The increment is equivalent to a set operation for a flag semaphore and a decrement is equivalent to a reset. It guarantees that only one process will be in a critical section at a time.Ī counting semaphore is associated with an integer count and two operations, namely, increment and decrement. The most common and simplest kind of semaphore is called a binary semaphore because they have two states locked or unlocked. signal(): increments the counter wakes up one waiting process.wait(): decreases the counter by one if the counter is negative, then it puts the thread on a queue and blocks.The increment operation is called V, or signal the decrement is called P, or wait. The critical section is a code segment where the shared variables can be accessed and the atomic action is required in this section.Ī semaphore is implemented as an integer variable with atomic increment and decrement operations so long as the value is not negative the thread will continue, it will block otherwise. To avoid a series of issues caused by multiple processes (or programs) accessing a shared resource at the same time, we need a method that can be authorised by generating and using a token, so that only one execution thread can access the critical section of the code at any given time. One simple way of solving this problem is semaphores. This problem arises due to preemptive multitasking, where OS takes care of managing when and for how long each process gets to execute. There is a problem with using shared memory - shared memory (aka shared resource) when accessed by two or more processes without the mechanism of guaranteeing safe read and write operations leaves your code open to nasty race conditions. Here I am talking more about semaphores and their usage, working with shared resources. If you are new here, please read my previous article IPC using shared memory this article is a prolongation to my previous article. Shared memory an IPC mechanism is about two processes sharing a common segment of memory that they can both read to and write from to communicate with one another.
0 Comments
Leave a Reply. |