What are semaphores?

Answer 1

Click on the link to your right for the answer. Signaling devices, typically to give permission to go ahead or to stop, but can also have other meanings. On a rail line, a lifted green flag can be the semaphore that means to go ahead. Traffic lights are a kind of semaphore. In computer RS232 communication lines the RTS and CTS lines are sometimes called semaphores.

Answer 2

In computer programming, a semaphore is a method of synchronizing access between asynchronous threads of execution to a shared resource.

Consider software which responds to a hardware-generated interrupt. For example, a serial driver's receive interrupt handler may be active when data has been received over the serial line, regardless of the application's current activity. The serial receiver interrupt handler would place the newly received data into a buffer, update the number of bytes currently held in that buffer. Part of the regular application algorithm would be to periodically check the status of that buffer, evaluate the number of pending bytes, obtain those, and decrement the count accordingly.

In such a scenario, semaphores are used to ensure that only one thread of execution is able to access, modify or reply upon data read from the shared buffer.

A semaphore typically supports two basic operations: request and release. When requesting the semaphore, the caller might need to wait in an endless loop until the semaphore becomes available (nobody else "owns" it). The caller then has exclusive access to the resource thus protected. When done, the caller must release the semaphore so that other code can get access to the shared resource in the same manner.

More advanced semaphores support non-blocking acquisition methods, allowing the caller to execute other tasks while waiting for the semaphore to be granted.

Some semaphores also support a notion of capacity. The above example describes a semaphore with a capacity of one; only one thread of execution can own the semaphore and access the shared resource at any given time. Higher capacity semaphores can allow a larger number of concurrent users: no more than 3 callers to access a given resource at any given time, etc.