The Termcap Library - 3. Definitions of the Terminal Capabilities

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# 3. Definitions of the Terminal Capabilities

This section is divided into many subsections, each for one aspect of use of display terminals. For writing a display program, you usually need only check the subsections for the operations you want to use. For writing a terminal description, you must read each subsection and fill in the capabilities described there.

String capabilities that are display commands may require numeric parameters (see section 1.6 Filling In Parameters). Most such capabilities do not use parameters. When a capability requires parameters, this is explicitly stated at the beginning of its definition. In simple cases, the first or second sentence of the definition mentions all the parameters, in the order they should be given, using a name in italics for each one. For example, the ``rp'` capability is a command that requires two parameters; its definition begins as follows:

> String of commands to output a graphic character c, repeated n times.

In complex cases or when there are many parameters, they are described explicitly.

When a capability is described as obsolete, this means that programs should not be written to look for it, but terminal descriptions should still be written to provide it.

When a capability is described as very obsolete, this means that it should be omitted from terminal descriptions as well.

## 3.1 Basic Characteristics

This section documents the capabilities that describe the basic and nature of the terminal, and also those that are relevant to the output of graphic characters.

``os'`
  Flag whose presence means that the terminal can overstrike. This means that outputting a graphic character does not erase whatever was present in the same character position before. The terminals that can overstrike include printing terminals, storage tubes (all obsolete nowadays), and many bit-map displays.
``eo'`
  Flag whose presence means that outputting a space erases a character position even if the terminal supports overstriking. If this flag is not present and overstriking is supported, output of a space has no effect except to move the cursor. (On terminals that do not support overstriking, you can always assume that outputting a space at a position erases whatever character was previously displayed there.)
``gn'`
  Flag whose presence means that this terminal type is a generic type which does not really describe any particular terminal. Generic types are intended for use as the default type assigned when the user connects to the system, with the intention that the user should specify what type he really has. One example of a generic type is the type ``network'`. Since the generic type cannot say how to do anything interesting with the terminal, termcap-using programs will always find that the terminal is too weak to be supported if the user has failed to specify a real terminal type in place of the generic one. The ``gn'` flag directs these programs to use a different error message: "You have not specified your real terminal type", rather than "Your terminal is not powerful enough to be used".
``hc'`
  Flag whose presence means this is a hardcopy terminal.
``rp'`
  String of commands to output a graphic character c, repeated n times. The first parameter value is the ASCII code for the desired character, and the second parameter is the number of times to repeat the character. Often this command requires padding proportional to the number of times the character is repeated. This effect can be had by using parameter arithmetic with ``%'`-sequences to compute the amount of padding, then generating the result as a number at the front of the string so that `tputs` will treat it as padding.
``hz'`
  Flag whose presence means that the ASCII character ``~'` cannot be output on this terminal because it is used for display commands. Programs handle this flag by checking all text to be output and replacing each ``~'` with some other character(s). If this is not done, the screen will be thoroughly garbled. The old Hazeltine terminals that required such treatment are probably very rare today, so you might as well not bother to support this flag.
``CC'`
  String whose presence means the terminal has a settable command character. The value of the string is the default command character (which is usually `ESC`). All the strings of commands in the terminal description should be written to use the default command character. If you are writing an application program that changes the command character, use the ``CC'` capability to figure out how to translate all the display commands to work with the new command character. Most programs have no reason to look at the ``CC'` capability.
``xb'`
  Flag whose presence identifies Superbee terminals which are unable to transmit the characters `ESC` and `Control-C`. Programs which support this flag are supposed to check the input for the code sequences sent by the `F1` and `F2` keys, and pretend that `ESC` or `Control-C` (respectively) had been read. But this flag is obsolete, and not worth supporting.

## 3.2 Screen Size

A terminal description has two capabilities, ``co'` and ``li'`, that describe the screen size in columns and lines. But there is more to the question of screen size than this.

On some operating systems the "screen" is really a window and the effective width can vary. On some of these systems, `tgetnum` uses the actual width of the window to decide what value to return for the ``co'` capability, overriding what is actually written in the terminal description. On other systems, it is up to the application program to check the actual window width using a system call. For example, on BSD 4.3 systems, the system call `ioctl` with code `TIOCGWINSZ` will tell you the current screen size.

On all window systems, termcap is powerless to advise the application program if the user resizes the window. Application programs must deal with this possibility in a system-dependent fashion. On some systems the C shell handles part of the problem by detecting changes in window size and setting the `TERMCAP` environment variable appropriately. This takes care of application programs that are started subsequently. It does not help application programs already running.

On some systems, including BSD 4.3, all programs using a terminal get a signal named `SIGWINCH` whenever the screen size changes. Programs that use termcap should handle this signal by using `ioctl TIOCGWINSZ` to learn the new screen size.

``co'`
  Numeric value, the width of the screen in character positions. Even hardcopy terminals normally have a ``co'` capability.
``li'`
  Numeric value, the height of the screen in lines.

## 3.3 Cursor Motion

Termcap assumes that the terminal has a cursor, a spot on the screen where a visible mark is displayed, and that most display commands take effect at the position of the cursor. It follows that moving the cursor to a specified location is very important.

There are many terminal capabilities for different cursor motion operations. A terminal description should define as many as possible, but most programs do not need to use most of them. One capability, ``cm'`, moves the cursor to an arbitrary place on the screen; this by itself is sufficient for any application as long as there is no need to support hardcopy terminals or certain old, weak displays that have only relative motion commands. Use of other cursor motion capabilities is an optimization, enabling the program to output fewer characters in some common cases.

If you plan to use the relative cursor motion commands in an application program, you must know what the starting cursor position is. To do this, you must keep track of the cursor position and update the records each time anything is output to the terminal, including graphic characters. In addition, it is necessary to know whether the terminal wraps after writing in the rightmost column. See section 3.4 Wrapping.

One other motion capability needs special mention: ``nw'` moves the cursor to the beginning of the following line, perhaps clearing all the starting line after the cursor, or perhaps not clearing at all. This capability is a least common denominator that is probably supported even by terminals that cannot do most other things such as ``cm'` or ``do'`. Even hardcopy terminals can support ``nw'`.

