What is "critical section"

n. (context computing programming English) A piece of code that accesses a shared resource that must not be concurrently accessed by more than one thread of execution.

In concurrent programming, a critical section or critical region is a part of a multi-threaded program that may not be concurrently executed by more than one of the program's processes. In other words, it is a piece of a program that requires mutual exclusion of access. Typically, the critical section accesses a shared resource, such as a data structure, a peripheral device, or a network connection, that does not allow multiple concurrent accesses.

A critical section may consist of multiple discontiguous parts of the program's code. For example, one part of a program might read from a file that another part wishes to modify. These parts together form a single critical section, since simultaneous readings and modifications may interfere with each other.

A critical section will usually terminate in finite time, and a thread, task, or process will have to wait for a fixed time to enter it (aka bounded waiting). Some synchronization mechanism is required at the entry and exit of the critical section to ensure exclusive use, for example a semaphore.

By carefully controlling which variables are modified inside and outside the critical section, concurrent access to that state is prevented. A critical section is typically used when a multithreaded program must update multiple related variables without a separate thread making conflicting changes to that data. In a related situation, a critical section may be used to ensure a shared resource, for example a printer, can only be accessed by one process at a time.

How critical sections are implemented varies among operating systems.

The simplest method is to prevent any change of processor control inside the critical section. On uni-processor systems, this can be done by disabling interrupts on entry into the critical section, avoiding system calls that can cause a context switch while inside the section, and restoring interrupts to their previous state on exit. Any thread of execution entering any critical section anywhere in the system will, with this implementation, prevent any other thread, including an interrupt, from being granted processing time on the CPU—and therefore from entering any other critical section or, indeed, any code whatsoever—until the original thread leaves its critical section.

This brute-force approach can be improved upon by using semaphores. To enter a critical section, a thread must obtain a semaphore, which it releases on leaving the section. Other threads are prevented from entering the critical section at the same time as the original thread, but are free to gain control of the CPU and execute other code, including other critical sections that are protected by different semaphores.