user_caps - user-defined terminfo capabilities
tic -x, infocmp -x
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.
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
- 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%;,
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
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(3NCURSES)
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.
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 which are used in these extended capabilities.
Thomas E. Dickey
beginning with ncurses 5.0 (1999)