user_caps - user-defined terminfo capabilities
tic -x, infocmp -x
Before ncurses 5.0, terminfo databases used a fixed repertoire
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
- 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 4096-byte entries.
- The format cannot represent very large numeric
capabilities, nor can it represent large numbers of special keyboard
- The tables of capability names differ between
- 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.
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 numbers.
Although termcap's extensibility was rarely used (it was never the
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
because no modifications to the toolset's predefined
capability names are needed.
The ncurses utilities tic
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
When compiling a terminal database, if “-x” is set, tic
will store a user-defined capability if the capability name is not one of the
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 to provide extended special-key names.
- Fitting that into termcap's limitation of 2-character names
would be pointless. These extended keys are available only with
The ncurses library uses the user-definable capabilities. While the terminfo
database may have other extensions, ncurses makes explicit checks for these:
- boolean, asserts that the terminal interprets SGR 39
and SGR 49 by resetting the foreground and background color, respectively,
to the default.
- This is a feature recognized by the screen program
- string, tells how to clear the terminal's scrollback
buffer. When present, the clear(1) program sends this before
clearing the terminal.
- The command “tput clear” does the same
- boolean, number or string, 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
- The capability type determines the values which ncurses
- 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
- 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
- explicitly list the number of bits used for red, green and
blue components as a slash-separated list of decimal integers.
- boolean, 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 mouse mode.
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.
Since 1999, xterm
has supported shift
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
“k” and that shifted special keys are an uppercase name,
ncurses' terminal database defines these names to which a suffix is added:
||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:
- 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 “-x” extension feature of tic
been adopted in NetBSD curses. That implementation stores user-defined
capabilities, but makes no use of these capabilities itself.
Thomas E. Dickey
beginning with ncurses 5.0 (1999)