Provided by: ncurses-bin_6.4+20240113-1ubuntu2_amd64 
NAME
user_caps - user-defined terminfo capability format
SYNOPSIS
infocmp -x
tic -x
DESCRIPTION
Background
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 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 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 capabilities.
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:
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). The box1 capability is used in tic to help with
terminal descriptions written for AIX.
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 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 tic and infocmp 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, tic 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.
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 uses the user-definable capabilities. While the terminfo database may
have other extensions, ncurses makes explicit checks for these:
AX 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 as well.
E3 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 thing.
NQ Boolean, used to suppress a consistency check in tic 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.
RGB
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:
Boolean
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.
number
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
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.
U8 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 ncurses(3NCURSES).
Set this capability to a nonzero value to enable it.
XM 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 data-driven.
xm shows the format of the mouse responses. In this experimental capability, the
parameters are
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
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(3NCURSES) for their values, and
• given the list of values, ask key_defined(3NCURSES) for the key-code which would be
returned for those keys by wgetch(3NCURSES).
PORTABILITY
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.
AUTHORS
Thomas E. Dickey
beginning with ncurses 5.0 (1999)
SEE ALSO
infocmp(1), tic(1)
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 that are
used in these extended capabilities.