``cm'`
  String of commands to position the cursor at line l, column c. Both parameters are origin-zero, and are defined relative to the screen, not relative to display memory. All display terminals except a few very obsolete ones support ``cm'`, so it is acceptable for an application program to refuse to operate on terminals lacking ``cm'`.
``ho'`
  String of commands to move the cursor to the upper left corner of the screen (this position is called the home position). In terminals where the upper left corner of the screen is not the same as the beginning of display memory, this command must go to the upper left corner of the screen, not the beginning of display memory. Every display terminal supports this capability, and many application programs refuse to operate if the ``ho'` capability is missing.
``ll'`
  String of commands to move the cursor to the lower left corner of the screen. On some terminals, moving up from home position does this, but programs should never assume that will work. Just output the ``ll'` string (if it is provided); if moving to home position and then moving up is the best way to get there, the ``ll'` command will do that.
``cr'`
  String of commands to move the cursor to the beginning of the line it is on. If this capability is not specified, many programs assume they can use the ASCII carriage return character for this.
``le'`
  String of commands to move the cursor left one column. Unless the ``bw'` flag capability is specified, the effect is undefined if the cursor is at the left margin; do not use this command there. If ``bw'` is present, this command may be used at the left margin, and it wraps the cursor to the last column of the preceding line.
``nd'`
  String of commands to move the cursor right one column. The effect is undefined if the cursor is at the right margin; do not use this command there, not even if ``am'` is present.
``up'`
  String of commands to move the cursor vertically up one line. The effect of sending this string when on the top line is undefined; programs should never use it that way.
``do'`
  String of commands to move the cursor vertically down one line. The effect of sending this string when on the bottom line is undefined; programs should never use it that way. Some programs do use ``do'` to scroll up one line if used at the bottom line, if ``sf'` is not defined but ``sr'` is. This is only to compensate for certain old, incorrect terminal descriptions. (In principle this might actually lead to incorrect behavior on other terminals, but that seems to happen rarely if ever.) But the proper solution is that the terminal description should define ``sf'` as well as ``do'` if the command is suitable for scrolling. The original idea was that this string would not contain a newline character and therefore could be used without disabling the kernel's usual habit of converting of newline into a carriage-return newline sequence. But many terminal descriptions do use newline in the ``do'` string, so this is not possible; a program which sends the ``do'` string must disable output conversion in the kernel (see section 1.4 Initialization for Use of Termcap).
``bw'`
  Flag whose presence says that ``le'` may be used in column zero to move to the last column of the preceding line. If this flag is not present, ``le'` should not be used in column zero.
``nw'`
  String of commands to move the cursor to start of next line, possibly clearing rest of line (following the cursor) before moving.
``DO'`, ``UP'`, ``LE'`, ``RI'`
  Strings of commands to move the cursor n lines down vertically, up vertically, or n columns left or right. Do not attempt to move past any edge of the screen with these commands; the effect of trying that is undefined. Only a few terminal descriptions provide these commands, and most programs do not use them.
``CM'`
  String of commands to position the cursor at line l, column c, relative to display memory. Both parameters are origin-zero. This capability is present only in terminals where there is a difference between screen-relative and memory-relative addressing, and not even in all such terminals.
``ch'`
  String of commands to position the cursor at column c in the same line it is on. This is a special case of ``cm'` in which the vertical position is not changed. The ``ch'` capability is provided only when it is faster to output than ``cm'` would be in this special case. Programs should not assume most display terminals have ``ch'`.
``cv'`
  String of commands to position the cursor at line l in the same column. This is a special case of ``cm'` in which the horizontal position is not changed. The ``cv'` capability is provided only when it is faster to output than ``cm'` would be in this special case. Programs should not assume most display terminals have ``cv'`.
``sc'`
  String of commands to make the terminal save the current cursor position. Only the last saved position can be used. If this capability is present, ``rc'` should be provided also. Most terminals have neither.
``rc'`
  String of commands to make the terminal restore the last saved cursor position. If this capability is present, ``sc'` should be provided also. Most terminals have neither.
``ff'`
  String of commands to advance to the next page, for a hardcopy terminal.
``ta'`
  String of commands to move the cursor right to the next hardware tab stop column. Missing if the terminal does not have any kind of hardware tabs. Do not send this command if the kernel's terminal modes say that the kernel is expanding tabs into spaces.
``bt'`
  String of commands to move the cursor left to the previous hardware tab stop column. Missing if the terminal has no such ability; many terminals do not. Do not send this command if the kernel's terminal modes say that the kernel is expanding tabs into spaces.

The following obsolete capabilities should be included in terminal descriptions when appropriate, but should not be looked at by new programs.

``nc'`
  Flag whose presence means the terminal does not support the ASCII carriage return character as ``cr'`. This flag is needed because old programs assume, when the ``cr'` capability is missing, that ASCII carriage return can be used for the purpose. We use ``nc'` to tell the old programs that carriage return may not be used. New programs should not assume any default for ``cr'`, so they need not look at ``nc'`. However, descriptions should contain ``nc'` whenever they do not contain ``cr'`.
``xt'`
  Flag whose presence means that the ASCII tab character may not be used for cursor motion. This flag exists because old programs assume, when the ``ta'` capability is missing, that ASCII tab can be used for the purpose. We use ``xt'` to tell the old programs not to use tab. New programs should not assume any default for ``ta'`, so they need not look at ``xt'` in connection with cursor motion. Note that ``xt'` also has implications for standout mode (see section 3.10 Standout and Appearance Modes). It is obsolete in regard to cursor motion but not in regard to standout. In fact, ``xt'` means that the terminal is a Teleray 1061.
``bc'`
  Very obsolete alternative name for the ``le'` capability.
``bs'`
  Flag whose presence means that the ASCII character backspace may be used to move the cursor left. Obsolete; look at ``le'` instead.
``nl'`
  Obsolete capability which is a string that can either be used to move the cursor down or to scroll. The same string must scroll when used on the bottom line and move the cursor when used on any other line. New programs should use ``do'` or ``sf'`, and ignore ``nl'`. If there is no ``nl'` capability, some old programs assume they can use the newline character for this purpose. These programs follow a bad practice, but because they exist, it is still desirable to define the ``nl'` capability in a terminal description if the best way to move down is not a newline.

## 3.4 Wrapping

Wrapping means moving the cursor from the right margin to the left margin of the following line. Some terminals wrap automatically when a graphic character is output in the last column, while others do not. Most application programs that use termcap need to know whether the terminal wraps. There are two special flag capabilities to describe what the terminal does when a graphic character is output in the last column.

