The Termcap Library - 1. The Termcap Library

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# 1. The Termcap Library

The termcap library is the application programmer's interface to the termcap data base. It contains functions for the following purposes:

- Finding the description of the user's terminal type (`tgetent`).
- Interrogating the description for information on various topics (`tgetnum`, `tgetflag`, `tgetstr`).
- Computing and performing padding (`tputs`).
- Encoding numeric parameters such as cursor positions into the terminal-specific form required for display commands (`tparam`, `tgoto`).

## 1.1 Preparing to Use the Termcap Library

To use the termcap library in a program, you need two kinds of preparation:

- The compiler needs declarations of the functions and variables in the library. On GNU systems, it suffices to include the header file ``termcap.h'` in each source file that uses these functions and variables. On Unix systems, there is often no such header file. Then you must explictly declare the variables as external. You can do likewise for the functions, or let them be implicitly declared and cast their values from type `int` to the appropriate type. We illustrate the declarations of the individual termcap library functions with ANSI C prototypes because they show how to pass the arguments. If you are not using the GNU C compiler, you probably cannot use function prototypes, so omit the argument types and names from your declarations.
- The linker needs to search the library. Usually either ``-ltermcap'` or ``-ltermlib'` as an argument when linking will do this.

## 1.2 Finding a Terminal Description: `tgetent`

An application program that is going to use termcap must first look up the description of the terminal type in use. This is done by calling `tgetent`, whose declaration in ANSI Standard C looks like:

    int tgetent (char *buffer, char *termtype);

This function finds the description and remembers it internally so that you can interrogate it about specific terminal capabilities (see section 1.3 Interrogating the Terminal Description).

The argument termtype is a string which is the name for the type of terminal to look up. Usually you would obtain this from the environment variable `TERM` using `getenv ("TERM")`.

If you are using the GNU version of termcap, you can alternatively ask `tgetent` to allocate enough space. Pass a null pointer for buffer, and `tgetent` itself allocates the storage using `malloc`. There is no way to get the address that was allocated, and you shouldn't try to free the storage.

With the Unix version of termcap, you must allocate space for the description yourself and pass the address of the space as the argument buffer. There is no way you can tell how much space is needed, so the convention is to allocate a buffer 2048 characters long and assume that is enough. (Formerly the convention was to allocate 1024 characters and assume that was enough. But one day, for one kind of terminal, that was not enough.)

No matter how the space to store the description has been obtained, termcap records its address internally for use when you later interrogate the description with `tgetnum`, `tgetstr` or `tgetflag`. If the buffer was allocated by termcap, it will be freed by termcap too if you call `tgetent` again. If the buffer was provided by you, you must make sure that its contents remain unchanged for as long as you still plan to interrogate the description.

The return value of `tgetent` is -1 if there is some difficulty accessing the data base of terminal types, 0 if the data base is accessible but the specified type is not defined in it, and some other value otherwise.

Here is how you might use the function `tgetent`:

    #ifdef unix
    static char term_buffer[2048];
    #else
    #define term_buffer 0
    #endif

    init_terminal_data ()
    {
      char *termtype = getenv ("TERM");
      int success;

      if (termtype == 0)
        fatal ("Specify a terminal type with `setenv TERM <yourtype>'.\n");

      success = tgetent (term_buffer, termtype);
      if (success < 0)
        fatal ("Could not access the termcap data base.\n");
      if (success == 0)
        fatal ("Terminal type `%s' is not defined.\n", termtype);
    }

Here we assume the function `fatal` prints an error message and exits.

If the environment variable `TERMCAP` is defined, its value is used to override the terminal type data base. The function `tgetent` checks the value of `TERMCAP` automatically. If the value starts with ``/'` then it is taken as a file name to use as the data base file, instead of ``/etc/termcap'` which is the standard data base. If the value does not start with ``/'` then it is itself used as the terminal description, provided that the terminal type termtype is among the types it claims to apply to. See section 2. The Format of the Data Base, for information on the format of a terminal description.

## 1.3 Interrogating the Terminal Description

Each piece of information recorded in a terminal description is called a capability. Each defined terminal capability has a two-letter code name and a specific meaning. For example, the number of columns is named ``co'`. See section 3. Definitions of the Terminal Capabilities, for definitions of all the standard capability names.

Once you have found the proper terminal description with `tgetent` (see section 1.2 Finding a Terminal Description: `tgetent`), your application program must interrogate it for various terminal capabilities. You must specify the two-letter code of the capability whose value you seek.

