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:

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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.

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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.

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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:

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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:

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:

These are the suffixes used to denote the modifiers:

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).