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NAME
user_caps - user-defined terminfo capabilities

SYNOPSIS
tic -x, infocmp -x

DESCRIPTION
Background
Before ncurses 5.0, terminfo databases used a fixed repertoire of terminal capabilities de‐
signed 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, nu‐
meric 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 en‐
sured by using a header which gave the number of items in the tables for each type of capa‐
bility. 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 for‐
mat, 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 ta‐
bles 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 re‐
installing 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. 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 ad‐
vantages over termcap:

• The fixed repertoire prevented users from adding features for unanticipated terminal im‐
provements (or required them to reuse existing capabilities as a workaround).

• The limitation to 16-bit signed integers was also mentioned. Because termcap stores ev‐
erything 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 nonstan‐
dard 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 capabili‐
ties 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 triv‐
ial 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 modi‐
fier combinations. The ncurses database uses a convention based on xterm to provide ex‐
tended 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 fore‐
ground 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.

RGB
boolean, number or string, to assert that the set_a_foreground and set_a_background ca‐
pabilities correspond to direct colors, using an RGB (red/green/blue) convention. This
capability allows the color_content function to return appropriate values without re‐
quiring 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(3X).

Set this capability to a nonzero value to enable it.

XM string, override ncurses's built-in string which enables/disables xterm 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 non‐
zero. 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 re‐
sponses are enough alike that only confusion would result.

The XM capability has a single parameter. If nonzero, the mouse protocol should be en‐
abled. If zero, the mouse protocol should be disabled. ncurses inspects this capabil‐
ity 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 custom‐
ized 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 pa‐
rameters 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 has supported shift, control, alt, and meta modifiers which produce dis‐
tinct 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 added:

Name Description
───────────────────────────────────────────────────────────────
kDC special form of kdch1 (delete character)
kDN special form of kcud1 (cursor down)
kEND special form of kend (End)
kHOM special form of khome (Home)
kLFT special form of kcub1 (cursor-left or cursor-back)
kNXT special form of knext (Next, or Page-Down)
kPRV special form of kprev (Prev, or Page-Up)
kRIT special form of kcuf1 (cursor-right, or cursor-forward)
kUP special form of kcuu1 (cursor-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 al‐
locate 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).

PORTABILITY
The “-x” extension feature of tic and infocmp has been adopted in NetBSD curses. That imple‐
mentation stores user-defined capabilities, but makes no use of these capabilities itself.