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
- Lacking a standard for the binary format, most implementations copy the
SVr2 binary format, which uses 16-bit signed integers, and is limited to
- 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
- 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:
- (meml) lock memory above cursor
- (memu) unlock memory
- (box1) box characters primary set
- The memory lock/unlock capabilities were included because they were used
in the X11R6 terminal description for xterm. The box1
capability is used in tic to help with terminal descriptions written for
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
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.
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.
- Fitting that into termcap's limitation of 2-character names would be
pointless. These extended keys are available only with terminfo.
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
||special form of kdch1 (delete character)
||special form of kcud1 (cursor down)
||special form of kend (End)
||special form of khome (Home)
||special form of kcub1 (cursor-left or cursor-back)
||special form of knext (Next, or Page-Down)
||special form of kprev (Prev, or Page-Up)
||special form of kcuf1 (cursor-right, or cursor-forward)
||special form of kcuu1 (cursor-up)
These are the suffixes used to denote the modifiers:
||Shift + Alt
||Shift + Control
||Alt + Control
||Shift + Alt + Control
||Meta + Shift
||Meta + Alt
||Meta + Alt + Shift
||Meta + Ctrl
||Meta + Ctrl + Shift
||Meta + Ctrl + Alt
||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