Capability values can be numeric, boolean (capability is either present or absent) or strings. Any particular capability always has the same value type; for example, ``co'` always has a numeric value, while ``am'` (automatic wrap at margin) is always a flag, and ``cm'` (cursor motion command) always has a string value. The documentation of each capability says which type of value it has.

There are three functions to use to get the value of a capability, depending on the type of value the capability has. Here are their declarations in ANSI C:

    int tgetnum (char *name);
    int tgetflag (char *name);
    char *tgetstr (char *name, char **area);

`tgetnum`
  Use `tgetnum` to get a capability value that is numeric. The argument name is the two-letter code name of the capability. If the capability is present, `tgetnum` returns the numeric value (which is nonnegative). If the capability is not mentioned in the terminal description, `tgetnum` returns -1.
`tgetflag`
  Use `tgetflag` to get a boolean value. If the capability name is present in the terminal description, `tgetflag` returns 1; otherwise, it returns 0.
`tgetstr`
  Use `tgetstr` to get a string value. It returns a pointer to a string which is the capability value, or a null pointer if the capability is not present in the terminal description. There are two ways `tgetstr` can find space to store the string value:
  - You can ask `tgetstr` to allocate the space. Pass a null pointer for the argument area, and `tgetstr` will use `malloc` to allocate storage big enough for the value. Termcap will never free this storage or refer to it again; you should free it when you are finished with it. This method is more robust, since there is no need to guess how much space is needed. But it is supported only by the GNU termcap library.
  - You can provide the space. Provide for the argument area the address of a pointer variable of type `char *`. Before calling `tgetstr`, initialize the variable to point at available space. Then `tgetstr` will store the string value in that space and will increment the pointer variable to point after the space that has been used. You can use the same pointer variable for many calls to `tgetstr`. There is no way to determine how much space is needed for a single string, and no way for you to prevent or handle overflow of the area you have provided. However, you can be sure that the total size of all the string values you will obtain from the terminal description is no greater than the size of the description (unless you get the same capability twice). You can determine that size with `strlen` on the buffer you provided to `tgetent`. See below for an example. Providing the space yourself is the only method supported by the Unix version of termcap.

Note that you do not have to specify a terminal type or terminal description for the interrogation functions. They automatically use the description found by the most recent call to `tgetent`.

Here is an example of interrogating a terminal description for various capabilities, with conditionals to select between the Unix and GNU methods of providing buffer space.

    char *tgetstr ();

    char *cl_string, *cm_string;
    int height;
    int width;
    int auto_wrap;

    char PC;   /* For tputs.  */
    char *BC;  /* For tgoto.  */
    char *UP;

    interrogate_terminal ()
    {
    #ifdef UNIX
      /* Here we assume that an explicit term_buffer
         was provided to tgetent.  */
      char *buffer
        = (char *) malloc (strlen (term_buffer));
    #define BUFFADDR &buffer
    #else
    #define BUFFADDR 0
    #endif

      char *temp;

      /* Extract information we will use.  */
      cl_string = tgetstr ("cl", BUFFADDR);
      cm_string = tgetstr ("cm", BUFFADDR);
      auto_wrap = tgetflag ("am");
      height = tgetnum ("li");
      width = tgetnum ("co");

      /* Extract information that termcap functions use.  */
      temp = tgetstr ("pc", BUFFADDR);
      PC = temp ? *temp : 0;
      BC = tgetstr ("le", BUFFADDR);
      UP = tgetstr ("up", BUFFADDR);
    }

See section 1.5 Padding, for information on the variable `PC`. See section 1.6.2 Sending Display Commands with Parameters, for information on `UP` and `BC`.

## 1.4 Initialization for Use of Termcap

Before starting to output commands to a terminal using termcap, an application program should do two things:

- Initialize various global variables which termcap library output functions refer to. These include `PC` and `ospeed` for padding (see section 1.5.4 Performing Padding with `tputs`) and `UP` and `BC` for cursor motion (see section 1.6.2.2 `tgoto`).
- Tell the kernel to turn off alteration and padding of horizontal-tab characters sent to the terminal.

To turn off output processing in Berkeley Unix you would use `ioctl` with code `TIOCLSET` to set the bit named `LLITOUT`, and clear the bits `ANYDELAY` using `TIOCSETN`. In POSIX or System V, you must clear the bit named `OPOST`. Refer to the system documentation for details.