``am'`
  Flag whose presence means that writing a character in the last column causes the cursor to wrap to the beginning of the next line. If ``am'` is not present, writing in the last column leaves the cursor at the place where the character was written. Writing in the last column of the last line should be avoided on terminals with ``am'`, as it may or may not cause scrolling to occur (see section 3.5 Scrolling). Scrolling is surely not what you would intend. If your program needs to check the ``am'` flag, then it also needs to check the ``xn'` flag which indicates that wrapping happens in a strange way. Many common terminals have the ``xn'` flag.
``xn'`
  Flag whose presence means that the cursor wraps in a strange way. At least two distinct kinds of strange behavior are known; the termcap data base does not contain anything to distinguish the two. On Concept-100 terminals, output in the last column wraps the cursor almost like an ordinary ``am'` terminal. But if the next thing output is a newline, it is ignored. DEC VT-100 terminals (when the wrap switch is on) do a different strange thing: the cursor wraps only if the next thing output is another graphic character. In fact, the wrap occurs when the following graphic character is received by the terminal, before the character is placed on the screen. On both of these terminals, after writing in the last column a following graphic character will be displayed in the first column of the following line. But the effect of relative cursor motion characters such as newline or backspace at such a time depends on the terminal. The effect of erase or scrolling commands also depends on the terminal. You can't assume anything about what they will do on a terminal that has ``xn'`. So, to be safe, you should never do these things at such a time on such a terminal. To be sure of reliable results on a terminal which has the ``xn'` flag, output a ``cm'` absolute positioning command after writing in the last column. Another safe thing to do is to output carriage-return newline, which will leave the cursor at the beginning of the following line.
``LP'`
  Flag whose presence means that it is safe to write in the last column of the last line without worrying about undesired scrolling. ``LP'` indicates the DEC flavor of ``xn'` strangeness.

## 3.5 Scrolling

Scrolling means moving the contents of the screen up or down one or more lines. Moving the contents up is forward scrolling; moving them down is reverse scrolling.

Scrolling happens after each line of output during ordinary output on most display terminals. But in an application program that uses termcap for random-access output, scrolling happens only when explicitly requested with the commands in this section.

Some terminals have a scroll region feature. This lets you limit the effect of scrolling to a specified range of lines. Lines outside the range are unaffected when scrolling happens. The scroll region feature is available if either ``cs'` or ``cS'` is present.

``sf'`
  String of commands to scroll the screen one line up, assuming it is output with the cursor at the beginning of the bottom line.
``sr'`
  String of commands to scroll the screen one line down, assuming it is output with the cursor at the beginning of the top line.
``do'`
  A few programs will try to use ``do'` to do the work of ``sf'`. This is not really correct--it is an attempt to compensate for the absence of a ``sf'` command in some old terminal descriptions. Since these terminal descriptions do define ``sr'`, perhaps at one time the definition of ``do'` was different and it could be used for scrolling as well. But it isn't desirable to combine these two functions in one capability, since scrolling often requires more padding than simply moving the cursor down. Defining ``sf'` and ``do'` separately allows you to specify the padding properly. Also, all sources agree that ``do'` should not be relied on to do scrolling. So the best approach is to add ``sf'` capabilities to the descriptions of these terminals, copying the definition of ``do'` if that does scroll.
``SF'`
  String of commands to scroll the screen n lines up, assuming it is output with the cursor at the beginning of the bottom line.
``SR'`
  String of commands to scroll the screen n lines down, assuming it is output with the cursor at the beginning of the top line.
``cs'`
  String of commands to set the scroll region. This command takes two parameters, start and end, which are the line numbers (origin-zero) of the first line to include in the scroll region and of the last line to include in it. When a scroll region is set, scrolling is limited to the specified range of lines; lines outside the range are not affected by scroll commands. Do not try to move the cursor outside the scroll region. The region remains set until explicitly removed. To remove the scroll region, use another ``cs'` command specifying the full height of the screen. The cursor position is undefined after the ``cs'` command is set, so position the cursor with ``cm'` immediately afterward.
``cS'`
  String of commands to set the scroll region using parameters in different form. The effect is the same as if ``cs'` were used. Four parameters are required:
  1. Total number of lines on the screen.
  2. Number of lines above desired scroll region.
  3. Number of lines below (outside of) desired scroll region.
  4. Total number of lines on the screen, the same as the first parameter.
  This capability is a GNU extension that was invented to allow the Ann Arbor Ambassador's scroll-region command to be described; it could also be done by putting non-Unix ``%'`-sequences into a ``cs'` string, but that would have confused Unix programs that used the ``cs'` capability with the Unix termcap. Currently only GNU Emacs uses the ``cS'` capability.
``ns'`
  Flag which means that the terminal does not normally scroll for ordinary sequential output. For modern terminals, this means that outputting a newline in ordinary sequential output with the cursor on the bottom line wraps to the top line. For some obsolete terminals, other things may happen. The terminal may be able to scroll even if it does not normally do so. If the ``sf'` capability is provided, it can be used for scrolling regardless of ``ns'`.
``da'`
  Flag whose presence means that lines scrolled up off the top of the screen may come back if scrolling down is done subsequently. The ``da'` and ``db'` flags do not, strictly speaking, affect how to scroll. But programs that scroll usually need to clear the lines scrolled onto the screen, if these flags are present.
``db'`
  Flag whose presence means that lines scrolled down off the bottom of the screen may come back if scrolling up is done subsequently.
``lm'`
  Numeric value, the number of lines of display memory that the terminal has. A value of zero means that the terminal has more display memory than can fit on the screen, but no fixed number of lines. (The number of lines may depend on the amount of text in each line.)

Any terminal description that defines ``SF'` should also define ``sf'`; likewise for ``SR'` and ``sr'`. However, many terminals can only scroll by one line at a time, so it is common to find ``sf'` and not ``SF'`, or ``sr'` without ``SR'`.

Therefore, all programs that use the scrolling facilities should be prepared to work with ``sf'` in the case that ``SF'` is absent, and likewise with ``sr'`. On the other hand, an application program that uses only ``sf'` and not ``SF'` is acceptable, though slow on some terminals.

When outputting a scroll command with `tputs`, the nlines argument should be the total number of lines in the portion of the screen being scrolled. Very often these commands require padding proportional to this number of lines. See section 1.5 Padding.

## 3.6 Windows

A window, in termcap, is a rectangular portion of the screen to which all display operations are restricted. Wrapping, clearing, scrolling, insertion and deletion all operate as if the specified window were all the screen there was.

``wi'`
  String of commands to set the terminal output screen window. This string requires four parameters, all origin-zero:
  1. The first line to include in the window.
  2. The last line to include in the window.
  3. The first column to include in the window.
  4. The last column to include in the window.

Most terminals do not support windows.

