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NAME | SYNOPSIS | DESCRIPTION | PORTABILITY | AUTHORS | SEE ALSO | COLOPHON |
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user_caps(5) File formats user_caps(5)
user_caps - user-defined terminfo capability format
infocmp -x
tic -x
Background
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.
• 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.
• 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.
• 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
terminfo 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.
• 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.
• 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).
• 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.
• 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.
• 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.
• 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 terminfo.
Recognized Capabilities
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%;,
Extended Key Definitions
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:
• 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 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.
This page is part of the ncurses (new curses) project.
Information about the project can be found at
⟨https://invisible-island.net/ncurses/ncurses.html⟩. If you have a
bug report for this manual page, send it to bug-ncurses@gnu.org.
This page was obtained from the tarball ncurses-6.6.tar.gz fetched
from ⟨https://ftp.gnu.org/gnu/ncurses/⟩ on 2026-01-16. If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for the
page, or you have corrections or improvements to the information
in this COLOPHON (which is not part of the original manual page),
send a mail to man-pages@man7.org
ncurses @NCURSES_MAJOR@.@NCU... 2025-11-11 user_caps(5)
Pages that refer to this page: infocmp(1m), tic(1m), tput(1), curs_termcap(3x), ncurses(3x), term(5), terminfo(5)
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HTML rendering created 2026-01-16 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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