Application Structure¶

A curses application uses information from the system locale; setlocale(3) prepares it for curses library calls.

setlocale(LC_ALL, "");

If the locale is not thus initialized, the library assumes that characters are printable as in ISO 8859-1, to work with certain legacy programs. You should initialize the locale; do not expect consistent behavior from the library when the locale has not been set up.

initscr(3NCURSES) or newterm(3NCURSES) must be called to initialize curses before use of any functions that deal with windows and screens.

To get character-at-a-time input without echoing—most interactive, screen-oriented programs want this—use the following sequence.

initscr(); cbreak(); noecho();

Most applications perform further setup as follows.

intrflush(stdscr, FALSE);
keypad(stdscr, TRUE);

A curses program then often enters an event loop of some sort. Call endwin(3NCURSES) before exiting.

Overview¶

A curses library abstracts the terminal screen by representing all or part of it as a WINDOW data structure. A window is a rectangular grid of character cells, addressed by row and column coordinates (y, x), with the upper left corner as (0, 0). A window called stdscr, the same size as the terminal screen, is always available. Create others with newwin(3NCURSES).

A curses library does not manage overlapping windows (but see below). You can either use stdscr to manage one screen-filling window, or tile the screen into non-overlapping windows and not use stdscr at all. Mixing the two approaches will result in unpredictable and undesired effects.

Functions permit manipulation of a window and the cursor identifying the cell within it at which the next output operation will occur. Among those, the most basic are move(3NCURSES) and addch(3NCURSES): these place the cursor and write a character to stdscr, respectively.

Frequent changes to the terminal screen can cause unpleasant flicker or inefficient use of the communication channel to the device, so the library does not generally update it automatically. Therefore, after using curses functions to accumulate a set of desired updates that make sense to present together, call refresh(3NCURSES) to tell the library to make the user's screen look like stdscr. The library optimizes its output by computing a minimal number of operations to mutate the screen from its state at the previous refresh to the new one. Effective optimization demands accurate information about the terminal device: the management of such information is the province of the terminfo(3NCURSES) API, a feature of every standard curses implementation.

Special windows called pads may also be manipulated. These are windows that are not constrained to the size of the terminal screen and whose contents need not be completely displayed. See pad(3NCURSES).

In addition to drawing characters on the screen, rendering attributes and colors may be supported, causing the characters to show up in such modes as underlined, in reverse video, or in color on terminals that support such display enhancements. See attr(3NCURSES).

curses predefines constants for a small set of forms-drawing graphics corresponding to the DEC Alternate Character Set (ACS), a feature of VT100 and other terminals. See waddch(3NCURSES).

curses is implemented using the operating system's terminal driver; keystroke events are received not as scan codes but as byte sequences. Graphical keycaps (alphanumeric and punctuation keys, and the space) appear as-is. Everything else, including the tab, enter/return, keypad, arrow, and function keys, appears as a control character or a multibyte escape sequence. curses translates these into unique key codes. See getch(3NCURSES).

ncurses provides reimplementations of the SVr4 panel(3NCURSES), form(3FORM), and menu(3MENU) libraries to ease construction of user interfaces with curses.

Initialization¶

The selection of an appropriate value of TERM in the process environment is essential to correct curses and terminfo library operation. A well-configured system selects a correct TERM value automatically; tset(1) may assist with troubleshooting exotic situations.

If you change the terminal type, export the TERM environment variable in the shell, then run tset(1) or the “tput init” command. See subsection “Tabs and Initialization” of terminfo(5).

If the environment variables LINES and COLUMNS are set, or if the curses program is executing in a graphical windowing environment, the information obtained thence overrides that obtained by terminfo. An ncurses extension supports resizable terminals; see wresize(3NCURSES).

If the environment variable TERMINFO is defined, a curses program checks first for a terminal type description in the location it identifies. TERMINFO is useful for developing experimental type descriptions or when write permission to /etc/terminfo is not available.

See section “ENVIRONMENT” below.

Naming Conventions¶

curses offers many functions in variant forms using a regular set of alternatives to the name of an elemental one. Those prefixed with “w” require a WINDOW pointer argument; those with a “mv” prefix first perform cursor movement using wmove(3NCURSES); a “mvw” prefix indicates both. The “w” function is typically the elemental one; the removal of this prefix usually indicates operation on stdscr.

