user_caps - user-defined terminfo capabilities
Before ncurses 5.0, terminfo databases used a fixed repertoire of
terminal capabilities designed for the SVr2 terminal database in 1984, and
extended in stages through SVr4 (1989), and standardized in the Single Unix
Specification beginning in 1995.
Most of the extensions in this fixed repertoire were
additions to the tables of boolean, numeric and string capabilities. Rather
than change the meaning of an existing capability, a new name was added. The
terminfo database uses a binary format; binary compatibility was ensured by
using a header which gave the number of items in the tables for each type of
capability. The standardization was incomplete:
- The binary format itself is not described in the X/Open Curses
documentation. Only the source format is described.
- Library developers rely upon the SVr4 documentation, and
reverse-engineering the compiled terminfo files to match the binary
- Lacking a standard for the binary format, most implementations copy the
SVr2 binary format, which uses 16-bit signed integers, and is limited to
- The format cannot represent very large numeric capabilities, nor can it
represent large numbers of special keyboard definitions.
- The tables of capability names differ between implementations.
- Although they may provide all of the standard capability names, the
position in the tables differs because some features were added as needed,
while others were added (out of order) to comply with X/Open Curses.
- While ncurses' repertoire of predefined capabilities is closest to
Solaris, Solaris's terminfo database has a few differences from the list
published by X/Open Curses. For example, ncurses can be configured with
tables which match the terminal databases for AIX, HP-UX or OSF/1, rather
than the default Solaris-like configuration.
- In SVr4 curses and ncurses, the terminal database is defined at
compile-time using a text file which lists the different terminal
- In principle, the text-file can be extended, but doing this requires
recompiling and reinstalling the library. The text-file used in ncurses
for terminal capabilities includes details for various systems past the
documented X/Open Curses features. For example, ncurses supports these
capabilities in each configuration:
- (meml) lock memory above cursor
- (memu) unlock memory
- (box1) box characters primary set
- The memory lock/unlock capabilities were included because they were used
in the X11R6 terminal description for xterm(1). The box1
capability is used in gtic to help with terminal descriptions written for
During the 1990s, some users were reluctant to use terminfo in
spite of its performance advantages over termcap:
- The fixed repertoire prevented users from adding features for
unanticipated terminal improvements (or required them to reuse existing
capabilities as a workaround).
- The limitation to 16-bit signed integers was also mentioned. Because
termcap stores everything as a string, it could represent larger
Although termcap's extensibility was rarely used (it was never the
speaker who had actually used the feature), the criticism had a
point. ncurses 5.0 provided a way to detect nonstandard capabilities,
determine their type and optionally store and retrieve them in a way which
did not interfere with other applications. These are referred to as
user-defined capabilities because no modifications to the toolset's
predefined capability names are needed.
The ncurses utilities gtic and ginfocmp have a
command-line option “-x” to control whether the nonstandard
capabilities are stored or retrieved. A library function
use_extended_names is provided for the same purpose.
When compiling a terminal database, if “-x” is set,
gtic will store a user-defined capability if the capability name is
not one of the predefined names.
Because ncurses provides a termcap library interface, these
user-defined capabilities may be visible to termcap applications:
- The termcap interface (like all implementations of termcap) requires that
the capability names are 2-characters.
- When the capability is simple enough for use in a termcap application, it
is provided as a 2-character name.
- There are other user-defined capabilities which refer to features not
usable in termcap, e.g., parameterized strings that use more than two
parameters or use more than the trivial expression support provided by
termcap. For these, the terminfo database should have only capability
names with 3 or more characters.
- Some terminals can send distinct strings for special keys (cursor-,
keypad- or function-keys) depending on modifier keys (shift, control,
etc.). While terminfo and termcap have a set of 60 predefined function-key
names, to which a series of keys can be assigned, that is insufficient for
more than a dozen keys multiplied by more than a couple of modifier
combinations. The ncurses database uses a convention based on
xterm(1) to provide extended special-key names.
The ncurses library uses the user-definable capabilities. While the terminfo
database may have other extensions, ncurses makes explicit checks for these:
- Fitting that into termcap's limitation of 2-character names would be
pointless. These extended keys are available only with terminfo.
Several terminals provide the ability to send distinct strings for combinations
of modified special keys. There is no standard for what those keys can send.
- boolean, asserts that the terminal interprets SGR 39 and SGR 49 by
resetting the foreground and background color, respectively, to the
- This is a feature recognized by the screen program as well.
- string, tells how to clear the terminal's scrollback buffer. When
present, the clear(1) program sends this before clearing the
- The command “tput clear” does the same thing.
- used to suppress a consistency check in gtic for the ncurses capabilities
in user6 through user9 (u6, u7, u8 and u9) which tell how to query the
terminal's cursor position and its device attributes.
