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{{short description|Legacy software library and database}}
[[File:Termapi.svg|frameless|right|440px]]

'''Termcap''' ("'''term'''inal '''cap'''ability") is a legacy software [[library (computing)|library]] and [[database]] used on [[Unix-like]] computers that enables programs to use display [[computer terminal]]s in a terminal-independent manner, which greatly simplifies the process of writing portable [[text mode]] applications.<ref name=":0" /> It was superseded by the [[terminfo]] database used by [[ncurses]], [[tput]], and other programs. 

A termcap database can describe the [[Terminal capabilities|capabilities]] of hundreds of different display terminals. This allows programs to have [[computer terminal#Programming interface|character-based display]] output, independent of the type of terminal. On-screen text editors such as [[Vi (text editor)|vi]] and [[Emacs]] are examples of programs that may use termcap. Other programs are listed in the  [[:Category:Termcap|Termcap]] category. Access to the termcap database was usually provided by separate libraries. e.g. GNU Termcap.<ref name=":0" />

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.

== History ==
[[Bill Joy]] wrote the first termcap library in 1978<ref>[http://landley.net/history/mirror/unix/art3.htm Peter H. Salus, "The history of Unix is as much about collaboration as it is about technology", Byte, October 1994.]</ref><ref>[http://www.mirbsd.org/cman/manPSD/19.curses.htm Kenneth C. R. C. Arnold and Elan Amir, "Screen Updating and Cursor Movement Optimization: A Library Package"]</ref> for the [[BSD|Berkeley Unix]] [[operating system]]; it has since been ported to most Unix and Unix-like environments, even [[OS-9]].<ref>{{cite web |author=Joel Mathew Hegberg |date=November 1994 |title=Tackling Termcap, Part I |url=http://www.joelhegberg.com/articles.html |accessdate=26 April 2021 |work=OS-9/OSK Answers!}}</ref> Joy's design was reportedly influenced by the design of the terminal data store in the earlier [[Incompatible Timesharing System]].<ref>{{cite newsgroup |url=https://groups.google.com/g/alt.sys.pdp10/c/NyI5x3yS9LA/m/U9KvbmXZ8nUJ |author=Mark Crispin |date=7 Oct 1998 |title=Re: KL Console Commands |newsgroup=alt.sys.pdp10}}</ref>{{better source|date=April 2021}}{{dubious|ITS terminal data store|date=April 2021}}

==Data model==
Termcap databases consist of one or more descriptions of terminals.

===Indices===
Each description must contain the [[canonical form|canonical]] 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]] 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]] 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,<ref name=":0">[https://www.gnu.org/software/termutils/manual/termcap-1.3/termcap.html The GNU Termcap Library]</ref> or by hiding the data which would not fit, e.g., via the '''ZZ''' capability in [[NetBSD]] termcap.<ref>[http://www.daemon-systems.org/man/termcap.5.html NetBSD termcap file format]</ref>  The [[terminfo]] 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.<ref>[http://freshmeat.net/projects/vi/ Discussion of termcap in vi]</ref>

==Obsolete features==
A special capability, the "hz" capability, was defined specifically to support the [[Hazeltine 1500]] terminal, which had the unfortunate characteristic of using the [[ASCII]] tilde character ('~') as a control sequence introducer.<ref>{{man|5|termcap|4.3BSD-Reno}}</ref>{{discuss|Hazeltine quirk}}  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.<ref>{{cite web|last=Stallman|first=Richard M.|title=The Termcap Library and Database, Second Edition|url=https://www.gnu.org/software/termutils/manual/termcap-1.3/html_mono/termcap.html|publisher=Gnu.org|accessdate=17 May 2014|year=1992}}</ref>  Additionally, attribute markers (such as start and end of underlining) themselves took up space on the screen.{{citation needed|date=May 2014}}  Comments in the database source code often referred to this as "Hazeltine braindamage".<ref>{{cite web|title=termcap.src|url=https://opensource.apple.com/source/emacs/emacs-78.2/emacs/etc/termcap.src|publisher=Apple Computer|accessdate=17 May 2014}}</ref>  Since the Hazeltine 1500 was a widely used terminal in the late 1970s,{{citation needed|date=May 2014}} it was important for applications to be able to deal with its limitations.

==See also==
*[[ANSI escape sequences]], attempts to unify the many sequences
*[[Computer terminal]]
*[[Curses (programming library)]]
*[[Terminfo]]
*[[Terminal emulator]]

==References==
{{reflist}}

==External links==
*[http://invisible-island.net/ncurses/ncurses.faq.html#which_terminfo Current termcap data]
*[http://www.catb.org/~esr/terminfo/ Termcap/Terminfo Resources Page] at [[Eric S. Raymond]]'s website

[[Category:Termcap| ]]
[[Category:Computer data]]
[[Category:Databases]]
[[Category:Text mode]]
[[Category:1978 software]]