SIGTERM

SIGTERM (signal number 15) is a termination request sent to a process in POSIX-compliant systems. It is intended to allow graceful shutdown, giving the process a chance to release resources and save state before exiting.

Delivery and handling: The kernel delivers SIGTERM to a process. The default action is to terminate the process, but the signal can be caught, blocked, or ignored by the process. If a handler is installed, it should perform minimal, async-signal-safe cleanup and then exit or return. Absent a handler, the process is terminated. SIGTERM is distinct from SIGKILL (signal 9), which cannot be caught or ignored and forces immediate termination, and from SIGINT (signal 2), typically generated by an interactive terminal with Ctrl-C.

Sending: The command “kill -TERM pid” or “kill -s TERM pid” sends SIGTERM. Utilities like killall or pkill also use it. Service managers such as systemd or init scripts use SIGTERM to stop services gracefully before escalating to SIGKILL after a timeout.

Programmatic use: In C, a handler can be installed with signal(SIGTERM, handler) or with sigaction for more control. A common pattern is to set a flag in the handler and perform cleanup in the main loop, since many operations are not async-signal-safe.

Notes: SIGTERM is portable across Linux, macOS, and BSD systems. Windows does not implement POSIX signals in the same way, so behavior can differ on non-POSIX platforms. In practice, SIGTERM is the preferred way to request a graceful shutdown, allowing applications to terminate cleanly or to prepare for shutdown.