## 3.7 Clearing Parts of the Screen

There are several terminal capabilities for clearing parts of the screen to blank. All display terminals support the ``cl'` string, and most display terminals support all of these capabilities.

``cl'`
  String of commands to clear the entire screen and position the cursor at the upper left corner.
``cd'`
  String of commands to clear the line the cursor is on, and all the lines below it, down to the bottom of the screen. This command string should be used only with the cursor in column zero; their effect is undefined if the cursor is elsewhere.
``ce'`
  String of commands to clear from the cursor to the end of the current line.
``ec'`
  String of commands to clear n characters, starting with the character that the cursor is on. This command string is expected to leave the cursor position unchanged. The parameter n should never be large enough to reach past the right margin; the effect of such a large parameter would be undefined.

Clear to end of line (``ce'`) is extremely important in programs that maintain an updating display. Nearly all display terminals support this operation, so it is acceptable for a an application program to refuse to work if ``ce'` is not present. However, if you do not want this limitation, you can accomplish clearing to end of line by outputting spaces until you reach the right margin. In order to do this, you must know the current horizontal position. Also, this technique assumes that writing a space will erase. But this happens to be true on all the display terminals that fail to support ``ce'`.

## 3.8 Insert/Delete Line

Inserting a line means creating a blank line in the middle of the screen, and pushing the existing lines of text apart. In fact, the lines above the insertion point do not change, while the lines below move down, and one is normally lost at the bottom of the screen.

Deleting a line means causing the line to disappear from the screen, closing up the gap by moving the lines below it upward. A new line appears at the bottom of the screen. Usually this line is blank, but on terminals with the ``db'` flag it may be a line previously moved off the screen bottom by scrolling or line insertion.

Insertion and deletion of lines is useful in programs that maintain an updating display some parts of which may get longer or shorter. They are also useful in editors for scrolling parts of the screen, and for redisplaying after lines of text are killed or inserted.

Many terminals provide commands to insert or delete a single line at the cursor position. Some provide the ability to insert or delete several lines with one command, using the number of lines to insert or delete as a parameter. Always move the cursor to column zero before using any of these commands.

``al'`
  String of commands to insert a blank line before the line the cursor is on. The existing line, and all lines below it, are moved down. The last line in the screen (or in the scroll region, if one is set) disappears and in most circumstances is discarded. It may not be discarded if the ``db'` is present (see section 3.5 Scrolling). The cursor must be at the left margin before this command is used. This command does not move the cursor.
``dl'`
  String of commands to delete the line the cursor is on. The following lines move up, and a blank line appears at the bottom of the screen (or bottom of the scroll region). If the terminal has the ``db'` flag, a nonblank line previously pushed off the screen bottom may reappear at the bottom. The cursor must be at the left margin before this command is used. This command does not move the cursor.
``AL'`
  String of commands to insert n blank lines before the line that the cursor is on. It is like ``al'` repeated n times, except that it is as fast as one ``al'`.
``DL'`
  String of commands to delete n lines starting with the line that the cursor is on. It is like ``dl'` repeated n times, except that it is as fast as one ``dl'`.

Any terminal description that defines ``AL'` should also define ``al'`; likewise for ``DL'` and ``dl'`. However, many terminals can only insert or delete one line at a time, so it is common to find ``al'` and not ``AL'`, or ``dl'` without ``DL'`.

Therefore, all programs that use the insert and delete facilities should be prepared to work with ``al'` in the case that ``AL'` is absent, and likewise with ``dl'`. On the other hand, it is acceptable to write an application that uses only ``al'` and ``dl'` and does not look for ``AL'` or ``DL'` at all.

If a terminal does not support line insertion and deletion directly, but does support a scroll region, the effect of insertion and deletion can be obtained with scrolling. However, it is up to the individual user program to check for this possibility and use the scrolling commands to get the desired result. It is fairly important to implement this alternate strategy, since it is the only way to get the effect of line insertion and deletion on the popular VT100 terminal.

Insertion and deletion of lines is affected by the scroll region on terminals that have a settable scroll region. This is useful when it is desirable to move any few consecutive lines up or down by a few lines. See section 3.5 Scrolling.

The line pushed off the bottom of the screen is not lost if the terminal has the ``db'` flag capability; instead, it is pushed into display memory that does not appear on the screen. This is the same thing that happens when scrolling pushes a line off the bottom of the screen. Either reverse scrolling or deletion of a line can bring the apparently lost line back onto the bottom of the screen. If the terminal has the scroll region feature as well as ``db'`, the pushed-out line really is lost if a scroll region is in effect.

When outputting an insert or delete command with `tputs`, the nlines argument should be the total number of lines from the cursor to the bottom of the screen (or scroll region). Very often these commands require padding proportional to this number of lines. See section 1.5 Padding.

For ``AL'` and ``DL'` the nlines argument should not depend on the number of lines inserted or deleted; only the total number of lines affected. This is because it is just as fast to insert two or n lines with ``AL'` as to insert one line with ``al'`.

## 3.9 Insert/Delete Character

Inserting a character means creating a blank space in the middle of a line, and pushing the rest of the line rightward. The character in the rightmost column is lost.

Deleting a character means causing the character to disappear from the screen, closing up the gap by moving the rest of the line leftward. A blank space appears in the rightmost column.

Insertion and deletion of characters is useful in programs that maintain an updating display some parts of which may get longer or shorter. It is also useful in editors for redisplaying the results of editing within a line.

Many terminals provide commands to insert or delete a single character at the cursor position. Some provide the ability to insert or delete several characters with one command, using the number of characters to insert or delete as a parameter.

Many terminals provide an insert mode in which outputting a graphic character has the added effect of inserting a position for that character. A special command string is used to enter insert mode and another is used to exit it. The reason for designing a terminal with an insert mode rather than an insert command is that inserting character positions is usually followed by writing characters into them. With insert mode, this is as fast as simply writing the characters, except for the fixed overhead of entering and leaving insert mode. However, when the line speed is great enough, padding may be required for the graphic characters output in insert mode.

Some terminals require you to enter insert mode and then output a special command for each position to be inserted. Or they may require special commands to be output before or after each graphic character to be inserted.

Deletion of characters is usually accomplished by a straightforward command to delete one or several positions; but on some terminals, it is necessary to enter a special delete mode before using the delete command, and leave delete mode afterward. Sometimes delete mode and insert mode are the same mode.

