notcurses

cleanroom TUI library for modern terminal emulators. definitely not curses.

• Introduction
• Basic use
  • Planes
  • Cells
• Differences from NCURSES
• Features missing relative to NCURSES

Introduction

• What it is: a library facilitating complex TUIs on modern terminal emulators, supporting vivid colors and Unicode to the maximum degree possible. Many tasks delegated to Curses can be achieved using notcurses (and vice versa).

• What it is not: a source-compatible X/Open Curses implementation, nor a replacement for NCURSES on existing systems, nor a widely-ported and -tested bedrock of Open Source, nor a battle-proven, veteran library.

notcurses abandons the X/Open Curses API bundled as part of the Single UNIX Specification. The latter shows its age, and seems not capable of making use of terminal functionality such as unindexed 24-bit color ("DirectColor", not to be confused with 8-bit indexed 24-bit color, aka "TrueColor" or (by NCURSES) as "extended color"). For some necessary background, consult Thomas E. Dickey's superb and authoritative NCURSES FAQ. As such, notcurses is not a drop-in Curses replacement. It is almost certainly less portable, and definitely tested on less hardware. Sorry about that. Ultimately, I hope to properly support all terminals supporting the features necessary for complex TUIs. I would argue that teletypes etc. are fundamentally unsuitable. Most operating systems seem reasonable targets, but I only have Linux and FreeBSD available for testing.

Whenever possible, notcurses makes use of the Terminfo library shipped with NCURSES, benefiting greatly from its portability and thoroughness.

notcurses opens up advanced functionality for the interactive user on workstations, phones, laptops, and tablets, at the expense of e.g. industrial and retail terminals (or even the Linux virtual console, which offers only eight colors and limited glyphs).

Why use this non-standard library?

• A svelter design than that codified in X/Open. All exported identifiers are prefixed to avoid namespace collisions. Far fewer identifiers are exported overall. All APIs natively suport UTF-8, and the cell API is based around Unicode's Extended Grapheme Cluster concept.

• Visual features not directly available via NCURSES, including images, fonts, video, high-contrast text, and transparent regions. All APIs natively support 24-bit color, quantized down as necessary for the terminal.

• Thread safety, and use in parallel programs, has been a design consideration from the beginning.

• It's Apache2-licensed in its entirety, as opposed to the drama in several acts that is the NCURSES license (the latter is summarized as "a restatement of MIT-X11").

On the other hand, if you're targeting industrial or critical applications, or wish to benefit from the time-tested reliability and portability of Curses, you should by all means use that fine library.

Basic use

A program wishing to use notcurses will need to link it, ideally using the output of pkg-config --libs notcurses. It is advised to compile with the output of pkg-config --cflags notcurses. If using CMake, a support file is provided, and can be accessed as notcurses.

Before calling into notcurses—and usually as one of the first calls of the program—be sure to call setlocale(3) with an appropriate UTF-8 LC_ALL locale. It is usually appropriate to pass NULL to setlocale(), relying on the user to properly set the LANG environment variable.

notcurses requires an available terminfo(5) definition appropriate for the terminal. It is usually appropriate to pass NULL in the termtype field of a notcurses_options struct, relying on the user to properly set the TERM environment variable. This variable is usually set by the terminal itself. It might be necessary to manually select a higher-quality definition for your terminal, i.e. xterm-direct as opposed to xterm or xterm-256color.