If you do not set the terminal flags properly, some older terminals will not work. This is because their commands may contain the characters that normally signify newline, carriage return and horizontal tab--characters which the kernel thinks it ought to modify before output.

When you change the kernel's terminal flags, you must arrange to restore them to their normal state when your program exits. This implies that the program must catch fatal signals such as `SIGQUIT` and `SIGINT` and restore the old terminal flags before actually terminating.

Modern terminals' commands do not use these special characters, so if you do not care about problems with old terminals, you can leave the kernel's terminal flags unaltered.

## 1.5 Padding

Padding means outputting null characters following a terminal display command that takes a long time to execute. The terminal description says which commands require padding and how much; the function `tputs`, described below, outputs a terminal command while extracting from it the padding information, and then outputs the padding that is necessary.

### 1.5.1 Why Pad, and How

Most types of terminal have commands that take longer to execute than they do to send over a high-speed line. For example, clearing the screen may take 20msec once the entire command is received. During that time, on a 9600 bps line, the terminal could receive about 20 additional output characters while still busy clearing the screen. Every terminal has a certain amount of buffering capacity to remember output characters that cannot be processed yet, but too many slow commands in a row can cause the buffer to fill up. Then any additional output that cannot be processed immediately will be lost.

To avoid this problem, we normally follow each display command with enough useless charaters (usually null characters) to fill up the time that the display command needs to execute. This does the job if the terminal throws away null characters without using up space in the buffer (which most terminals do). If enough padding is used, no output can ever be lost. The right amount of padding avoids loss of output without slowing down operation, since the time used to transmit padding is time that nothing else could be done.

The number of padding characters needed for an operation depends on the line speed. In fact, it is proportional to the line speed. A 9600 baud line transmits about one character per msec, so the clear screen command in the example above would need about 20 characters of padding. At 1200 baud, however, only about 3 characters of padding are needed to fill up 20msec.

### 1.5.2 When There Is Not Enough Padding

There are several common manifestations of insufficient padding.

- Emacs displays ``I-search: ^Q-'` at the bottom of the screen. This means that the terminal thought its buffer was getting full of display commands, so it tried to tell the computer to stop sending any.
- The screen is garbled intermittently, or the details of garbling vary when you repeat the action. (A garbled screen could be due to a command which is simply incorrect, or to user option in the terminal which doesn't match the assumptions of the terminal description, but this usually leads to reproducible failure.) This means that the buffer did get full, and some commands were lost. Many changeable factors can change which ones are lost.
- Screen is garbled at high output speeds but not at low speeds. Padding problems nearly always go away at low speeds, usually even at 1200 baud. This means that a high enough speed permits commands to arrive faster than they can be executed.

Although any obscure command on an obscure terminal might lack padding, in practice problems arise most often from the clearing commands ``cl'` and ``cd'` (see section 3.7 Clearing Parts of the Screen), the scrolling commands ``sf'` and ``sr'` (see section 3.5 Scrolling), and the line insert/delete commands ``al'` and ``dl'` (see section 3.8 Insert/Delete Line).

Occasionally the terminal description fails to define ``sf'` and some programs will use ``do'` instead, so you may get a problem with ``do'`. If so, first define ``sf'` just like ``do'`, then add some padding to ``sf'`.

The best strategy is to add a lot of padding at first, perhaps 200 msec. This is much more than enough; in fact, it should cause a visible slowdown. (If you don't see a slowdown, the change has not taken effect; see section 2.5 When Changes in the Data Base Take Effect.) If this makes the problem go away, you have found the right place to add padding; now reduce the amount until the problem comes back, then increase it again. If the problem remains, either it is in some other capability or it is not a matter of padding at all.

Keep in mind that on many terminals the correct padding for insert/delete line or for scrolling is cursor-position dependent. If you get problems from scrolling a large region of the screen but not from scrolling a small part (just a few lines moving), it may mean that fixed padding should be replaced with position-dependent padding.

### 1.5.3 Specifying Padding in a Terminal Description

In the terminal description, the amount of padding required by each display command is recorded as a sequence of digits at the front of the command. These digits specify the padding time in milliseconds (msec). They can be followed optionally by a decimal point and one more digit, which is a number of tenths of msec.

Sometimes the padding needed by a command depends on the cursor position. For example, the time taken by an "insert line" command is usually proportional to the number of lines that need to be moved down or cleared. An asterisk (``*'`) following the padding time says that the time should be multiplied by the number of screen lines affected by the command.

