Written by Mattt October 14th, 2013 This article has been translated into: 中文 To err is human. To NSError is Cocoa. All programs on a Unix system are a child process of another process, forking all the way from the original process, the unmoved mover: pid 1 (which in the case of OS X is launchd). When the executable finishes, it communicates a status code between 0 and 255 to its parent, as a way