Each terminal can be prepared via a call to notcurses_init(), which is supplied a struct of type notcurses_options:

// Get a human-readable string describing the running notcurses version.
const char* notcurses_version(void);

struct cell;      // a coordinate on an ncplane: an EGC plus styling
struct ncplane;   // a drawable notcurses surface, composed of cells
struct notcurses; // notcurses state for a given terminal, composed of ncplanes

// Configuration for notcurses_init().
typedef struct notcurses_options {
  // The name of the terminfo database entry describing this terminal. If NULL,
  // the environment variable TERM is used. Failure to open the terminal
  // definition will result in failure to initialize notcurses.
  const char* termtype;
  // A file descriptor for this terminal on which we will generate output.
  // Must be a valid file descriptor attached to a terminal, or notcurses will
  // refuse to start. You'll usually want STDOUT_FILENO.
  int outfd;
  // If smcup/rmcup capabilities are indicated, notcurses defaults to making
  // use of the "alternate screen". This flag inhibits use of smcup/rmcup.
  bool inhibit_alternate_screen;
} notcurses_options;

// Initialize a notcurses context, corresponding to a connected terminal.
// Returns NULL on error, including any failure to initialize terminfo.
struct notcurses* notcurses_init(const notcurses_options* opts);

// Destroy a notcurses context.
int notcurses_stop(struct notcurses* nc);

notcurses_stop should be called before exiting your program to restore the terminal settings and free resources.

The vast majority of the notcurses API draws into virtual buffers. Only upon a call to notcurses_render will the visible terminal display be updated to reflect the changes:

// Make the physical screen match the virtual screen. Changes made to the
// virtual screen (i.e. most other calls) will not be visible until after a
// successful call to notcurses_render().
int notcurses_render(struct notcurses* nc);

Planes

Fundamental to notcurses is a z-buffer of rectilinear virtual screens, known as ncplanes. An ncplane can be larger than the physical screen, or smaller, or the same size; it can be entirely contained within the physical screen, or overlap in part, or lie wholly beyond the boundaries, never to be rendered. Each ncplane has a current writing state (cursor position, foreground and background color, etc.), a backing array of UTF-8 EGCs, and a z-index. If opaque, a cell on a higher ncplane completely obstructs a corresponding cell from a lower ncplane from being seen. An ncplane corresponds loosely to an NCURSES Panel, but is the primary drawing surface of notcurses—there is no object corresponding to a bare NCURSES WINDOW.

Cells

Unlike the notcurses or ncplane objects, the definition of cell is available to the user:

// A cell corresponds to a single character cell on some plane, which can be
// occupied by a single grapheme cluster (some root spacing glyph, along with
// possible combining characters, which might span multiple columns). At any
// cell, we can have a theoretically arbitrarily long UTF-8 string, a foreground
// color, a background color, and an attribute set. Valid grapheme cluster
// contents include:
//
//  * A NUL terminator,
//  * A single control character, followed by a NUL terminator,
//  * At most one spacing character, followed by zero or more nonspacing
//    characters, followed by a NUL terminator.
//
// Multi-column characters can only have a single style/color throughout.
//
// Each cell occupies 16 static bytes (128 bits). The surface is thus ~1.6MB
// for a (pretty large) 500x200 terminal. At 80x43, it's less than 64KB.
// Dynamic requirements can add up to 16MB to an ncplane, but such large pools
// are unlikely in common use.
typedef struct cell {
  // These 32 bits are either a single-byte, single-character grapheme cluster
  // (values 0--0x7f), or a pointer into a per-ncplane attached pool of
  // varying-length UTF-8 grapheme clusters. This pool may thus be up to 16MB.
  uint32_t gcluster;          // 1 * 4b -> 4b
  // The classic NCURSES WA_* attributes (16 bits), plus 16 bits of alpha.
  uint32_t attrword;          // + 4b -> 8b
  // (channels & 0x8000000000000000ull): inherit styling from prior cell
  // (channels & 0x4000000000000000ull): foreground is *not* "default color"
  // (channels & 0x3f00000000000000ull): reserved, must be 0
  // (channels & 0x00ffffff00000000ull): foreground in 3x8 RGB (rrggbb)
  // (channels & 0x0000000080000000ull): in the middle of a multicolumn glyph
  // (channels & 0x0000000040000000ull): background is *not* "default color"
  // (channels & 0x000000003f000000ull): reserved, must be 0
  // (channels & 0x0000000000ffffffull): background in 3x8 RGB (rrggbb)
  // At render time, these 24-bit values are quantized down to terminal
  // capabilities, if necessary. There's a clear path to 10-bit support should
  // we one day need it, but keep things cagey for now. "default color" is
  // best explained by color(3NCURSES). ours is the same concept. until the
  // "not default color" bit is set, any color you load will be ignored.
  uint64_t channels;          // + 8b == 16b
} cell;