    :al=1.3*\E[L:

is used to describe the "insert line" command for a certain terminal. The padding required is 1.3 msec per line affected. The command itself is ```ESC` [ L'`.

The padding time specified in this way tells `tputs` how many pad characters to output. See section 1.5.4 Performing Padding with `tputs`.

Two special capability values affect padding for all commands. These are the ``pc'` and ``pb'`. The variable ``pc'` specifies the character to pad with, and ``pb'` the speed below which no padding is needed. The defaults for these variables, a null character and 0, are correct for most terminals. See section 3.17 Padding Capabilities.

### 1.5.4 Performing Padding with `tputs`

Use the termcap function `tputs` to output a string containing an optional padding spec of the form described above (see section 1.5.3 Specifying Padding in a Terminal Description). The function `tputs` strips off and decodes the padding spec, outputs the rest of the string, and then outputs the appropriate padding. Here is its declaration in ANSI C:

    char PC;
    short ospeed;

    int tputs (char *string, int nlines, int (*outfun) ());

Here string is the string (including padding spec) to be output; nlines is the number of lines affected by the operation, which is used to multiply the amount of padding if the padding spec ends with a ``*'`. Finally, outfun is a function (such as `fputchar`) that is called to output each character. When actually called, outfun should expect one argument, a character.

The operation of `tputs` is controlled by two global variables, `ospeed` and `PC`. The value of `ospeed` is supposed to be the terminal output speed, encoded as in the `ioctl` system call which gets the speed information. This is needed to compute the number of padding characters. The value of `PC` is the character used for padding.

You are responsible for storing suitable values into these variables before using `tputs`. The value stored into the `PC` variable should be taken from the ``pc'` capability in the terminal description (see section 3.17 Padding Capabilities). Store zero in `PC` if there is no ``pc'` capability.

The argument nlines requires some thought. Normally, it should be the number of lines whose contents will be cleared or moved by the command. For cursor motion commands, or commands that do editing within one line, use the value 1. For most commands that affect multiple lines, such as ``al'` (insert a line) and ``cd'` (clear from the cursor to the end of the screen), nlines should be the screen height minus the current vertical position (origin 0). For multiple insert and scroll commands such as ``AL'` (insert multiple lines), that same value for nlines is correct; the number of lines being inserted is not correct.

If a "scroll window" feature is used to reduce the number of lines affected by a command, the value of nlines should take this into account. This is because the delay time required depends on how much work the terminal has to do, and the scroll window feature reduces the work. See section 3.5 Scrolling.

Commands such as ``ic'` and ``dc'` (insert or delete characters) are problematical because the padding needed by these commands is proportional to the number of characters affected, which is the number of columns from the cursor to the end of the line. It would be nice to have a way to specify such a dependence, and there is no need for dependence on vertical position in these commands, so it is an obvious idea to say that for these commands nlines should really be the number of columns affected. However, the definition of termcap clearly says that nlines is always the number of lines affected, even in this case, where it is always 1. It is not easy to change this rule now, because too many programs and terminal descriptions have been written to follow it.

Because nlines is always 1 for the ``ic'` and ``dc'` strings, there is no reason for them to use ``*'`, but some of them do. These should be corrected by deleting the ``*'`. If, some day, such entries have disappeared, it may be possible to change to a more useful convention for the nlines argument for these operations without breaking any programs.

## 1.6 Filling In Parameters

Some terminal control strings require numeric parameters. For example, when you move the cursor, you need to say what horizontal and vertical positions to move it to. The value of the terminal's ``cm'` capability, which says how to move the cursor, cannot simply be a string of characters; it must say how to express the cursor position numbers and where to put them within the command.

The specifications of termcap include conventions as to which string-valued capabilities require parameters, how many parameters, and what the parameters mean; for example, it defines the ``cm'` string to take two parameters, the vertical and horizontal positions, with 0,0 being the upper left corner. These conventions are described where the individual commands are documented.

Termcap also defines a language used within the capability definition for specifying how and where to encode the parameters for output. This language uses character sequences starting with ``%'`. (This is the same idea as `printf`, but the details are different.) The language for parameter encoding is described in this section.

A program that is doing display output calls the functions `tparam` or `tgoto` to encode parameters according to the specifications. These functions produce a string containing the actual commands to be output (as well a padding spec which must be processed with `tputs`; see section 1.5 Padding).

