term - compiled terminfo terminal description

tic(1) compiles a terminfo terminal type description, and setupterm(3NCURSES) reads it. A compiled description may be stored in a file or in a database of, potentially, many such descriptions. Further, a compiled description may be in one of two formats: one similar to that used by System V, and a newer, extensible format employed exclusively by ncurses.

Storage Location

Compiled terminfo descriptions are placed under the directory /etc/terminfo. One of two configurations is selected when building the ncurses libraries.

directory tree

A two-level scheme is used to avoid a linear search of a huge Unix system directory: /etc/terminfo/c/name where name is the name of the terminal, and c is the first character of name. Thus, the compiled description of terminal type act4 is found in the file /etc/terminfo/a/act4. Synonyms for the same terminal are implemented by multiple links to the same compiled file.

hashed database

Using the Berkeley database API, two types of records are stored: the terminfo data in the same format as that stored in a directory tree with the terminal's primary type name as a key, and records containing only aliases pointing to the primary name.

If built to write hashed databases, ncurses can still read terminfo databases organized as a directory tree, but cannot write entries into the directory tree. It can write (or rewrite) entries in the hashed database.

ncurses distinguishes the two cases in the TERMINFO and TERMINFO_DIRS environment variable by assuming a directory tree for entries that correspond to an existing directory, and a hashed database otherwise.

Legacy Storage Format

The format has been chosen so that it will be the same on all hardware. A byte of at least eight bits' width is assumed, but no assumptions about bit ordering or sign extension are made.

The file is divided into six parts:

1. header,

2. terminal names,

3. Boolean flags,

4. numbers,

5. strings, and

6. a string table.

The header section begins the file. This section contains six short integers in the format described below. These integers are

(1) the magic number

(octal 0432);

(2) the size,

in bytes, of the terminal names section;

(3) the number of bytes in the Boolean flags section;

(4) the number of short integers in the numbers section;

(5) the number of offsets

(short integers) in the strings section;

(6) the size,

in bytes, of the string table.

The capabilities in the Boolean flags, numbers, and strings sections are in the same order as in the header file term.h.

Short integers are signed, in the range -32768 to 32767, and stored in little-endian format.

Numbers in a terminal description, whether they are entries in the numbers or strings table, are positive integers. Boolean flags are treated as positive one-byte integers. In each case, those positive integers represent a terminal capability. The terminal compiler tic uses negative integers to handle the cases where a capability is not available:

• If a capability is absent from this terminal, tic stores a -1 in the corresponding table.

  The integer value -1 is represented by two bytes 0377, 0377.
  Absent Boolean values are represented by the byte 0 (false).

• If a capability has been canceled from this terminal, tic stores a -2 in the corresponding table.

  The integer value -2 is represented by two bytes 0377, 0376.
  The Boolean value -2 is represented by the byte 0376.
  

• Other negative values are illegal.

The terminal names section comes after the header. It contains the first line of the terminfo description, listing the various names for the terminal, separated by the | character. The terminal names section is terminated with an ASCII NUL character.

The Boolean flags section has one byte for each flag. Boolean capabilities are either 1 or 0 (true or false) according to whether the terminal supports the given capability or not.

Between the Boolean flags section and the number section, a null byte will be inserted, if necessary, to ensure that the number section begins on an even byte This is a relic of the PDP-11's word-addressed architecture, originally designed to avoid traps induced by addressing a word on an odd byte boundary. All short integers are aligned on a short word boundary.

The numbers section is similar to the Boolean flags section. Each capability takes up two bytes, and is stored as a little-endian short integer.

The strings section is also similar. Each capability is stored as a short integer. The capability value is an index into the string table.

The string table is the last section. It contains all of the values of string capabilities referenced in the strings section. Each string is null-terminated. Special characters in X or \c notation are stored in their interpreted form, not the printing representation. Padding information $<nn> and parameter information %x are stored intact in uninterpreted form.

Extended Storage Format

The previous section describes the conventional terminfo binary format. With some minor variations of the offsets (see PORTABILITY), the same binary format is used in all modern Unix systems. Each system uses a standard set of Boolean, numeric, or string capabilities.

The ncurses libraries and applications support extended terminfo binary format, allowing users to define capabilities that are loaded at runtime. This extension is made possible by using the fact that the other implementations stop reading the terminfo data when they reach the end of the size given in the header. ncurses checks the size, and if it exceeds that specified in the header, continues to parse according to its own scheme.

First, it reads the extended header (5 short integers):

(1)

count of extended Boolean capabilities

(2)

count of extended numeric capabilities

(3)

count of extended string capabilities

(4)

count of the items in extended string table

(5)

size of the extended string table in bytes

The count- and size-values for the extended string table include the extended capability names as well as extended capability values.

Using the counts and sizes, ncurses allocates arrays and reads data for the extended capabilities in the same order as the header information.

The extended string table contains values for string capabilities. After the end of these values, it contains the names for each of the extended capabilities in order: Boolean, numeric, and string.

By storing terminal descriptions in this way, ncurses is able to provide a database useful with legacy applications, as well as providing data for applications that require more information about a terminal type than was anticipated by X/Open Curses. See user_caps(5) for an overview of the way ncurses uses this extended information.

Applications that manipulate terminal data can use the definitions described in terminfo_variables(3NCURSES) associating the long capability names with members of a TERMTYPE structure.

Extended Number Format

On occasion, 16-bit signed integers are not large enough. ncurses 6.1 introduced a new format by making a few changes to the legacy format:

• a different magic number (octal 01036)

• changing the type for the number array from signed 16-bit integers to signed 32-bit integers.

To maintain compatibility, the library presents the same data structures to direct users of the TERMTYPE structure as in previous formats. However, that cannot provide callers with the extended numbers. The library uses a similar but hidden data structure TERMTYPE2 to provide data for the terminfo functions.

Here is a terminfo description of the Lear-Siegler ADM-3, a popular though rather stupid early terminal.

adm3a|lsi adm3a,
        am,
        cols#80, lines#24,
        bel=G, clear=\032$<1>, cr=M, cub1=H, cud1=J,
        cuf1=L, cup=\E=%p1%{32}%+%c%p2%{32}%+%c, cuu1=K,
        home=, ind=J,

A hexadecimal dump of its compiled terminal description (in legacy format) follows.

TABLE

ncurses(3NCURSES), terminfo(3NCURSES), terminfo(5), user_caps(5)