Four functions prefixed with “p” require a pad argument.

In function synopses, ncurses man pages apply the following names to parameters.

Wide and Non-wide Character Configurations¶

This manual page describes functions that appear in any configuration of the library. There are two common configurations; see section “ALTERNATE CONFIGURATIONS” below.

ncurses

is the library in its “non-wide” configuration, handling only eight-bit characters. It stores a character combined with attributes in a chtype datum, which is often an alias of int.

Attributes alone (with no corresponding character) can be stored in variables of chtype or attr_t type. In either case, they are represented as an integral bit mask.

Each cell of a WINDOW is stored as a chtype.

ncursesw

is the library in its “wide” configuration, which handles character encodings requiring a larger data type than char (a byte-sized type) can represent. It adds about one third more calls using additional data types that can store such multibyte characters.

The wide library provides additional functions that complement those in the non-wide library where the size of the underlying character type is significant. A somewhat regular naming convention relates many of the wide variants to their non-wide counterparts; where a non-wide function name contains “ch” or “str”, prefix it with “_w” to obtain the wide counterpart. For example, waddch becomes wadd_wch. (Exceptions that add only “w” comprise addwstr, inwstr, and their variants.)

This convention is inapplicable to some non-wide function names, so other transformations are used for the wide configuration: the window background management function “bkgd” becomes “bkgrnd”; the window border-drawing and -clearing functions are suffixed with “_set”; and character attribute manipulation functions like “attron” become “attr_on”.

Function Name Index¶

The following table lists the curses functions provided in the non-wide and wide APIs and the corresponding man pages that describe them. Those flagged with “*” are ncurses-specific, neither described by X/Open Curses nor present in SVr4.

ncurses's screen-pointer extension adds additional functions corresponding to many of the above, each with an “_sp” suffix; see sp_funcs(3NCURSES).

The availability of some extensions is configurable when ncurses is compiled; see sections “ALTERNATE CONFIGURATIONS” and “EXTENSIONS” below.

BAUDRATE¶

The debugging library checks this variable when the application has redirected output to a file. Its integral value is used for the baud rate. If that value is absent or invalid, ncurses uses 9600. This feature allows testers to construct repeatable test cases that take into account optimization decisions that depend on baud rate.

CC (command character)¶

When set, the command_character (cmdch) capability value of loaded terminfo entries changes to the value of this variable. Very few terminfo entries provide this feature.

Because this name is also used in development environments to represent the C compiler's name, ncurses ignores its value if it is not one character in length.

COLUMNS¶

This variable specifies the width of the screen in characters. Applications running in a windowing environment usually are able to obtain the width of the window in which they are executing. If COLUMNS is not defined and the terminal's screen size is not available from the terminal driver, ncurses uses the size specified by the columns (cols) capability of the terminal type's entry in the terminfo database, if any.

It is important that your application use the correct screen size. Automatic detection thereof is not always possible because an application may be running on a host that does not honor NAWS (Negotiations About Window Size) or as a different user ID than the owner of the terminal device file. Setting COLUMNS and/or LINES overrides the library's use of the screen size obtained from the operating system.

The COLUMNS and LINES variables may be specified independently. This property is useful to circumvent misfeatures of legacy terminal type descriptions; xterm(1) descriptions specifying 65 lines were once notorious. For best results, avoid specifying cols and lines capability codes in terminfo descriptions of terminal emulators.

use_env(3NCURSES) can disable use of the process environment in determining the screen size. use_tioctl(3NCURSES) can update COLUMNS and LINES to match the screen size obtained from system calls or the terminal database.

ESCDELAY¶

For curses to distinguish the ESC character resulting from a user's press of the “Escape” key on the input device from one beginning an escape sequence (as commonly produced by function keys), it waits after receiving the escape character to see if further characters are available on the input stream within a short interval. A global variable ESCDELAY stores this interval in milliseconds. The default value of 1000 (one second) is adequate for most uses. This environment variable overrides it.

The most common instance where you may wish to change this value is to work with a remote host over a slow communication channel. If the host running a curses application does not receive the characters of an escape sequence in a timely manner, the library can interpret them as multiple key stroke events.

xterm(1) mouse events are a form of escape sequence; therefore, if your application makes heavy use of multiple-clicking, you may wish to lengthen the default value because the delay applies to the composite multi-click event as well as the individual clicks.

