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user_caps(5) File formats user_caps(5)
user_caps - user-defined terminfo capability format
infocmp -x
tic -x
Prior to ncurses 5.0 (1999), terminfo databases used a fixed repertoire
of terminal capabilities designed for the SVr2 terminal database in
1984, added to in stages through SVr4 (1989), and standardized in
X/Open Curses starting in 1995.
Most such additions to this fixed repertoire suppelmented the tables of
Boolean, numeric, and string capabilities. Rather than changing 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 that counted the number of items in the tables for each
type of capability. Because each curses vendor extended the standard
capability lists in distinct ways, a library could be programmed to
recognize only compiled terminfo entries that it was prepared to
interpret. Standardization was incomplete.
o X/Open Curses describes only the source format, not its binary
representation on disk.
Library developers rely upon SVr4 documentation and reverse
engineering of compiled terminfo files to match the binary format.
o 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 SVr2 format cannot represent very large numeric capability
values, nor can it represent large numbers of key definitions, as
are required to distinguish multiple modifier keys used in
combination with a function key.
o The tables of capability names differ between implementations.
Although they may provide all of the standard capability names,
each arranges its table entries differently because some features
were added as needed, while others were added -- out of order --
for X/Open Curses conformance.
While ncurses's capability repertoire is closest to that of
Solaris, the set of capabilities supported by each vendor's term-
info database differs from the list published by X/Open Curses.
ncurses can be configured with tables that match the terminal
databases for AIX, HP-UX, or OSF/1, rather than the default
Solaris-like configuration.
o In SVr4 curses and ncurses, the terminal database is defined at
compile time by interpolating a text file that lists the different
terminal capabilities.
In principle, the text file can be extended, but doing so requires
recompiling and reinstalling the library. The text file used by
ncurses for terminal capabilities includes details of extensions to
X/Open Curses made by various systems. For example, ncurses
supports the following nonstandard capabilities in each
configuration.
memory_lock
(meml) lock memory above cursor
memory_unlock
(memu) unlock memory
box_chars_1
(box1) box characters primary set
The memory lock/unlock capabilities were included because they were
used in the X11R6 terminal description for xterm(1). tic uses the
box1 capability to cope with terminal descriptions written for AIX.
During the 1990s, some application developers were reluctant to use
terminfo in spite of its performance (and other) advantages over
termcap.
o The fixed repertoire prevented users from adding support for
terminal features unanticipated by X/Open Curses (or required them
to reuse existing capabilities as a workaround).
o 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 -- the claimant was
never an implementor who had actually exercised it -- the criticism had
a point. ncurses 5.0 provided a way to detect nonstandard
capabilities, to determine their type, and to optionally store and
retrieve them in a way that did not interfere with other applications.
ncurses terms these user-defined capabilities because no modifications
to the standard capability list are needed.
The ncurses utilities tic and infocmp have a command-line option "-x"
to control whether the nonstandard capabilities are stored or
retrieved. ncurses provides use_extended_names(3x) to programs for the
same purpose.
When compiling a terminal database, if "-x" is used, tic stores a user-
defined capability if the capability name is not standard.
Because ncurses provides a termcap library interface, these user-
defined capabilities may be visible to termcap applications.
o The termcap interface (like all implementations of termcap)
restricts capability names to two characters.
When the capability is simple enough for use in a termcap
application, it is provided as a two-character name.
o Other user-defined capabilities employ features not usable in
termcap, such as parameterized strings that use more than two
parameters or require more powerful expressions than termcap
supports. Such capabilities should, in the terminfo database, have
names at least three characters in length.
o 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 define a set of sixty
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.
Fitting that into termcap's limitation of 2-character names would
be pointless. These extended keys are available only with term-
info.
The ncurses library employs user-definable capabilities. While the
terminfo database may have other extensions, ncurses makes explicit
checks for the following.
AX (Boolean) asserts that the terminal interprets SGR 39 and SGR 49
by resetting the foreground and background colors, respectively,
to the default.
screen(1) recognizes this capability as well.
E3 (string) tells an application 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 thing.
NQ (Boolean) suppresses a consistency check in tic for the ncurses
string capabilities user6 (u6) through user9 (u9), which tell an
application how to query the terminal's cursor position and its
device attributes.
RGB
(Boolean, numeric, or string) asserts that the set_a_foreground
(setaf) and set_a_background (setab) capabilities employ direct
colors, using an RGB (red/green/blue) convention. This
capability allows color_content(3x) to return appropriate values
without requiring the application to initialize colors using
init_color(3x).
