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)