Portable applications should not rely upon the presence of ESCDELAY in either form, but setting the environment variable rather than the global variable does not create problems when compiling an application.

If keypad(3NCURSES) is disabled for the curses window receiving input, a program must disambiguate escape sequences itself.

HOME¶

ncurses may read and write auxiliary terminal descriptions in .termcap and .terminfo files in the user's home directory.

LINES¶

This counterpart to COLUMNS specifies the height of the screen in characters. The corresponding terminfo capability and code is lines. See the description of the COLUMNS variable above.

MOUSE_BUTTONS_123¶

(OS/2 EMX port only) OS/2 numbers a three-button mouse inconsistently with other platforms, such that 1 is the left button, 2 the right, and 3 the middle. This variable customizes the mouse button numbering. Its value must be three digits 1-3 in any order. By default, ncurses assumes a numbering of “132”.

NCURSES_ASSUMED_COLORS¶

If set, this variable overrides the ncurses library's compiled-in assumption that the terminal's default colors are white on black; see default_colors(3NCURSES). Set the foreground and background color values with this environment variable by assigning it two integer values separated by a comma, indicating foregound and background color numbers, respectively.

For example, to tell ncurses not to assume anything about the colors, use a value of “-1,-1”. To make the default color scheme green on black, use “2,0”. ncurses accepts integral values from -1 up to the value of the terminfo max_colors (colors) capability.

NCURSES_CONSOLE2¶

(MinGW port only) The Console2 program defectively handles the Microsoft Console API call CreateConsoleScreenBuffer. Applications that use it will hang. However, it is possible to simulate the action of this call by mapping coordinates, explicitly saving and restoring the original screen contents. Setting the environment variable NCGDB has the same effect.

NCURSES_GPM_TERMS¶

(Linux only) When ncurses is configured to use the GPM interface, this variable may list one or more terminal names against which the TERM variable (see below) is matched. An empty value disables the GPM interface, using ncurses's built-in support for xterm(1) mouse protocols instead. If the variable is absent, ncurses attempts to open GPM if TERM contains “linux”.

NCURSES_NO_HARD_TABS¶

ncurses may use tab characters in cursor movement optimization. In some cases, your terminal driver may not handle them properly. Set this environment variable to any value to disable the feature. You can also adjust your stty(1) settings to avoid the problem.

NCURSES_NO_MAGIC_COOKIE¶

Many terminals store video attributes as a property of a character cell, as curses does. Historically, some recorded changes in video attributes as data that logically occupies character cells on the display, switching attributes on or off, similarly to tags in a markup language; these are termed “magic cookies”, and must be subsequently overprinted. If the terminfo entry for your terminal type does not adequately describe its handling of magic cookies, set this variable to any value to instruct ncurses to disable attributes entirely.

NCURSES_NO_PADDING¶

Most terminal type descriptions in the terminfo database detail hardware devices. Many people use curses-based applications in terminal emulator programs that run in a windowing environment. These programs can duplicate all of the important features of a hardware terminal, but often lack their limitations. Chief among these absent drawbacks is the problem of data flow management; that is, limiting the speed of communication to what the hardware could handle. Unless a hardware terminal is interfaced into a terminal concentrator (which does flow control), an application must manage flow control itself to prevent overruns and data loss.

A solution that comes at no hardware cost is for an application to pause after directing a terminal to execute an operation that it performs slowly, such as clearing the display. Many terminal type descriptions, including that for the VT100, embed delay specifications in capabilities. You may wish to use these terminal descriptions without paying the performance penalty. Set NCURSES_NO_PADDING to any value to disable all but mandatory padding. Mandatory padding is used by such terminal capabilities as flash_screen (flash).

NCURSES_NO_SETBUF¶

(Obsolete) Prior to internal changes developed in ncurses 5.9 (patches 20120825 through 20130126), the library used setbuf(3) to enable fully buffered output when initializing the terminal. This was done, as in SVr4 curses, to increase performance. For testing purposes, both of ncurses and of certain applications, this feature was made optional. Setting this variable disabled output buffering, leaving the output stream in the original (usually line-buffered) mode.