### 1.6.1 Describing the Encoding

A terminal command string that requires parameters contains special character sequences starting with ``%'` to say how to encode the parameters. These sequences control the actions of `tparam` and `tgoto`.

The parameters values passed to `tparam` or `tgoto` are considered to form a vector. A pointer into this vector determines the next parameter to be processed. Some of the ``%'`-sequences encode one parameter and advance the pointer to the next parameter. Other ``%'`-sequences alter the pointer or alter the parameter values without generating output.

For example, the ``cm'` string for a standard ANSI terminal is written as ``\E[%i%d;%dH'`. (``\E'` stands for `ESC`.) ``cm'` by convention always requires two parameters, the vertical and horizontal goal positions, so this string specifies the encoding of two parameters. Here ``%i'` increments the two values supplied, and each ``%d'` encodes one of the values in decimal. If the cursor position values 20,58 are encoded with this string, the result is ``\E[21;59H'`.

First, here are the ``%'`-sequences that generate output. Except for ``%%'`, each of them encodes one parameter and advances the pointer to the following parameter.

``%%'`
  Output a single ``%'`. This is the only way to represent a literal ``%'` in a terminal command with parameters. ``%%'` does not use up a parameter.
``%d'`
  As in `printf`, output the next parameter in decimal.
``%2'`
  Like ``%02d'` in `printf`: output the next parameter in decimal, and always use at least two digits.
``%3'`
  Like ``%03d'` in `printf`: output the next parameter in decimal, and always use at least three digits. Note that ``%4'` and so on are not defined.
``%.'`
  Output the next parameter as a single character whose ASCII code is the parameter value. Like ``%c'` in `printf`.
``%+char'`
  Add the next parameter to the character char, and output the resulting character. For example, ``%+ '` represents 0 as a space, 1 as ``!'`, etc.

The following ``%'`-sequences specify alteration of the parameters (their values, or their order) rather than encoding a parameter for output. They generate no output; they are used only for their side effects on the parameters. Also, they do not advance the "next parameter" pointer except as explicitly stated. Only ``%i'`, ``%r'` and ``%>'` are defined in standard Unix termcap. The others are GNU extensions.

``%i'`
  Increment the next two parameters. This is used for terminals that expect cursor positions in origin 1. For example, ``%i%d,%d'` would output two parameters with ``1'` for 0, ``2'` for 1, etc.
``%r'`
  Interchange the next two parameters. This is used for terminals whose cursor positioning command expects the horizontal position first.
``%s'`
  Skip the next parameter. Do not output anything.
``%b'`
  Back up one parameter. The last parameter used will become once again the next parameter to be output, and the next output command will use it. Using ``%b'` more than once, you can back up any number of parameters, and you can refer to each parameter any number of times.
``%>c1c2'`
  Conditionally increment the next parameter. Here c1 and c2 are characters which stand for their ASCII codes as numbers. If the next parameter is greater than the ASCII code of c1, the ASCII code of c2 is added to it.
``%a op type pos'`
  Perform arithmetic on the next parameter, do not use it up, and do not output anything. Here op specifies the arithmetic operation, while type and pos together specify the other operand. Spaces are used above to separate the operands for clarity; the spaces don't appear in the data base, where this sequence is exactly five characters long. The character op says what kind of arithmetic operation to perform. It can be any of these characters:
  ``='`
    assign a value to the next parameter, ignoring its old value. The new value comes from the other operand.
  ``+'`
    add the other operand to the next parameter.
  ``-'`
    subtract the other operand from the next parameter.
  ``*'`
    multiply the next parameter by the other operand.
  ``/'`
    divide the next parameter by the other operand.
  The "other operand" may be another parameter's value or a constant; the character type says which. It can be:
  ``p'`
    Use another parameter. The character pos says which parameter to use. Subtract 64 from its ASCII code to get the position of the desired parameter relative to this one. Thus, the character ``A'` as pos means the parameter after the next one; the character ``?'` means the parameter before the next one.
  ``c'`
    Use a constant value. The character pos specifies the value of the constant. The 0200 bit is cleared out, so that 0200 can be used to represent zero.

The following ``%'`-sequences are special purpose hacks to compensate for the weird designs of obscure terminals. They modify the next parameter or the next two parameters but do not generate output and do not use up any parameters. ``%m'` is a GNU extension; the others are defined in standard Unix termcap.