A cell ought be initialized with CELL_TRIVIAL_INITIALIZER or the cell_init() function before it is further used. These just zero out the cell. A cell has three fundamental elements:

• The EGC displayed at this coordinate, encoded in UTF-8. If the EGC is a single ASCII character (value less than 0x80), it is stored inline in the cell's gcluster field. Otherwise, gcluster's top 24 bits are a 128-biased offset into the associated ncplane's egcpool. This implies that cells are associated with ncplanes once prepared.
• The Curses-style attributes of the text, and a 16-bit alpha channel.
• The 48 bits of foreground and background RGB, plus a few flags.

The EGC should be loaded using cell_load(). Either a single NUL-terminated EGC can be provided, or a string composed of multiple EGCs. In the latter case, the first EGC from the string is loaded. Remember, backing storage for the EGC is provided by the ncplane passed to cell_load(); if this ncplane is destroyed (or even erased), the cell cannot safely be used. If you're done using the cell before being done with the ncplane, call cell_release() to free up the EGC resources.

#define CELL_TRIVIAL_INITIALIZER { .gcluster = '\0', .attrword = 0, .channels = 0, }

static inline void
cell_init(cell* c){
  memset(c, 0, sizeof(*c));
}

// Breaks the UTF-8 string in 'gcluster' down, setting up the cell 'c'.
int cell_load(struct ncplane* n, cell* c, const char* gcluster);

// Release resources held by the cell 'c'.
void cell_release(struct ncplane* n, cell* c);

#define CELL_STYLE_MASK 0xffff0000ul
#define CELL_ALPHA_MASK 0x0000fffful

// Set the specified style bits for the cell 'c', whether they're actively
// supported or not.
static inline void
cell_set_style(cell* c, unsigned stylebits){
  c->attrword = (c->attrword & ~CELL_STYLE_MASK) |
                ((stylebits & 0xffff) << 16u);
}

// Add the specified styles to the cell's existing spec.
static inline void
cell_enable_styles(cell* c, unsigned stylebits){
  c->attrword |= ((stylebits & 0xffff) << 16u);
}

// Remove the specified styles from the cell's existing spec.
static inline void
cell_disable_styles(cell* c, unsigned stylebits){
  c->attrword &= ~((stylebits & 0xffff) << 16u);
}

static inline uint32_t
cell_fg_rgb(uint64_t channel){
  return (channel & 0x00ffffff00000000ull) >> 32u;
}

static inline uint32_t
cell_bg_rgb(uint64_t channel){
  return (channel & 0x0000000000ffffffull);
}

static inline unsigned
cell_rgb_red(uint32_t rgb){
  return (rgb & 0xff0000ull) >> 16u;
}

static inline unsigned
cell_rgb_green(uint32_t rgb){
  return (rgb & 0xff00ull) >> 8u;
}

static inline unsigned
cell_rgb_blue(uint32_t rgb){
  return (rgb & 0xffull);
}

#define CELL_FGDEFAULT_MASK 0x4000000000000000ull
#define CELL_BGDEFAULT_MASK 0x0000000040000000ull

static inline void
cell_rgb_set_fg(uint64_t* channels, unsigned r, unsigned g, unsigned b){
  uint64_t rgb = (r & 0xffull) << 48u;
  rgb |= (g & 0xffull) << 40u;
  rgb |= (b & 0xffull) << 32u;
  rgb |= CELL_FGDEFAULT_MASK;
  *channels = (*channels & ~0x40ffffff00000000ull) | rgb;
}

