resizeterm(3X) Library calls resizeterm(3X)

is_term_resized, resize_term, resizeterm - manage the terminal dimensions understood by curses

#include <curses.h>
bool is_term_resized(int lines, int columns);
int resize_term(int lines, int columns);
int resizeterm(int lines, int columns);

This is an extension to the curses library. It provides callers with a hook into the ncurses data to resize windows, primarily for use by programs running in an X Window terminal (e.g., xterm) when the terminal's screen size is changed by the user:

  • curses windows cannot extend outside the screen. If the terminal is shrunk, curses windows must be shrunk to fit.
  • If the terminal is stretched, rows and/or columns can be added to existing windows. The added cells should match the current attributes of the windows.

If the calling program has not set up a handler for SIGWINCH when it initializes ncurses (e.g., using initscr(3X) or newterm(3X)), then ncurses sets a handler for SIGWINCH which notifies the library when a window-size event has occurred. The library checks for this notification

When the library has found that the terminal's window-size has changed, it calls resizeterm to update its data structures.

An application which establishes its own SIGWINCH handler can call resizeterm, but in that case, the library will not see SIGWINCH, and proper layout will rely upon the application.

The function resizeterm resizes the standard and current windows (i.e., stdscr and curscr) to the specified dimensions, and adjusts other bookkeeping data used by the ncurses library that record the window dimensions such as the LINES and COLS variables.

Most of the work for resizeterm is done by the inner function resize_term. The outer function resizeterm adds bookkeeping for the SIGWINCH handler, as well as repainting the soft-key area (see slk_touch(3X)).

The resize_term function attempts to resize all windows. This helps with simple applications. However:

  • It is not possible to automatically resize pads.
  • Applications which have complicated layouts should check for KEY_RESIZE returned from wgetch, and adjust their layout, e.g., using wresize and mvwin, or by recreating the windows.

When resizing windows, resize_term recursively adjusts subwindows, keeping them within the updated parent window's limits. If a top-level window happens to extend to the screen's limits, then on resizing the window, resize_term will keep the window extending to the corresponding limit, regardless of whether the screen has shrunk or grown.

A support function is_term_resized is provided so that applications can check if the resize_term function would modify the window structures. It returns TRUE if the windows would be modified, and FALSE otherwise.

Except as noted, these functions return the integer ERR upon failure and OK on success. They will fail if either of the dimensions are less than or equal to zero, or if an error occurs while (re)allocating memory for the windows.

While these functions are intended to be used to support a signal handler (i.e., for SIGWINCH), care should be taken to avoid invoking them in a context where malloc or realloc may have been interrupted, since it uses those functions.

If ncurses is configured to supply its own SIGWINCH handler,

  • on receipt of a SIGWINCH, the handler sets a flag
  • which is tested in wgetch(3X), doupdate(3X) and restartterm(3X),
  • in turn, calling the resizeterm function,
  • which ungetch's a KEY_RESIZE which will be read on the next call to wgetch.
The KEY_RESIZE alerts an application that the screen size has changed, and that it should repaint special features such as pads that cannot be done automatically.
Calling resizeterm or resize_term directly from a signal handler is unsafe. This indirect method is used to provide a safe way to resize the ncurses data structures.

If the environment variables LINES or COLUMNS are set, this overrides the library's use of the window size obtained from the operating system. Thus, even if a SIGWINCH is received, no screen size change may be recorded.

It is possible to resize the screen with SVr4 curses, by

Doing that clears the screen and is visually distracting.

This extension of ncurses was introduced in mid-1995. It was adopted in NetBSD curses (2001) and PDCurses (2003).

Thomas Dickey (from an equivalent function written in 1988 for BSD curses)

curs_getch(3X), curs_variables(3X), wresize(3X)

2024-03-16 ncurses 6.5