- boolean, number or string, used to assert that
the set_a_foreground and set_a_background capabilities
correspond to direct colors, using an RGB (red/green/blue)
convention. This capability allows the color_content function to
return appropriate values without requiring the application to initialize
colors using init_color.
- The capability type determines the values which ncurses sees:
- implies that the number of bits for red, green and blue are the same.
Using the maximum number of colors, ncurses adds two, divides that sum by
three, and assigns the result to red, green and blue in that order.
- If the number of bits needed for the number of colors is not a multiple of
three, the blue (and green) components lose in comparison to red.
- tells ncurses what result to add to red, green and blue. If ncurses runs
out of bits, blue (and green) lose just as in the boolean
- explicitly list the number of bits used for red, green and blue components
as a slash-separated list of decimal integers.
- Because there are several RGB encodings in use, applications which make
assumptions about the number of bits per color are unlikely to work
reliably. As a trivial case, for example, one could define RGB#1 to
represent the standard eight ANSI colors, i.e., one bit per color.
- number, asserts that ncurses must use Unicode values for
line-drawing characters, and that it should ignore the alternate character
set capabilities when the locale uses UTF-8 encoding. For more
information, see the discussion of NCURSES_NO_UTF8_ACS in
- Set this capability to a nonzero value to enable it.
- string, override ncurses's built-in string which enables/disables
xterm(1) mouse mode.
- ncurses sends a character sequence to the terminal to initialize mouse
mode, and when the user clicks the mouse buttons or (in certain modes)
moves the mouse, handles the characters sent back by the terminal to tell
it what was done with the mouse.
- The mouse protocol is enabled when the mask passed in the
mousemask function is nonzero. By default, ncurses handles the
responses for the X11 xterm mouse protocol. It also knows about the SGR
1006 xterm mouse protocol, but must to be told to look for this
specifically. It will not be able to guess which mode is used, because the
responses are enough alike that only confusion would result.
- The XM capability has a single parameter. If nonzero, the mouse
protocol should be enabled. If zero, the mouse protocol should be
disabled. ncurses inspects this capability if it is present, to see
whether the 1006 protocol is used. If so, it expects the responses to use
the SGR 1006 xterm mouse protocol.
- The xterm mouse protocol is used by other terminal emulators. The terminal
database uses building-blocks for the various xterm mouse protocols which
can be used in customized terminal descriptions.
- The terminal database building blocks for this mouse feature also have an
experimental capability xm. The “xm” capability
describes the mouse response. Currently there is no interpreter which
would use this information to make the mouse support completely
- xm shows the format of the mouse responses. In this experimental
capability, the parameters are
- state, e.g., pressed or released
- y-ordinate starting region
- x-ordinate starting region
- y-ordinate ending region
- x-ordinate ending region
- Here are examples from the terminal database for the most commonly used
xterm mouse protocols:
xterm+x11mouse|X11 xterm mouse protocol,
Since 1999, xterm(1) has supported shift,
control, alt, and meta modifiers which produce distinct
special-key strings. In a terminal description, ncurses has no special
knowledge of the modifiers used. Applications can use the naming
convention established for xterm to find these special keys in
the terminal description.
Starting with the curses convention that key names begin
with “k” and that shifted special keys are an uppercase name,
ncurses' terminal database defines these names to which a suffix is
||special form of kdch1 (delete character)
||special form of kcud1 (cursor down)
||special form of kend (End)
||special form of khome (Home)
||special form of kcub1 (cursor-left or cursor-back)
||special form of knext (Next, or Page-Down)
||special form of kprev (Prev, or Page-Up)
||special form of kcuf1 (cursor-right, or cursor-forward)
||special form of kcuu1 (cursor-up)
These are the suffixes used to denote the modifiers:
||Shift + Alt
||Shift + Control
||Alt + Control
||Shift + Alt + Control
||Meta + Shift
||Meta + Alt
||Meta + Alt + Shift
||Meta + Ctrl
||Meta + Ctrl + Shift
||Meta + Ctrl + Alt
||Meta + Ctrl + Alt + Shift
None of these are predefined; terminal descriptions can refer to
names which ncurses will allocate at runtime to key-codes. To
use these keys in an ncurses program, an application could do this:
The “-x” extension feature of gtic and ginfocmp has
been adopted in NetBSD curses. That implementation stores user-defined
capabilities, but makes no use of these capabilities itself.
- using a list of extended key names, ask tigetstr(3X) for
their values, and
- given the list of values, ask key_defined(3X) for the
key-code which would be returned for those keys by
The terminal database section NCURSES USER-DEFINABLE
CAPABILITIES summarizes commonly-used user-defined capabilities which
are used in the terminal descriptions. Some of those features are mentioned
in screen(1) or tmux(1).
XTerm Control Sequences provides further information on the
xterm(1) features which are used in these extended capabilities.
Thomas E. Dickey
beginning with ncurses 5.0 (1999)