static inline void
cell_rgb_set_bg(uint64_t* channels, unsigned r, unsigned g, unsigned b){
  uint64_t rgb = (r & 0xffull) << 16u;
  rgb |= (g & 0xffull) << 8u;
  rgb |= (b & 0xffull);
  rgb |= CELL_BGDEFAULT_MASK;
  *channels = (*channels & ~0x0000000040ffffffull) | rgb;
}

static inline void
cell_set_fg(cell* c, unsigned r, unsigned g, unsigned b){
  cell_rgb_set_fg(&c->channels, r, g, b);
}

static inline void
cell_set_bg(cell* c, unsigned r, unsigned g, unsigned b){
  cell_rgb_set_bg(&c->channels, r, g, b);
}

static inline void
cell_get_fg(const cell* c, unsigned* r, unsigned* g, unsigned* b){
  *r = cell_rgb_red(cell_fg_rgb(c->channels));
  *g = cell_rgb_green(cell_fg_rgb(c->channels));
  *b = cell_rgb_blue(cell_fg_rgb(c->channels));
}

static inline void
cell_get_bg(const cell* c, unsigned* r, unsigned* g, unsigned* b){
  *r = cell_rgb_red(cell_bg_rgb(c->channels));
  *g = cell_rgb_green(cell_bg_rgb(c->channels));
  *b = cell_rgb_blue(cell_bg_rgb(c->channels));
}

static inline bool
cell_fg_default_p(const cell* c){
  return !(c->channels & CELL_FGDEFAULT_MASK);
}

static inline bool
cell_bg_default_p(const cell* c){
  return !(c->channels & CELL_BGDEFAULT_MASK);
}

Differences from NCURSES

The biggest difference, of course, is that notcurses is not an implementation of X/Open (aka XSI) Curses, nor part of SUS4-2018.

The detailed differences between notcurses and NCURSES probably can't be fully enumerated, and if they could, no one would want to read it. With that said, some design decisions might surprise NCURSES programmers:

• The screen is not cleared on entry.
• There is no distinct PANEL type. The z-buffer is a fundamental property, and all drawable surfaces are ordered along the z axis. There is no equivalent to update_panels().
• Scrolling is disabled by default, and cannot be globally enabled.
• The Curses cchar_t has a fixed-size array of wchar_t. The notcurses cell instead supports a UTF-8 encoded extended grapheme cluster of arbitrary length. The only supported charsets are C and UTF-8. notcurses does not generally make use of wchar_t.
• The hardware cursor is disabled by default, when supported (civis capability).
• Echoing of input is disabled by default, and cbreak mode is used by default.
• Colors are always specified as 24 bits in 3 components (RGB). If necessary, these will be quantized for the actual terminal. There are no "color pairs".
• There is no distinct "pad" concept (these are NCURSES WINDOWs created with the newpad() function). All drawable surfaces can exceed the display size.
• Multiple threads can freely call into notcurses, so long as they're not accessing the same data. In particular, it is always safe to concurrently mutate different ncplanes in different threads.
• NCURSES has thread-ignorant and thread-semi-safe versions, trace-enabled and traceless versions, and versions with and without support for wide characters. notcurses is one library: no tracing, UTF-8, thread safety.

Features missing relative to NCURSES

This isn't "features currently missing", but rather "features I do not intend to implement".

• There is no immediate-output mode (immedok(), echochar() etc.). ncplane_putc() followed by notcurses_render() ought be just as fast as echochar().
• There is no support for soft labels (slk_init(), etc.).
• There is no concept of subwindows which share memory with their parents.
• There is no tracing functionality ala trace(3NCURSES). Superior external tracing solutions exist, such as bpftrace.
• There is no timeout functionality for input (timeout(), halfdelay(), etc.). Roll your own with any of the four thousand ways to do it.