| Why one would like to have an atomic operation is easier to understand. For example, one can use the atomic nature of creating a symbolic link on nix to create a lock file to prevent a race condition in a forking shell script. Say you have two or more processes wanting to do something that can (or should) only be done by one process at a time; one naive solution is to manage access of each process to said action by using a lock file. However, writing or touching a file itself is not atomic. The answer is to throw a symbolic link into the mix. In this scenario, the lock file already exists. However, the lock is not the file itself, but a symbolic link to the file. The protocol for each process to follow is: 1. try to create a symbolic link to lock file (any file really) 2. if successful, proceed; if failed, wait (or exit) 3. when process is done, delete symbolic link to lock file Simply checking for the existence of the symlink is not sufficient since there is a period of time between checking for the symlink (or file) and proceeding with said action where another process can think it has the lock. The OS ensures that one and only one symlink (of the same name) can exist; attempts to create it again (even simultaneously) will result in a failure of one process to create the symlink. There is one winner; all others are losers. This is to say, the kernel ensures that the operation is atomic. As a result, the OS is now arbitrating what process can proceed to action, at the very lowest level. Another way to think about it is that it provides a way to make competing processes serialize - or get in line so that they may complete their action one at a time. In my experience, it is important to experiment and test to make sure that the atomic primitive you're using is actually working as expected. I've run up against some inconsistent implementations of symlink creation that make this action not as straightforward to use as one is lead to believe. |