Some terminals make a distinction between character positions in which a space character has been output and positions which have been cleared. On these terminals, the effect of insert or delete character runs to the first cleared position rather than to the end of the line. In fact, the effect may run to more than one line if there is no cleared position to stop the shift on the first line. These terminals are identified by the ``in'` flag capability.

On terminals with the ``in'` flag, the technique of skipping over characters that you know were cleared, and then outputting text later on in the same line, causes later insert and delete character operations on that line to do nonstandard things. A program that has any chance of doing this must check for the ``in'` flag and must be careful to write explicit space characters into the intermediate columns when ``in'` is present.

A plethora of terminal capabilities are needed to describe all of this complexity. Here is a list of them all. Following the list, we present an algorithm for programs to use to take proper account of all of these capabilities.

``im'`
  String of commands to enter insert mode. If the terminal has no special insert mode, but it can insert characters with a special command, ``im'` should be defined with a null value, because the ``vi'` editor assumes that insertion of a character is impossible if ``im'` is not provided. New programs should not act like ``vi'`. They should pay attention to ``im'` only if it is defined.
``ei'`
  String of commands to leave insert mode. This capability must be present if ``im'` is. On a few old terminals the same string is used to enter and exit insert mode. This string turns insert mode on if it was off, and off it it was on. You can tell these terminals because the ``ei'` string equals the ``im'` string. If you want to support these terminals, you must always remember accurately whether insert mode is in effect. However, these terminals are obsolete, and it is reasonable to refuse to support them. On all modern terminals, you can safely output ``ei'` at any time to ensure that insert mode is turned off.
``ic'`
  String of commands to insert one character position at the cursor. The cursor does not move. If outputting a graphic character while in insert mode is sufficient to insert the character, then the ``ic'` capability should be defined with a null value. If your terminal offers a choice of ways to insert--either use insert mode or use a special command--then define ``im'` and do not define ``ic'`, since this gives the most efficient operation when several characters are to be inserted. Do not define both strings, for that means that both must be used each time insertion is done.
``ip'`
  String of commands to output following an inserted graphic character in insert mode. Often it is used just for a padding spec, when padding is needed after an inserted character (see section 1.5 Padding).
``IC'`
  String of commands to insert n character positions at and after the cursor. It has the same effect as repeating the ``ic'` string and a space, n times. If ``IC'` is provided, application programs may use it without first entering insert mode.
``mi'`
  Flag whose presence means it is safe to move the cursor while in insert mode and assume the terminal remains in insert mode.
``in'`
  Flag whose presence means that the terminal distinguishes between character positions in which space characters have been output and positions which have been cleared.

An application program can assume that the terminal can do character insertion if any one of the capabilities ``IC'`, ``im'`, ``ic'` or ``ip'` is provided.

To insert n blank character positions, move the cursor to the place to insert them and follow this algorithm:

1. If an ``IC'` string is provided, output it with parameter n and you are finished. Otherwise (or if you don't want to bother to look for an ``IC'` string) follow the remaining steps.
2. Output the ``im'` string, if there is one, unless the terminal is already in insert mode.
3. Repeat steps 4 through 6, n times.
4. Output the ``ic'` string if any.
5. Output a space.
6. Output the ``ip'` string if any.
7. Output the ``ei'` string, eventually, to exit insert mode. There is no need to do this right away. If the ``mi'` flag is present, you can move the cursor and the cursor will remain in insert mode; then you can do more insertion elsewhere without reentering insert mode.

To insert n graphic characters, position the cursor and follow this algorithm:

1. If an ``IC'` string is provided, output it with parameter n, then output the graphic characters, and you are finished. Otherwise (or if you don't want to bother to look for an ``IC'` string) follow the remaining steps.
2. Output the ``im'` string, if there is one, unless the terminal is already in insert mode.
3. For each character to be output, repeat steps 4 through 6.
4. Output the ``ic'` string if any.
5. Output the next graphic character.
6. Output the ``ip'` string if any.
7. Output the ``ei'` string, eventually, to exit insert mode. There is no need to do this right away. If the ``mi'` flag is present, you can move the cursor and the cursor will remain in insert mode; then you can do more insertion elsewhere without reentering insert mode.

Note that this is not the same as the original Unix termcap specifications in one respect: it assumes that the ``IC'` string can be used without entering insert mode. This is true as far as I know, and it allows you be able to avoid entering and leaving insert mode, and also to be able to avoid the inserted-character padding after the characters that go into the inserted positions.

Deletion of characters is less complicated; deleting one column is done by outputting the ``dc'` string. However, there may be a delete mode that must be entered with ``dm'` in order to make ``dc'` work.

``dc'`
  String of commands to delete one character position at the cursor. If ``dc'` is not present, the terminal cannot delete characters.
``DC'`
  String of commands to delete n characters starting at the cursor. It has the same effect as repeating the ``dc'` string n times. Any terminal description that has ``DC'` also has ``dc'`.
``dm'`
  String of commands to enter delete mode. If not present, there is no delete mode, and ``dc'` can be used at any time (assuming there is a ``dc'`).
``ed'`
  String of commands to exit delete mode. This must be present if ``dm'` is.

To delete n character positions, position the cursor and follow these steps:

1. If the ``DC'` string is present, output it with parameter n and you are finished. Otherwise, follow the remaining steps.
2. Output the ``dm'` string, unless you know the terminal is already in delete mode.
3. Output the ``dc'` string n times.
4. Output the ``ed'` string eventually. If the flag capability ``mi'` is present, you can move the cursor and do more deletion without leaving and reentering delete mode.

As with the ``IC'` string, we have departed from the original termcap specifications by assuming that ``DC'` works without entering delete mode even though ``dc'` would not.

If the ``dm'` and ``im'` capabilities are both present and have the same value, it means that the terminal has one mode for both insertion and deletion. It is useful for a program to know this, because then it can do insertions after deletions, or vice versa, without leaving insert/delete mode and reentering it.

## 3.10 Standout and Appearance Modes

Appearance modes are modifications to the ways characters are displayed. Typical appearance modes include reverse video, dim, bright, blinking, underlined, invisible, and alternate character set. Each kind of terminal supports various among these, or perhaps none.

For each type of terminal, one appearance mode or combination of them that looks good for highlighted text is chosen as the standout mode. The capabilities ``so'` and ``se'` say how to enter and leave standout mode. Programs that use appearance modes only to highlight some text generally use the standout mode so that they can work on as many terminals as possible. Use of specific appearance modes other than "underlined" and "alternate character set" is rare.

Terminals that implement appearance modes fall into two general classes as to how they do it.