Nowadays, ncurses performs its own buffering and does not require this workaround; it does not modify the buffering of the standard output stream. This approach makes signal handling, as for interrupts, more robust. A drawback is that certain unconventional programs mixed stdio(3) calls with ncurses calls and (usually) got the behavior they expected. This is no longer the case; ncurses does not write to the standard output file descriptor through a stdio-buffered stream.

As a special case, low-level API calls such as putp(3NCURSES) still use the standard output stream. High-level curses calls such as printw(3NCURSES) do not.

NCURSES_NO_UTF8_ACS¶

At initialization, ncurses inspects the TERM environment variable for special cases where VT100 forms-drawing characters (and the corresponding alternate character set terminfo capabilities) are known to be unsupported by terminal types that otherwise claim VT100 compatibility. Specifically, when running in a UTF-8 locale, the Linux virtual console device and the GNU screen(1) program ignore them. Set this variable to a nonzero value to instruct ncurses that the terminal's ACS support is broken; the library then outputs Unicode code points that correspond to the forms-drawing characters. Set it to zero (or a non-integer) to disable the special check for terminal type names matching “linux” or “screen”, directing ncurses to assume that the ACS feature works if the terminal type description advertises it.

As an alternative to use of this variable, ncurses checks for an extended terminfo numeric capability U8 that can be compiled using “tic -x”. Examples follow.

# linux console, if patched to provide working
# VT100 shift-in/shift-out, with corresponding font.
linux-vt100|linux console with VT100 line-graphics,


        U8#0, use=linux,
# uxterm with vt100Graphics resource set to false
xterm-utf8|xterm relying on UTF-8 line-graphics,


        U8#1, use=xterm,

The two-character name “U8” was chosen to permit its use via ncurses's termcap interface.

NCURSES_TRACE¶

At initialization, ncurses (in its debugging configuration) checks for this variable's presence. If defined with an integral value, the library calls curses_trace(3NCURSES) with that value as the argument.

TERM¶

The TERM variable denotes the terminal type. Each is distinct, though many are similar. It is commonly set by terminal emulators to help applications find a workable terminal description. Some choose a popular approximation such as “ansi”, “vt100”, or “xterm” rather than an exact fit to their capabilities. Not infrequently, an application will have problems with that approach; for example, a key stroke may not operate correctly, or produce no effect but seeming garbage characters on the screen.

Setting TERM has no effect on hardware operation; it affects the way applications communicate with the terminal. Likewise, as a general rule (xterm(1) being a rare exception), terminal emulators that allow you to specify TERM as a parameter or configuration value do not change their behavior to match that setting.

TERMCAP¶

If ncurses is configured with termcap support, it checks for a terminal type description in termcap format if one in terminfo format is not available. Setting this variable directs ncurses to ignore the usual termcap database location, /etc/termcap; see TERMPATH below. TERMCAP should contain either a terminal description (with newlines stripped out), or a file name indicating where the information required by the TERM environment variable is stored.

TERMINFO¶

ncurses can be configured to read terminal type description databases in various locations using different formats. This variable overrides the default location.

• Descriptions in terminfo format are normally stored in a directory tree using subdirectories named by the common first letters of the terminal types named therein. This is the scheme used in System V.
• If ncurses is configured to use hashed databases, then TERMINFO may name its location, such as /usr/share/terminfo.db, rather than /usr/share/terminfo/.

The hashed database uses less disk space and is a little faster than the directory tree. However, some applications assume the existence of the directory tree, and read it directly rather than using the terminfo API.

• If ncurses is configured with termcap support, this variable may contain the location of a termcap file.
• If the value of TERMINFO begins with “hex:” or “b64:”, ncurses uses the remainder of the value as a compiled terminfo description. You might produce the base64 format using infocmp(1).

The compiled description is used only if it corresponds to the terminal type identified by TERM.

Setting TERMINFO is the simplest, but not the only, way to direct ncurses to a terminal database. The search path is as follows.

• the last terminal database to which the running ncurses application wrote, if any
• the location specified by the TERMINFO environment variable
• $HOME/.terminfo
• locations listed in the TERMINFO_DIRS environment variable
• location(s) configured and compiled into ncurses

TERMINFO_DIRS¶

This variable specifies a list of locations, akin to PATH, in which ncurses searches for the terminal type descriptions described by TERMINFO above. The list items are separated by colons on Unix and semicolons on OS/2 EMX. System V terminfo lacks a corresponding feature; TERMINFO_DIRS is an ncurses extension.

