SIGWINCH

SIGWINCH is a signal in Unix and Unix‑like operating systems that notifies a process that the size of its controlling terminal window has changed. The name stands for “window change.” It is defined in POSIX. The signal number is platform‑dependent; on many systems it is 4.

When a terminal’s dimensions change—for example, when a user resizes a terminal emulator or switches displays—the kernel sends SIGWINCH to each process that has the terminal as its controlling terminal. Processes can handle this signal to adjust layout, re‑draw screens, or update internal data structures. If the signal is not caught, the default action is to terminate the process, so programs that wish to respond gracefully typically install a handler.

SIGWINCH is mainly used by full‑screen text editors, command‑line interfaces, and terminal multiplexer programs. The curses library provides a convenient interface (wresize, getmaxyx) that automatically interprets the new window size after a SIGWINCH. Shells such as Bash and Zsh also use SIGWINCH to invoke tnset to reset the terminal state when a terminal size change occurs.

Historically, SIGWINCH was introduced by the BSD operating system to add interactivity to terminal‑based applications. It is supported by most modern shells, editors, and libraries, and is considered an essential part of terminal application development on Unix‑like platforms.