Termcap (''terminal capability'') is a software

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and

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used on

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computers. It enables programs to use display

computer terminal A computer terminal is an electronic or electromechanical hardware device that can be used for entering data into, and transcribing data from, a computer or a computing system. The teletype was an example of an early-day hard-copy terminal and ...

s in a device-independent manner, which greatly simplifies the process of writing portable

text mode Text mode is a computer display mode in which content is internally represented on a computer screen in terms of characters rather than individual pixels. Typically, the screen consists of a uniform rectangular grid of ''character cells'', each ...

applications.

Bill Joy William Nelson Joy (born November 8, 1954) is an American computer engineer and venture capitalist. He co-founded Sun Microsystems in 1982 along with Scott McNealy, Vinod Khosla, and Andy Bechtolsheim, and served as Chief Scientist and CTO at ...

wrote the first termcap library in 1978 for the

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; it has since been ported to most Unix and Unix-like environments, even

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. Joy's design was reportedly influenced by the design of the terminal data store in the earlier

Incompatible Timesharing System Incompatible Timesharing System (ITS) is a time-sharing operating system developed principally by the MIT Artificial Intelligence Laboratory, with help from Project MAC. The name is the jocular complement of the MIT Compatible Time-Sharing Sy ...

. A termcap database can describe the capabilities of hundreds of different display terminals. This allows programs to have character-based display output, independent of the type of terminal. On-screen text editors such as vi and

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are examples of programs that may use termcap. Other programs are listed in the

Termcap Termcap (''terminal capability'') is a software library and database used on Unix-like computers. It enables programs to use display computer terminals in a device-independent manner, which greatly simplifies the process of writing portable text ...

category. Examples of what the database describes: *how many columns wide the display is *what string to send to move the cursor to an arbitrary position (including how to encode the row and column numbers) *how to scroll the screen up one or several lines *how much padding is needed for such a scrolling operation.

Data model

Termcap databases consist of one or more descriptions of terminals.

Indices

Each description must contain the

canonical The adjective canonical is applied in many contexts to mean "according to the canon" the standard, rule or primary source that is accepted as authoritative for the body of knowledge or literature in that context. In mathematics, "canonical example ...

name of the terminal. It may also contain one or more aliases for the name of the terminal. The canonical name or aliases are the keys by which the library searches the termcap database.

Data values

The description contains one or more capabilities, which have conventional names. The capabilities are typed: ''boolean'', ''numeric'' and ''string''. The termcap library has no predetermined type for each capability name. It determines the types of each capability by the syntax: * ''string'' capabilities have an "=" between the capability name and its value, * ''numeric'' capabilities have a "#" between the capability name and its value, and * ''boolean'' capabilities have no associated value (they are always ''true'' if specified). Applications which use termcap do expect specific types for the commonly used capabilities, and obtain the values of capabilities from the termcap database using library calls that return successfully only when the database contents matches the assumed type.

Hierarchy

Termcap descriptions can be constructed by including the contents of one description in another, suppressing capabilities from the included description or overriding or adding capabilities. No matter what storage model is used, the termcap library constructs the terminal description from the requested description, including, suppressing or overriding at the time of the request.

Storage model

Termcap data is stored as text, making it simple to modify. The text can be retrieved by the termcap library from files or environment variables.

Environment variables

The TERM

environment variable An environment variable is a dynamic-named value that can affect the way running processes will behave on a computer. They are part of the environment in which a process runs. For example, a running process can query the value of the TEMP env ...

contains the terminal type name. The TERMCAP environment variable may contain a termcap database. It is most often used to store a single termcap description, set by a terminal emulator to provide the terminal's characteristics to the shell and dependent programs. The TERMPATH environment variable is supported by newer termcap implementations and defines a search path for termcap files.

Flat file

The original (and most common) implementation of the termcap library retrieves data from a flat text file. Searching a large termcap file, e.g., 500 kB, can be slow. To aid performance, a utility such as reorder is used to put the most frequently used entries near the beginning of the file.

Hashed database

4.4BSD based implementations of termcap store the terminal description in a hashed database (e.g., something like

Berkeley DB Berkeley DB (BDB) is an unmaintained embedded database software library for key/value data, historically significant in open source software. Berkeley DB is written in C with API bindings for many other programming languages. BDB stores arbitr ...

version 1.85). These store two types of records: aliases which point to the canonical entry, and the canonical entry itself. The text of the termcap entry is stored literally.

Limitations and extensions

The original termcap implementation was designed to use little memory: * the first name is two characters, to fit in 16 bits * capability names are two characters * descriptions are limited to 1023 characters. * only one termcap entry with its definitions can be included, and must be at the end. Newer implementations of the termcap interface generally do not require the two-character name at the beginning of the entry. Capability names are still two characters in all implementations. The tgetent function used to read the terminal description uses a buffer whose size must be large enough for the data, and is assumed to be 1024 characters. Newer implementations of the termcap interface may relax this constraint by allowing a null pointer in place of the fixed buffer, or by hiding the data which would not fit, e.g., via the ZZ capability in

NetBSD NetBSD is a free and open-source Unix operating system based on the Berkeley Software Distribution (BSD). It was the first open-source BSD descendant officially released after 386BSD was forked. It continues to be actively developed and is a ...

termcap. The

terminfo Terminfo is a library and database that enables programs to use display terminals in a device-independent manner. Mary Ann Horton implemented the first terminfo library in 1981–1982 as an improvement over termcap. The improvements include * fast ...

library interface also emulates the termcap interface, and does not actually use the fixed-size buffer. The terminfo library's emulation of termcap allows multiple other entries to be included without restricting the position. A few other newer implementations of the termcap library may also provide this ability, though it is not well documented.

Obsolete features

A special capability, the "hz" capability, was defined specifically to support the

Hazeltine 1500 The Hazeltine 1500 was a popular smart terminal introduced by Hazeltine Corporation in April 1977 at a price of . Using a microprocessor and semiconductor random access memory, it implemented the basic features of the earlier Hazeltine 2000 in a ...

terminal, which had the unfortunate characteristic of using the

ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...

tilde character ('~') as a control sequence introducer. In order to support that terminal, not only did code that used the database have to know about using the tilde to introduce certain control sequences, but it also had to know to substitute another printable character for any tildes in the displayed text, since a tilde in the text would be interpreted by the terminal as the start of a control sequence, resulting in missing text and screen garbling. Additionally, attribute markers (such as start and end of underlining) themselves took up space on the screen. Comments in the database source code often referred to this as "Hazeltine braindamage". Since the Hazeltine 1500 was a widely used terminal in the late 1970s, it was important for applications to be able to deal with its limitations.

References

{{reflist

External links

Current termcap dataTermcap/Terminfo Resources Page
at

Eric S. Raymond Eric Steven Raymond (born December 4, 1957), often referred to as ESR, is an American software developer, open-source software advocate, and author of the 1997 essay and 1999 book ''The Cathedral and the Bazaar''. He wrote a guidebook for the ...

's website Computer data Databases Text mode 1978 software