In some terminals, the presence or absence of any appearance mode is recorded separately for each character position. In these terminals, each graphic character written is given the appearance modes current at the time it is written, and keeps those modes until it is erased or overwritten. There are special commands to turn the appearance modes on or off for characters to be written in the future.

In other terminals, the change of appearance modes is represented by a marker that belongs to a certain screen position but affects all following screen positions until the next marker. These markers are traditionally called magic cookies.

The same capabilities (``so'`, ``se'`, ``mb'` and so on) for turning appearance modes on and off are used for both magic-cookie terminals and per-character terminals. On magic cookie terminals, these give the commands to write the magic cookies. On per-character terminals, they change the current modes that affect future output and erasure. Some simple applications can use these commands without knowing whether or not they work by means of cookies.

However, a program that maintains and updates a display needs to know whether the terminal uses magic cookies, and exactly what their effect is. This information comes from the ``sg'` capability.

The ``sg'` capability is a numeric capability whose presence indicates that the terminal uses magic cookies for appearance modes. Its value is the number of character positions that a magic cookie occupies. Usually the cookie occupies one or more character positions on the screen, and these character positions are displayed as blank, but in some terminals the cookie has zero width.

The ``sg'` capability describes both the magic cookie to turn standout on and the cookie to turn it off. This makes the assumption that both kinds of cookie have the same width on the screen. If that is not true, the narrower cookie must be "widened" with spaces until it has the same width as the other.

On some magic cookie terminals, each line always starts with normal display; in other words, the scope of a magic cookie never extends over more than one line. But on other terminals, one magic cookie affects all the lines below it unless explicitly canceled. Termcap does not define any way to distinguish these two ways magic cookies can work. To be safe, it is best to put a cookie at the beginning of each line.

On some per-character terminals, standout mode or other appearance modes may be canceled by moving the cursor. On others, moving the cursor has no effect on the state of the appearance modes. The latter class of terminals are given the flag capability ``ms'` ("can move in standout"). All programs that might have occasion to move the cursor while appearance modes are turned on must check for this flag; if it is not present, they should reset appearance modes to normal before doing cursor motion.

A program that has turned on only standout mode should use ``se'` to reset the standout mode to normal. A program that has turned on only alternate character set mode should use ``ae'` to return it to normal. If it is possible that any other appearance modes are turned on, use the ``me'` capability to return them to normal.

Note that the commands to turn on one appearance mode, including ``so'` and ``mb'` ... ``mr'`, if used while some other appearance modes are turned on, may combine the two modes on some terminals but may turn off the mode previously enabled on other terminals. This is because some terminals do not have a command to set or clear one appearance mode without changing the others. Programs should not attempt to use appearance modes in combination except with ``sa'`, and when switching from one single mode to another should always turn off the previously enabled mode and then turn on the new desired mode.

On some old terminals, the ``so'` and ``se'` commands may be the same command, which has the effect of turning standout on if it is off, or off it is on. It is therefore risky for a program to output extra ``se'` commands for good measure. Fortunately, all these terminals are obsolete.

Programs that update displays in which standout-text may be replaced with non-standout text must check for the ``xs'` flag. In a per-character terminal, this flag says that the only way to remove standout once written is to clear that portion of the line with the ``ce'` string or something even more powerful (see section 3.7 Clearing Parts of the Screen); just writing new characters at those screen positions will not change the modes in effect there. In a magic cookie terminal, ``xs'` says that the only way to remove a cookie is to clear a portion of the line that includes the cookie; writing a different cookie at the same position does not work.

Such programs must also check for the ``xt'` flag, which means that the terminal is a Teleray 1061. On this terminal it is impossible to position the cursor at the front of a magic cookie, so the only two ways to remove a cookie are (1) to delete the line it is on or (2) to position the cursor at least one character before it (possibly on a previous line) and output the ``se'` string, which on these terminals finds and removes the next ``so'` magic cookie on the screen. (It may also be possible to remove a cookie which is not at the beginning of a line by clearing that line.) The ``xt'` capability also has implications for the use of tab characters, but in that regard it is obsolete (See section 3.3 Cursor Motion).

``so'`
  String of commands to enter standout mode.
``se'`
  String of commands to leave standout mode.
``sg'`
  Numeric capability, the width on the screen of the magic cookie. This capability is absent in terminals that record appearance modes character by character.
``ms'`
  Flag whose presence means that it is safe to move the cursor while the appearance modes are not in the normal state. If this flag is absent, programs should always reset the appearance modes to normal before moving the cursor.
``xs'`
  Flag whose presence means that the only way to reset appearance modes already on the screen is to clear to end of line. On a per-character terminal, you must clear the area where the modes are set. On a magic cookie terminal, you must clear an area containing the cookie. See the discussion above.
``xt'`
  Flag whose presence means that the cursor cannot be positioned right in front of a magic cookie, and that ``se'` is a command to delete the next magic cookie following the cursor. See discussion above.
``mb'`
  String of commands to enter blinking mode.
``md'`
  String of commands to enter double-bright mode.
``mh'`
  String of commands to enter half-bright mode.
``mk'`
  String of commands to enter invisible mode.
``mp'`
  String of commands to enter protected mode.
``mr'`
  String of commands to enter reverse-video mode.
``me'`
  String of commands to turn off all appearance modes, including standout mode and underline mode. On some terminals it also turns off alternate character set mode; on others, it may not. This capability must be present if any of ``mb'` ... ``mr'` is present.
``as'`
  String of commands to turn on alternate character set mode. This mode assigns some or all graphic characters an alternate picture on the screen. There is no standard as to what the alternate pictures look like.
``ae'`
  String of commands to turn off alternate character set mode.
``sa'`
  String of commands to turn on an arbitrary combination of appearance modes. It accepts 9 parameters, each of which controls a particular kind of appearance mode. A parameter should be 1 to turn its appearance mode on, or zero to turn that mode off. Most terminals do not support the ``sa'` capability, even among those that do have various appearance modes. The nine parameters are, in order, standout, underline, reverse, blink, half-bright, double-bright, blank, protect, alt char set.

## 3.11 Underlining

Underlining on most terminals is a kind of appearance mode, much like standout mode. Therefore, it may be implemented using magic cookies or as a flag in the terminal whose current state affects each character that is output. See section 3.10 Standout and Appearance Modes, for a full explanation.

The ``ug'` capability is a numeric capability whose presence indicates that the terminal uses magic cookies for underlining. Its value is the number of character positions that a magic cookie for underlining occupies; it is used for underlining just as ``sg'` is used for standout. Aside from the simplest applications, it is impossible to use underlining correctly without paying attention to the value of ``ug'`.

