Provided by: ncurses-bin_6.1-1ubuntu1.18.04.1_amd64 
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
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.
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 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.
RGB
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 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.
U8 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 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.
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
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 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
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 wgetch(3X).
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.
SEE ALSO
tic(1), infocmp(1).
AUTHORS
Thomas E. Dickey
beginning with ncurses 5.0 (1999)
user_caps(5)