TERMPATH¶

If TERMCAP does not hold a terminal type description or file name, then ncurses checks the contents of TERMPATH, a list of locations, akin to PATH, in which it searches for termcap terminal type descriptions. The list items are separated by colons on Unix and semicolons on OS/2 EMX.

If both TERMCAP and TERMPATH are unset or invalid, ncurses searches for the files /etc/termcap, /usr/share/misc/termcap, and $HOME/.termcap, in that order.

Error Checking¶

In many cases, X/Open Curses is vague about error conditions, omitting some of the SVr4 documentation.

Unlike other implementations, ncurses checks pointer parameters, such as those to WINDOW structures, to ensure that they are not null. This is done primarily to guard against programmer error. The standard interface does not provide a way for the library to tell an application which of several possible errors occurred. Relying on this (or some other) extension adversely affects the portability of curses applications.

Padding Differences¶

In historical curses implementations, delays embedded in the terminfo capabilities carriage_return (cr), scroll_forward (ind), cursor_left (cub1), form_feed (ff), and tab (ht) activated corresponding delay bits in the Unix terminal driver. ncurses performs all padding by sending NUL bytes to the device. This method is slightly more expensive, but narrows the interface to the Unix kernel significantly and correspondingly increases the package's portability.

Header Files¶

The header file curses.h itself includes the header files stdio.h and unctrl.h.

X/Open Curses has more to say,

but does not finish the story. A more complete account follows.

•

Starting with 4BSD curses (1980) all implementations have provided a curses.h file.

BSD curses code included curses.h and unctrl.h from an internal header file curses.ext, where “ext” abbreviated “externs”.

The implementations of printw and scanw used undocumented internal functions of the standard I/O library (_doprnt and _doscan), but nothing in curses.h itself relied upon stdio.h.

•

SVr2 curses added newterm, which relies upon stdio.h because its function prototype employs the FILE type.

SVr4 curses added putwin and getwin, which also use stdio.h.

X/Open Curses specifies all three of these functions.

SVr4 curses and X/Open Curses do not require the developer to include stdio.h before curses.h. Both document use of curses as requiring only curses.h.

As a result, standard curses.h always includes stdio.h.

•

X/Open Curses and SVr4 curses are inconsistent with respect to unctrl.h.

As noted in util(3NCURSES), ncurses includes unctrl.h from curses.h (as SVr4 does).

•

X/Open Curses's comments about term.h and termios.h may refer to HP-UX and AIX.

HP-UX curses includes term.h from curses.h to declare setupterm in curses.h, but ncurses and Solaris curses do not.

AIX curses includes term.h and termios.h. Again, ncurses and Solaris curses do not.

•

X/Open Curses says that curses.h may include term.h, but does not require it to do so.

Some programs use functions declared in both curses.h and term.h, and must include both header files in the same module. Very old versions of AIX curses required inclusion of curses.h before term.h.

The header files supplied by ncurses include the standard library headers required for its declarations, so ncurses's own header files can be included in any order. But for portability, you should include curses.h before term.h.

•

X/Open Curses says “may make visible” because including a header file does not necessarily make visible all of the symbols in it (consider #ifdef and similar).

For instance, ncurses's curses.h may include wchar.h if the proper symbol is defined, and if ncurses is configured for wide-character support. If wchar.h is included, its symbols may be made visible depending on the value of the _XOPEN_SOURCE feature test macro.

•

X/Open Curses mandates an application's inclusion of one standard C library header in a special case: stdarg.h before curses.h to prototype the functions vw_printw and vw_scanw (as well as the obsolete vwprintw and vwscanw). Each of these takes a variadic argument list, a va_list parameter, like that of printf(3).

SVr3 curses introduced the two obsolete functions, and X/Open Curses the others. In between, SVr4 curses provided for the possibility that an application might include either varargs.h or stdarg.h. These represented contrasting approaches to handling variadic argument lists. The older interface, varargs.h, used a pointer to char for variadic functions' va_list parameter. Later, the list acquired its own standard data type, va_list, defined in stdarg.h, empowering the compiler to check the types of a function call's actual parameters against the formal ones declared in its prototype.

No conforming implementations of X/Open Curses require an application to include stdarg.h before curses.h because they either have allowed for a special type, or, like ncurses, they include stdarg.h themselves to provide a portable interface.