes can access the same resource (for example, a data structure or a file), the portion of code that performs those accesses or modifications is the critical region. Allowing concurrent entry to that code can lead to incorrect results, data corruption, or other errors. To prevent such problems, concurrent programs use synchronization mechanisms that permit only one thread or process at a time to enter and execute the code in the critical region.

Examples

Updating a shared data structure (linked list)

Multiple threads trying to update a shared data structure, such as a linked list, create a critical region in the portion of code where the threads modify the linked list. If multiple threads were allowed to access and modify the linked list at the same time, it could result in data corruption or other errors. Therefore, the critical region must be managed so that only one thread at a time is allowed to access and modify the linked list.

Accessing a shared file

When multiple processes try to access a shared resource such as a file on a hard drive, the critical region is the portion of code where the processes read and write to the file. If multiple processes were allowed to access and modify the file simultaneously, it could result in data corruption or other errors. Therefore, the critical region must be managed so that only one process at a time is allowed to access and modify the file.

Notes or pitfalls

• Concurrent entry to a critical region can cause data corruption or other errors; careful management is required to maintain consistency and correctness.

Related terms

• Critical section (synonym)
• Locks
• Semaphores
• Monitors