``us'`
  String of commands to turn on underline mode or to output a magic cookie to start underlining.
``ue'`
  String of commands to turn off underline mode or to output a magic cookie to stop underlining.
``ug'`
  Width of magic cookie that represents a change of underline mode; or missing, if the terminal does not use a magic cookie for this.
``ms'`
  Flag whose presence means that it is safe to move the cursor while the appearance modes are not in the normal state. Underlining is an appearance mode. If this flag is absent, programs should always turn off underlining before moving the cursor.

There are two other, older ways of doing underlining: there can be a command to underline a single character, or the output of ``_'`, the ASCII underscore character, as an overstrike could cause a character to be underlined. New programs need not bother to handle these capabilities unless the author cares strongly about the obscure terminals which support them. However, terminal descriptions should provide these capabilities when appropriate.

``uc'`
  String of commands to underline the character under the cursor, and move the cursor right.
``ul'`
  Flag whose presence means that the terminal can underline by overstriking an underscore character (``_'`); some terminals can do this even though they do not support overstriking in general. An implication of this flag is that when outputting new text to overwrite old text, underscore characters must be treated specially lest they underline the old text instead.

## 3.12 Cursor Visibility

Some terminals have the ability to make the cursor invisible, or to enhance it. Enhancing the cursor is often done by programs that plan to use the cursor to indicate to the user a position of interest that may be anywhere on the screen--for example, the Emacs editor enhances the cursor on entry. Such programs should always restore the cursor to normal on exit.

``vs'`
  String of commands to enhance the cursor.
``vi'`
  String of commands to make the cursor invisible.
``ve'`
  String of commands to return the cursor to normal.

If you define either ``vs'` or ``vi'`, you must also define ``ve'`.

## 3.13 Bell

Here we describe commands to make the terminal ask for the user to pay attention to it.

``bl'`
  String of commands to cause the terminal to make an audible sound. If this capability is absent, the terminal has no way to make a suitable sound.
``vb'`
  String of commands to cause the screen to flash to attract attention ("visible bell"). If this capability is absent, the terminal has no way to do such a thing.

## 3.14 Keypad and Function Keys

Many terminals have arrow and function keys that transmit specific character sequences to the computer. Since the precise sequences used depend on the terminal, termcap defines capabilities used to say what the sequences are. Unlike most termcap string-valued capabilities, these are not strings of commands to be sent to the terminal, rather strings that are received from the terminal.

Programs that expect to use keypad keys should check, initially, for a ``ks'` capability and send it, to make the keypad actually transmit. Such programs should also send the ``ke'` string when exiting.

``ks'`
  String of commands to make the keypad keys transmit. If this capability is not provided, but the others in this section are, programs may assume that the keypad keys always transmit.
``ke'`
  String of commands to make the keypad keys work locally. This capability is provided only if ``ks'` is.
``kl'`
  String of input characters sent by typing the left-arrow key. If this capability is missing, you cannot expect the terminal to have a left-arrow key that transmits anything to the computer.
``kr'`
  String of input characters sent by typing the right-arrow key.
``ku'`
  String of input characters sent by typing the up-arrow key.
``kd'`
  String of input characters sent by typing the down-arrow key.
``kh'`
  String of input characters sent by typing the "home-position" key.
``K1'` ... ``K5'`
  Strings of input characters sent by the five other keys in a 3-by-3 array that includes the arrow keys, if the keyboard has such a 3-by-3 array. Note that one of these keys may be the "home-position" key, in which case one of these capabilities will have the same value as the ``kh'` key.
``k0'`
  String of input characters sent by function key 10 (or 0, if the terminal has one labeled 0).
``k1'` ... ``k9'`
  Strings of input characters sent by function keys 1 through 9, provided for those function keys that exist.
``kn'`
  Number: the number of numbered function keys, if there are more than 10.
``l0'` ... ``l9'`
  Strings which are the labels appearing on the keyboard on the keys described by the capabilities ``k0'` ... ``l9'`. These capabilities should be left undefined if the labels are ``f0'` or ``f10'` and ``f1'` ... ``f9'`.
``kH'`
  String of input characters sent by the "home down" key, if there is one.
``kb'`
  String of input characters sent by the "backspace" key, if there is one.
``ka'`
  String of input characters sent by the "clear all tabs" key, if there is one.
``kt'`
  String of input characters sent by the "clear tab stop this column" key, if there is one.
``kC'`
  String of input characters sent by the "clear screen" key, if there is one.
``kD'`
  String of input characters sent by the "delete character" key, if there is one.
``kL'`
  String of input characters sent by the "delete line" key, if there is one.
``kM'`
  String of input characters sent by the "exit insert mode" key, if there is one.
``kE'`
  String of input characters sent by the "clear to end of line" key, if there is one.
``kS'`
  String of input characters sent by the "clear to end of screen" key, if there is one.
``kI'`
  String of input characters sent by the "insert character" or "enter insert mode" key, if there is one.
``kA'`
  String of input characters sent by the "insert line" key, if there is one.
``kN'`
  String of input characters sent by the "next page" key, if there is one.
``kP'`
  String of input characters sent by the "previous page" key, if there is one.
``kF'`
  String of input characters sent by the "scroll forward" key, if there is one.
``kR'`
  String of input characters sent by the "scroll reverse" key, if there is one.
``kT'`
  String of input characters sent by the "set tab stop in this column" key, if there is one.
``ko'`
  String listing the other function keys the terminal has. This is a very obsolete way of describing the same information found in the ``kH'` ... ``kT'` keys. The string contains a list of two-character termcap capability names, separated by commas. The meaning is that for each capability name listed, the terminal has a key which sends the string which is the value of that capability. For example, the value ``:ko=cl,ll,sf,sr:'` says that the terminal has four function keys which mean "clear screen", "home down", "scroll forward" and "scroll reverse".

## 3.15 Meta Key

A Meta key is a key on the keyboard that modifies each character you type by controlling the 0200 bit. This bit is on if and only if the Meta key is held down when the character is typed. Characters typed using the Meta key are called Meta characters. Emacs uses Meta characters as editing commands.

``km'`
  Flag whose presence means that the terminal has a Meta key.
``mm'`
  String of commands to enable the functioning of the Meta key.
``mo'`
  String of commands to disable the functioning of the Meta key.

