The alternate screen

Many terminals provide an "alternate screen" with its own contents, no scrollback buffer, and no scrolling. Entering the alternate screen replaces the current visible contents wholesale, as does returning to the regular screen. Notcurses refers to the alternate screen's semantics as "TUI mode", and the regular screen's semantics as "CLI mode". It is possible to swap between the two modes at runtime using notcurses_leave_alternate_screen(3) and notcurses_enter_alternate_screen(3). Notcurses will enter TUI mode by default on startup; to prevent this, use NCOPTION_NO_ALTERNATE_SCREEN as described in notcurses_init(3). On program exit, Notcurses will always return to the regular screen, independent of the screen being used on program start.

Construction

Before calling into Notcurses—and usually as one of the first calls of the program—be sure to call setlocale with an appropriate UTF-8 LC_ALL locale. It is usually appropriate to use setlocale(LC_ALL, ""), relying on the user to properly set the LANG environment variable. Notcurses will refuse to start if nl_langinfo(3) doesn't indicate UTF-8 or ANSI_X3.4-1968 (aka US-ASCII). Be aware that capabilities are substantially reduced in ASCII.

notcurses_init(3) accepts a struct notcurses_options allowing fine-grained control of Notcurses behavior, including signal handlers, alternative screens, and overriding the TERM environment variable. A terminfo entry appropriate for the actual terminal must be available.

ncdirect_init(3) makes available a restricted subset of Notcurses functionality. This subset is intended to be interleaved with user-generated output, and is limited to coloring and styling. Direct mode is documented in notcurses_direct(3).

Only one context can be active in a process at a time, whether direct mode (struct ncdirect) or rendered mode (struct notcurses).

Output

All output is performed on struct ncplanes (see Ncplanes below). Output is not visible until explicitly rendered via notcurses_render(3). Information on drawing functions is available at notcurses_output(3).

Input

Notcurses supports input from keyboards (via stdin) and pointing devices (via a broker such as GPM, X, or Wayland). Input is delivered as 32-bit Unicode code points. Synthesized events such as mouse button presses and arrow keys are mapped into Unicode's Supplementary Private Use Area-B (https://unicode.org/charts/PDF/U100000.pdf). Information on input is available at notcurses_input(3). The included tool notcurses-input(1) can be used to test input decoding.

Ncpiles

A given notcurses context is made up of one or more piles. Piles provide distinct rendering contexts: a thread can be rendering or mutating one pile, while another thread concurrently renders or mutates another pile. A pile is made up of planes, totally ordered on a z-axis. In addition to the z-ordering, the planes of a pile are bound in a forest (a set of directed, acyclic graphs). Those planes which are not bound to some other plane constitute the root planes of a pile. A pile is destroyed when all its planes are destroyed, or moved to other piles. Since the standard plane (see below) always exists, and cannot be moved to another pile, one pile always exists, known as the standard pile.

Note that rasterizing a pile will replace all content within its margins.

For more information, see notcurses_pile(3).

Ncplanes

Following initialization, a single ncplane exists, the "standard plane" (see notcurses_stdplane(3)). This plane cannot be destroyed nor manually resized, and is always exactly as large as the screen (if run without a TTY, the "screen" is assumed to be 80x24 cells). Further ncplanes can be created with ncplane_create(3). A total z-ordering always exists on the set of ncplanes, and new ncplanes are placed at the top of the z-buffer. Ncplanes can be larger, smaller, or the same size as the physical screen, and can be placed anywhere relative to it (including entirely off-screen). Ncplanes are made up of nccells (see NcCells below). Information on ncplanes is available at notcurses_plane(3).

NcCells

nccells make up the framebuffers backing each ncplane, one cell per coordinate, one extended grapheme cluster (see unicode(7)) per cell. An nccell consists of a gcluster (either a directly-encoded 7-bit ASCII character (see ascii(7)), or a 25-bit index into the ncplane's egcpool), a set of attributes, and two channels (one for the foreground, and one for the background—see notcurses_channels(3)). Information on cells is available at notcurses_cell(3).

It is not usually necessary for users to interact directly with nccells. They are typically encountered when retrieving data from ncplanes or the rendered scene (see e.g. ncplane_at_yx(3)), or to achieve peak performance when a particular EGC is heavily reused within a plane.

Visuals

Bitmaps can be loaded from disk or memory, or even synthesized from the content of existing planes. These are stored in ncvisual objects, described in notcurses_visual(3). Visuals can be rendered to arbitrarily many planes using a variety of blitters, varying in their aspect ratios and resolution. If the terminal supports a pixel protocol such as Sixel or Kitty, it is possible to render bitmaps at the pixel level (as opposed to the cell level, using geometric glyphs). Otherwise, various Unicode-based blitters are available to render bitmaps in the text paradigm.

Widgets

A few high-level widgets are included, all built atop ncplanes:

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notcurses_fds(3) for dumping file descriptors/subprocesses to a plane

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notcurses_menu(3) for menu bars at the top or bottom of the screen

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notcurses_multiselector(3) for selecting one or more items from a set

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notcurses_plot(3) for drawing histograms and lineplots

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notcurses_progbar(3) for drawing progress bars

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notcurses_reader(3) for free-form input data

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notcurses_reel(3) for hierarchal display of block-based data

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notcurses_tabbed(3) for tabbed interfaces

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notcurses_selector(3) for selecting one item from a set

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notcurses_tree(3) for hierarchal display of line-based data

Threads

Notcurses explicitly supports use in multithreaded environments, but it does not itself perform any locking.

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Only one pile's rendered frame can be rasterized at a time, and it is not safe to concurrently render that pile. It is safe to rasterize a frame while rendering some other pile.

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It is otherwise always safe to operate concurrently on distinct piles.

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It is not safe to render a pile while concurrently modifying that pile.

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It is safe to output to multiple distinct ncplanes at the same time, even within the same pile.

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It is safe to output to ncplanes while adding or deleting some other ncplane.

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It is not safe for multiple threads to output to the same ncplane.

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It is not safe to add, delete, or reorder ncplanes within a single pile from multiple threads.

Only one thread may call notcurses_get(3) or any other input-related thread at a time, but it is safe to call for input while another thread renders.

Since multiple threads can concurrently manipulate distinct ncplanes, peak performance might require dividing the screen into several planes, and manipulating them from multiple threads.

Destruction

Before exiting, notcurses_stop(3) should be called. In addition to freeing up resources, this is necessary to restore the terminal to a state useful for the shell. By default, notcurses_init(3) installs signal handlers to catch all signals which would normally terminate the process. The new handlers will try to call notcurses_stop(3), and then propagate the received signal to the previous action.