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The Termcap Library

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2.2 Writing the Capabilities

There are three kinds of capabilities: flags, numbers, and strings. Each kind has its own way of being written in the description. Each defined capability has by convention a particular kind of value; for example, `li' always has a numeric value and `cm' always a string value.

A flag capability is thought of as having a boolean value: the value is true if the capability is present, false if not. When the capability is present, just write its name between two colons.

A numeric capability has a value which is a nonnegative number. Write the capability name, a `#', and the number, between two colons. For example, `...:li#48:...' is how you specify the `li' capability for 48 lines.

A string-valued capability has a value which is a sequence of characters. Usually these are the characters used to perform some display operation. Write the capability name, a `=', and the characters of the value, between two colons. For example, `...:cm=\E[%i%d;%dH:...' is how the cursor motion command for a standard ANSI terminal would be specified.

Special characters in the string value can be expressed using `\'-escape sequences as in C; in addition, `\E' stands for ESC. `^' is also a kind of escape character; `^' followed by char stands for the control-equivalent of char. Thus, `^a' stands for the character control-a, just like `\001'. `\' and `^' themselves can be represented as `\\' and `\^'.

To include a colon in the string, you must write `\072'. You might ask, "Why can't `\:' be used to represent a colon?" The reason is that the interrogation functions do not count slashes while looking for a capability. Even if `:ce=ab\:cd:' were interpreted as giving the `ce' capability the value `ab:cd', it would also appear to define `cd' as a flag.

The string value will often contain digits at the front to specify padding (see section 1.5 Padding) and/or `%'-sequences within to specify how to encode parameters (see section 1.6 Filling In Parameters). Although these things are not to be output literally to the terminal, they are considered part of the value of the capability. They are special only when the string value is processed by tputs, tparam or tgoto. By contrast, `\' and `^' are considered part of the syntax for specifying the characters in the string.

Let's look at the VT52 example again:

dw|vt52|DEC vt52:\
        :cm=\EY%+ %+ :co#80:li#24:\     

Here we see the numeric-valued capabilities `co' and `li', the flags `bs' and `pt', and many string-valued capabilities. Most of the strings start with ESC represented as `\E'. The rest contain control characters represented using `^'. The meanings of the individual capabilities are defined elsewhere (see section 3. Definitions of the Terminal Capabilities).

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