In computer science, a semaphore is a variable or abstract data type used to control access to a common resource by multiple threads and avoid critical section problems in a concurrent system such as a multitasking operating system). Semaphores are a type of synchronization primitive. A semaphore is an integer variable, shared among multiple processes, used for process synchronization and access control for a common resource in a concurrent environment. Semaphores are used to implement critical sections, which are regions of code that must be executed by only one process at a time. By using semaphores, processes can coordinate access to shared resources, such as shared memory or I/O devices.

There are two main types of semaphores: counting semaphores and binary semaphores. Counting semaphores are integer value semaphores and have an unrestricted value domain. These semaphores are used to coordinate the resource access, where the semaphore count is the number of available resources. If the resources are added, semaphore count is automatically incremented, and if the resources are removed, the count is decremented. Binary semaphores are restricted to the values 0 and 1 (or locked/unlocked, unavailable/available) and are used to implement locks).

Semaphores have two indivisible (atomic) operations, namely wait and signal, which are used for process synchronization. The wait operation decrements the value of its argument S, if it is positive. If S is negative or zero, then no operation is performed. The signal operation increments the value of its argument S.

Semaphores are a useful tool in the prevention of race conditions; however, their use is not a guarantee that a program is free from these problems. Semaphores are complicated, so the wait and signal operations must be implemented in the correct order to prevent deadlocks. Semaphores are implemented in the machine-independent code of the microkernel, so they are machine-independent.

In summary, a semaphore is a synchronization primitive used to control access to a common resource by multiple threads and avoid critical section problems in a concurrent system. Semaphores are an integer variable, shared among multiple processes, used for process synchronization and access control for a common resource in a concurrent environment. There are two main types of semaphores: counting semaphores and binary semaphores. Semaphores have two atomic operations, wait and signal, which are used for process synchronization.