The capability type determines the values ncurses sees.
Boolean
implies that the number of bits for red, green, and blue are
the same. Starting with the value of the capability
max_colors (colors; termcap: co), ncurses adds two, divides
the 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) color channels lose in
comparison to red.
numeric
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 case.
string
specify the quantity of bits used for red, green, and blue
color channels as a slash-separated list of decimal integers.
Because there are several RGB encodings in use, applications that
make assumptions about the number of bits per color channel are
unlikely to work reliably. As a trivial case, one could define
RGB#1 to represent the standard eight ANSI X3.64/ECMA-48/ISO 6429
colors using one bit per color channel.
U8 (numeric) asserts whether ncurses must use Unicode values for
line-drawing characters, and that it should ignore the alternate
character set (ACS) capabilities when the locale uses UTF-8
encoding. See the discussion of NCURSES_NO_UTF8_ACS in section
"ENVIRONMENT" of ncurses(3x).
Set this capability to a nonzero value to enable it.
XM (string) override ncurses's built-in string that directs xterm(1)
to enable or disable 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 the application what was done with the
mouse.
The mouse protocol is enabled when the mask argument to the
mousemask(3x) 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 it specifically. ncurses is not be able to guess which
of the two modes is used, because the responses are enough alike
that only confusion would result.
The XM capability has a single numeric 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 usable in customized terminal descriptions.
The terminal database building blocks for this mouse feature also
have an experimental capability, xm, that describes the mouse
response. No known interpreter uses this information, which
could make mouse support completely data-driven.
xm shows the format of the mouse responses. In this experimental
capability, the parameters are as follows.
p1 y-ordinate
p2 x-ordinate
p3 button
p4 state, e.g., pressed or released
p5 y-ordinate starting region
p6 x-ordinate starting region
p7 y-ordinate ending region
p8 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,
kmous=\E[M, XM=\E[?1000%?%p1%{1}%=%th%el%;,
xm=\E[M
%?%p4%t%p3%e%{3}%;%' '%+%c
%p2%'!'%+%c
%p1%'!'%+%c,
xterm+sm+1006|xterm SGR-mouse,
kmous=\E[<, XM=\E[?1006;1000%?%p1%{1}%=%th%el%;,
xm=\E[<%i%p3%d;
%p1%d;
%p2%d;
%?%p4%tM%em%;,
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(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 capability codes describing
the input generated by a terminal's key caps begin with "k", and that
shifted special keys use uppercase letters in their names, ncurses's
terminal database defines the following names and codes to which a
suffix is added.
Code Description
-------------------------------------------------------------------
kDC shifted kdch1 (delete character)
kDN shifted kcud1 (cursor down)
kEND shifted kend (end)
kHOM shifted khome (home)
kLFT shifted kcub1 (cursor back)
kNXT shifted knext (next)
kPRV shifted kprev (previous)
kRIT shifted kcuf1 (cursor forward)
kUP shifted kcuu1 (cursor up)
Keycap nomenclature on the Unix systems for which curses was developed
differs from today's ubiquitous descendants of the IBM PC/AT keyboard
layout. In the foregoing, interpret "backward" as "left", "forward" as
"right", "next" as "page down", and "prev(ious)" as "page up".
These are the suffixes used to denote the modifiers:
Value Description
----------------------------------
2 Shift
3 Alt
4 Shift + Alt
5 Control
6 Shift + Control
7 Alt + Control
8 Shift + Alt + Control
9 Meta
10 Meta + Shift
11 Meta + Alt
12 Meta + Alt + Shift
13 Meta + Ctrl
14 Meta + Ctrl + Shift
15 Meta + Ctrl + Alt
16 Meta + Ctrl + Alt + Shift
ncurses defines no capabilities for modified F-keys; terminal
descriptions can refer to names that ncurses allocates at runtime to
key codes. To use these keys in an ncurses program, an application
could do this:
o using a list of extended key names, ask tigetstr(3x) for their
values, and
o given the list of values, ask key_defined(3x) for the key-code
which would be returned for those keys by wgetch(3x).
The "-x" extension feature of tic and infocmp has 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)
infocmp(1m), tic(1m)
In the source form of the terminal database, terminfo.src, the section
"NCURSES USER-DEFINABLE CAPABILITIES". summarizes commonly-used user-
defined capabilities employed 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 that are used in these extended capabilities.
ncurses 6.5 2025-08-16 user_caps(5)