user_caps(5) - Man pages

user_caps(5) File formats user_caps(5)

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 termi-
nal capabilities designed for the SVr2 terminal database in 1984, and
extended in stages through SVr4 (1989), and standardized in X/Open
Curses starting 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 ter-
minfo 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 docu-
mentation. Only the source format is described.

Library developers rely upon the SVr4 documentation, and reverse-en-
gineering 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 con-
figured 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 com-
pile-time using a text file which lists the different terminal capa-
bilities.

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 sys-
tems 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 capa-
bility 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 unan-
ticipated terminal improvements (or required them to reuse existing
capabilities as a workaround).

• The limitation to 16-bit signed integers was also mentioned. Be-
cause 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-de-
fined 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 prede-
fined 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 applica-
tion, 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, con-
trol, 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 con-
vention 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 ter-
minal.

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 de-
vice attributes.

RGB
Boolean, number or string, used to assert that the set_a_fore-
ground and set_a_background capabilities correspond to direct col-
ors, 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 com-
parison 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 de-
fine 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 char-
acter 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 cer-
tain 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 mouse-
mask function is nonzero. By default, ncurses handles the re-
sponses 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 re-
sponses 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 com-
binations 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 modi-
fiers which produce distinct special-key strings. In a terminal de-
scription, ncurses has no special knowledge of the modifiers used. Ap-
plications 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 suf-
fix 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)