winsize

Winsize refers to the terminal window size data structure used in Unix-like systems to describe the dimensions of a terminal in characters and, optionally, pixels. It is central to programs that need to adapt their text-based user interfaces to the current size of the terminal, such as full-screen editors, pagers, and terminal multiplexers.

The data structure is commonly exposed as struct winsize and typically contains four fields: ws_row for the number of text rows, ws_col for the number of text columns, ws_xpixel for the width in pixels, and ws_ypixel for the height in pixels. The ws_row and ws_col fields provide the core dimensions used by layout and wrapping logic, while ws_xpixel and ws_ypixel offer optional pixel dimensions that may be reported by some terminals or terminal emulators. On many systems the pixel fields may be zero if pixel data is not available.

Access to the terminal size is performed via the ioctl system call using the TIOCGWINSZ request to get the current size and TIOCSWINSZ to set a new size. A typical usage involves passing a file descriptor (often STDIN_FILENO or the controlling terminal) and a pointer to a struct winsize to ioctl. The resulting ws_row and ws_col values are then used to adjust interface layout accordingly.

Header declarations for struct winsize appear in system headers such as <sys/ioctl.h>, with some systems providing related definitions in other headers like <termios.h>. The exact presence and layout of fields can vary slightly between platforms.

Applications relying on winsize include terminal-based editors and applications, screen and tmux, and libraries such as ncurses, all of which use terminal dimensions to manage windows, panels, and dynamic content.