when a user wishes to interrupt the process. This is typically initiated by pressing , but on some systems, the "delete" character or "break" key can be used. "Signal kill" The SIGKILL signal is sent to a process to cause it to terminate immediately (kill). In contrast to SIGTERM and SIGINT, this signal cannot be caught or ignored, and the receiving process cannot perform any clean-up upon receiving this signal. The following exceptions apply: Zombie processes cannot be killed since they are already dead and waiting for their parent processes to reap them. Processes that are in the blocked state will not die until they wake up again. The init process is special: It does not get signals that it does not want to handle, and thus it can ignore SIGKILL. An exception from this rule is while init is ptraced on Linux. An uninterruptibly sleeping process may not terminate (and free its resources) even when sent SIGKILL. This is one of the few cases in which a UNIX system may have to be rebooted to solve a temporary software problem. SIGKILL is used as a last resort when terminating processes in most system shutdown procedures if it does not voluntarily exit in response to SIGTERM. To speed the computer shutdown procedure, Mac OS X 10.6, aka Snow Leopard, will send SIGKILL to applications that have marked themselves "clean" resulting in faster shutdown times with, presumably, no ill effects. The command has a similar, while dangerous effect, when executed e.g. in Linux; it doesn't let programs save unsaved data. It has other options, and with none, uses the safer SIGTERM signal. "Signal pipe" The SIGPIPE signal is sent to a process when it attempts to write to a pipe without a process connected to the other end. "Signal poll" The SIGPOLL signal is sent when an event occurred on an explicitly watched file descriptor. Using it effectively leads to making asynchronous I/O requests since the kernel will poll the descriptor in place of the caller. It provides an alternative to active polling. to "Signal real-time minimum", "signal real-time maximum" The SIGRTMIN to SIGRTMAX signals are intended to be used for user-defined purposes. They are real-time signals. "Signal quit" The SIGQUIT signal is sent to a process by its controlling terminal when the user requests that the process quit and perform a core dump. "Signal segmentation violation" The SIGSEGV signal is