SIGKILL

SIGKILL is a signal used in Unix-like operating systems to immediately terminate a process. It cannot be caught, blocked, or ignored by the target process, and it is intended as a last resort when other termination requests fail.

On most systems, SIGKILL corresponds to signal number 9. It is delivered by commands such as kill -9 PID, pkill -9 name, or kill -KILL PID. When SIGKILL is sent, the operating system stops the process without giving it a chance to run any termination handlers, run atexit functions, or perform cleanup. The kernel will release the process's resources and remove it from the process table. Because cleanup code cannot run, data in memory or in buffers could be left in an inconsistent state.

In practice, SIGKILL should be used only after SIGTERM (and possibly SIGINT) have been attempted and the process remains unresponsive. If a process is stuck in uninterruptible I/O (uninterruptible sleep), termination may be delayed until it returns to user space; eventual termination is still guaranteed by the kernel in normal conditions, but not guaranteed at that moment.

After termination, the parent process may receive a SIGCHLD signal. If the parent does not wait, the child may become a zombie until waited on. Routine system administration guidance encourages first attempting graceful shutdowns and reserving SIGKILL for stubborn processes.

See also: SIGTERM, kill command.