Storage Location¶

Compiled terminfo descriptions are placed under the directory /usr/share/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: /usr/share/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 /usr/share/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:

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

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 predefined set of Boolean, number 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 due to the predefined data, continues to parse according to its own scheme.

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

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.

setupterm¶

Note that it is possible for setupterm to expect a different set of capabilities than are actually present in the file. Either the database may have been updated since setupterm was recompiled (resulting in extra unrecognized entries in the file) or the program may have been recompiled more recently than the database was updated (resulting in missing entries). The routine setupterm must be prepared for both possibilities - this is why the numbers and sizes are included. Also, new capabilities must always be added at the end of the lists of Boolean, number, and string capabilities.

Binary Format¶

X/Open Curses does not specify a format for the terminfo database. System V curses used a directory-tree of binary files, one per terminal description.

Despite the consistent use of little-endian numbers and the otherwise self-describing format, it is not wise to count on portability of binary terminfo entries between commercial Unix versions. The problem is that there are at least three versions of terminfo (under HP-UX, AIX, and OSF/1) each of which diverged from System V terminfo after SVr1, and added extension capabilities to the string table that (in the binary format) collide with System V and X/Open Curses extensions. See terminfo(5) for detailed discussion of terminfo source compatibility issues.

This implementation is by default compatible with the binary terminfo format used by Solaris curses, except in a few less-used details where it was found that the latter did not match X/Open Curses. The format used by the other Unix versions can be matched by building ncurses with different configuration options.

Magic Codes¶

The magic number in a binary terminfo file is the first 16 bits (two bytes). Besides making it more reliable for the library to check that a file is terminfo, utilities such as file(1) also use that to tell what the file-format is. System V defined more than one magic number, with 0433, 0435 as screen-dumps (see scr_dump(5)). This implementation uses 01036 as a continuation of that sequence, but with a different high-order byte to avoid confusion.

The TERMTYPE Structure¶

Direct access to the TERMTYPE structure is provided for legacy applications. Portable applications should use tigetflag(3NCURSES) and related functions to read terminal capabilities.

Mixed-case Terminal Names¶

A small number of terminal descriptions use uppercase characters in their names. If the underlying file system ignores the difference between uppercase and lowercase, ncurses represents the “first character” of the terminal name used as the intermediate level of a directory tree in (two-character) hexadecimal form.

Limits¶

ncurses stores compiled terminal descriptions in three related formats, described in the subsections

• Legacy Storage Format, and
• Extended Storage Format, and
• Extended Number Format.

The legacy storage format and the extended number format differ by the types of numeric capability that they can store (for example, 16- versus 32-bit integers). The extended storage format introduced by ncurses 5.0 adds data to either of these formats.

Some limitations apply:

• total compiled entries cannot exceed 4096 bytes in the legacy format.
• total compiled entries cannot exceed 32768 bytes in the extended format.
• the name field cannot exceed 128 bytes.

Compiled entries are limited to 32768 bytes because offsets into the strings table use two-byte integers. The legacy format could have supported 32768-byte entries, but was limited to a virtual memory page's 4096 bytes.