user_caps(5) File Formats Manual user_caps(5)
user_caps - user-defined terminfo capabilities
tic -x, infocmp -x
Before ncurses 5.0, terminfo databases used a fixed repertoire of ter-
minal 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 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:
o The binary format itself is not described in the X/Open Curses doc-
umentation. 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.
o 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.
o 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
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:
o The fixed repertoire prevented users from adding features for unan-
ticipated terminal improvements (or required them to reuse existing
capabilities as a workaround).
o The limitation to 16-bit signed integers was also mentioned.
Because termcap stores everything as a string, it could represent
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, deter-
mine 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
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-
Because ncurses provides a termcap library interface, these user-
defined capabilities may be visible to termcap applications:
o 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.
o 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.
o 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 prede-
fined function-key names, to which a series of keys can be
assigned, that is insufficient for more than a dozen keys multi-
plied by more than a couple of modifier combinations. The ncurses
database uses a convention based on xterm to provide extended spe-
Fitting that into termcap's limitation of 2-character names would
be pointless. These extended keys are available only with ter-
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
The command "tput clear" does the same thing.
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:
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.
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
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 char-
acter set capabilities when the locale uses UTF-8 encoding. For
more information, see the discussion of NCURSES_NO_UTF8_ACS in
Set this capability to a nonzero value to enable it.
XM string, override ncurses's built-in string which enables/disables
xterm mouse mode.
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 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 spe-
cial 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:
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:
4 Shift + Alt
6 Shift + Control
7 Alt + Control
8 Shift + Alt + Control
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:
o using a list of extended key names, ask tigetstr(3X) for their val-
o given the list of values, ask key_defined(3X) 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 Net-
BSD curses. That implementation stores user-defined capabilities, but
makes no use of these capabilities itself.
Thomas E. Dickey
beginning with ncurses 5.0 (1999)
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