If the terminal has ``km'` but does not have ``mm'` and ``mo'`, it means that the Meta key always functions. If it has ``mm'` and ``mo'`, it means that the Meta key can be turned on or off. Send the ``mm'` string to turn it on, and the ``mo'` string to turn it off. I do not know why one would ever not want it to be on.

## 3.16 Initialization

``ti'`
  String of commands to put the terminal into whatever special modes are needed or appropriate for programs that move the cursor nonsequentially around the screen. Programs that use termcap to do full-screen display should output this string when they start up.
``te'`
  String of commands to undo what is done by the ``ti'` string. Programs that output the ``ti'` string on entry should output this string when they exit.
``is'`
  String of commands to initialize the terminal for each login session.
``if'`
  String which is the name of a file containing the string of commands to initialize the terminal for each session of use. Normally ``is'` and ``if'` are not both used.
``i1'`
``i3'`
  Two more strings of commands to initialize the terminal for each login session. The ``i1'` string (if defined) is output before ``is'` or ``if'`, and the ``i3'` string (if defined) is output after. The reason for having three separate initialization strings is to make it easier to define a group of related terminal types with slightly different initializations. Define two or three of the strings in the basic type; then the other types can override one or two of the strings.
``rs'`
  String of commands to reset the terminal from any strange mode it may be in. Normally this includes the ``is'` string (or other commands with the same effects) and more. What would go in the ``rs'` string but not in the ``is'` string are annoying or slow commands to bring the terminal back from strange modes that nobody would normally use.
``it'`
  Numeric value, the initial spacing between hardware tab stop columns when the terminal is powered up. Programs to initialize the terminal can use this to decide whether there is a need to set the tab stops. If the initial width is 8, well and good; if it is not 8, then the tab stops should be set; if they cannot be set, the kernel is told to convert tabs to spaces, and other programs will observe this and do likewise.
``ct'`
  String of commands to clear all tab stops.
``st'`
  String of commands to set tab stop at current cursor column on all lines.
``NF'`
  Flag whose presence means that the terminal does not support XON/XOFF flow control. Programs should not send XON (`C-q`) or XOFF (`C-s`) characters to the terminal.

## 3.17 Padding Capabilities

There are two terminal capabilities that exist just to explain the proper way to obey the padding specifications in all the command string capabilities. One, ``pc'`, must be obeyed by all termcap-using programs.

``pb'`
  Numeric value, the lowest baud rate at which padding is actually needed. Programs may check this and refrain from doing any padding at lower speeds.
``pc'`
  String of commands for padding. The first character of this string is to be used as the pad character, instead of using null characters for padding. If ``pc'` is not provided, use null characters. Every program that uses termcap must look up this capability and use it to set the variable `PC` that is used by `tputs`. See section 1.5 Padding.

Some termcap capabilities exist just to specify the amount of padding that the kernel should give to cursor motion commands used in ordinary sequential output.

``dC'`
  Numeric value, the number of msec of padding needed for the carriage-return character.
``dN'`
  Numeric value, the number of msec of padding needed for the newline (linefeed) character.
``dB'`
  Numeric value, the number of msec of padding needed for the backspace character.
``dF'`
  Numeric value, the number of msec of padding needed for the formfeed character.
``dT'`
  Numeric value, the number of msec of padding needed for the tab character.

In some systems, the kernel uses the above capabilities; in other systems, the kernel uses the paddings specified in the string capabilities ``cr'`, ``sf'`, ``le'`, ``ff'` and ``ta'`. Descriptions of terminals which require such padding should contain the ``dC'` ... ``dT'` capabilities and also specify the appropriate padding in the corresponding string capabilities. Since no modern terminals require padding for ordinary sequential output, you probably won't need to do either of these things.

## 3.18 Status Line

A status line is a line on the terminal that is not used for ordinary display output but instead used for a special message. The intended use is for a continuously updated description of what the user's program is doing, and that is where the name "status line" comes from, but in fact it could be used for anything. The distinguishing characteristic of a status line is that ordinary output to the terminal does not affect it; it changes only if the special status line commands of this section are used.

``hs'`
  Flag whose presence means that the terminal has a status line. If a terminal description specifies that there is a status line, it must provide the ``ts'` and ``fs'` capabilities.
``ts'`
  String of commands to move the terminal cursor into the status line. Usually these commands must specifically record the old cursor position for the sake of the ``fs'` string.
``fs'`
  String of commands to move the cursor back from the status line to its previous position (outside the status line).
``es'`
  Flag whose presence means that other display commands work while writing the status line. In other words, one can clear parts of it, insert or delete characters, move the cursor within it using ``ch'` if there is a ``ch'` capability, enter and leave standout mode, and so on.
``ds'`
  String of commands to disable the display of the status line. This may be absent, if there is no way to disable the status line display.
``ws'`
  Numeric value, the width of the status line. If this capability is absent in a terminal that has a status line, it means the status line is the same width as the other lines. Note that the value of ``ws'` is sometimes as small as 8.

## 3.19 Half-Line Motion

Some terminals have commands for moving the cursor vertically by half-lines, useful for outputting subscripts and superscripts. Mostly it is hardcopy terminals that have such features.

``hu'`
  String of commands to move the cursor up half a line. If the terminal is a display, it is your responsibility to avoid moving up past the top line; however, most likely the terminal that supports this is a hardcopy terminal and there is nothing to be concerned about.
``hd'`
  String of commands to move the cursor down half a line. If the terminal is a display, it is your responsibility to avoid moving down past the bottom line, etc.

## 3.20 Controlling Printers Attached to Terminals

Some terminals have attached hardcopy printer ports. They may be able to copy the screen contents to the printer; they may also be able to redirect output to the printer. Termcap does not have anything to tell the program whether the redirected output appears also on the screen; it does on some terminals but not all.

``ps'`
  String of commands to cause the contents of the screen to be printed. If it is absent, the screen contents cannot be printed.
``po'`
  String of commands to redirect further output to the printer.
``pf'`
  String of commands to terminate redirection of output to the printer. This capability must be present in the description if ``po'` is.
``pO'`
  String of commands to redirect output to the printer for next n characters of output, regardless of what they are. Redirection will end automatically after n characters of further output. Until then, nothing that is output can end redirection, not even the ``pf'` string if there is one. The number n should not be more than 255. One use of this capability is to send non-text byte sequences (such as bit-maps) to the printer.

Most terminals with printers do not support all of ``ps'`, ``po'` and ``pO'`; any one or two of them may be supported. To make a program that can send output to all kinds of printers, it is necessary to check for all three of these capabilities, choose the most convenient of the ones that are provided, and use it in its own appropriate fashion.

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