``%n'`
  Exclusive-or the next parameter with 0140, and likewise the parameter after next.
``%m'`
  Complement all the bits of the next parameter and the parameter after next.
``%B'`
  Encode the next parameter in BCD. It alters the value of the parameter by adding six times the quotient of the parameter by ten. Here is a C statement that shows how the new value is computed:

      parm = (parm / 10) * 16 + parm % 10;

``%D'`
  Transform the next parameter as needed by Delta Data terminals. This involves subtracting twice the remainder of the parameter by 16.

      parm -= 2 * (parm % 16);

### 1.6.2 Sending Display Commands with Parameters

The termcap library functions `tparam` and `tgoto` serve as the analog of `printf` for terminal string parameters. The newer function `tparam` is a GNU extension, more general but missing from Unix termcap. The original parameter-encoding function is `tgoto`, which is preferable for cursor motion.

#### 1.6.2.1 `tparam`

The function `tparam` can encode display commands with any number of parameters and allows you to specify the buffer space. It is the preferred function for encoding parameters for all but the ``cm'` capability. Its ANSI C declaration is as follows:

    char *tparam (char *ctlstring, char *buffer, int size, int parm1,...)

The arguments are a control string ctlstring (the value of a terminal capability, presumably), an output buffer buffer and size, and any number of integer parameters to be encoded. The effect of `tparam` is to copy the control string into the buffer, encoding parameters according to the ``%'` sequences in the control string.

You describe the output buffer by its address, buffer, and its size in bytes, size. If the buffer is not big enough for the data to be stored in it, `tparam` calls `malloc` to get a larger buffer. In either case, `tparam` returns the address of the buffer it ultimately uses. If the value equals buffer, your original buffer was used. Otherwise, a new buffer was allocated, and you must free it after you are done with printing the results. If you pass zero for size and buffer, `tparam` always allocates the space with `malloc`.

All capabilities that require parameters also have the ability to specify padding, so you should use `tputs` to output the string produced by `tparam`. See section 1.5 Padding. Here is an example.

    {
    char *buf;
    char buffer[40];

    buf = tparam (command, buffer, 40, parm);
    tputs (buf, 1, fputchar);
    if (buf != buffer)
    free (buf);
    }

If a parameter whose value is zero is encoded with ``%.'`-style encoding, the result is a null character, which will confuse `tputs`. This would be a serious problem, but luckily ``%.'` encoding is used only by a few old models of terminal, and only for the ``cm'` capability. To solve the problem, use `tgoto` rather than `tparam` to encode the ``cm'` capability.

#### 1.6.2.2 `tgoto`

The special case of cursor motion is handled by `tgoto`. There are two reasons why you might choose to use `tgoto`:

- For Unix compatibility, because Unix termcap does not have `tparam`.
- For the ``cm'` capability, since `tgoto` has a special feature to avoid problems with null characters, tabs and newlines on certain old terminal types that use ``%.'` encoding for that capability.

Here is how `tgoto` might be declared in ANSI C:

    char *tgoto (char *cstring, int hpos, int vpos)

There are three arguments, the terminal description's ``cm'` string and the two cursor position numbers; `tgoto` computes the parametrized string in an internal static buffer and returns the address of that buffer. The next time you use `tgoto` the same buffer will be reused.

Parameters encoded with ``%.'` encoding can generate null characters, tabs or newlines. These might cause trouble: the null character because `tputs` would think that was the end of the string, the tab because the kernel or other software might expand it into spaces, and the newline becaue the kernel might add a carriage-return, or padding characters normally used for a newline. To prevent such problems, `tgoto` is careful to avoid these characters. Here is how this works: if the target cursor position value is such as to cause a problem (that is to say, zero, nine or ten), `tgoto` increments it by one, then compensates by appending a string to move the cursor back or up one position.

The compensation strings to use for moving back or up are found in global variables named `BC` and `UP`. These are actual external C variables with upper case names; they are declared `char *`. It is up to you to store suitable values in them, normally obtained from the ``le'` and ``up'` terminal capabilities in the terminal description with `tgetstr`. Alternatively, if these two variables are both zero, the feature of avoiding nulls, tabs and newlines is turned off.

It is safe to use `tgoto` for commands other than ``cm'` only if you have stored zero in `BC` and `UP`.

Note that `tgoto` reverses the order of its operands: the horizontal position comes before the vertical position in the arguments to `tgoto`, even though the vertical position comes before the horizontal in the parameters of the ``cm'` string. If you use `tgoto` with a command such as ``AL'` that takes one parameter, you must pass the parameter to `tgoto` as the "vertical position".

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