xref: /freebsd/contrib/ncurses/doc/ncurses-intro.doc (revision 7a69bbfb278952228c9aa1eca241e65fcdef425e)

                         Writing Programs with NCURSES

     by Eric S. Raymond and Zeyd M. Ben-Halim
     updates since release 1.9.9e by Thomas Dickey

                                   Contents

     * Introduction
          + A Brief History of Curses
          + Scope of This Document
          + Terminology
     * The Curses Library
          + An Overview of Curses
               o Compiling Programs using Curses
               o Updating the Screen
               o Standard Windows and Function Naming Conventions
               o Variables
          + Using the Library
               o Starting up
               o Output
               o Input
               o Using Forms Characters
               o Character Attributes and Color
               o Mouse Interfacing
               o Finishing Up
          + Function Descriptions
               o Initialization and Wrapup
               o Causing Output to the Terminal
               o Low-Level Capability Access
               o Debugging
          + Hints, Tips, and Tricks
               o Some Notes of Caution
               o Temporarily Leaving ncurses Mode
               o Using ncurses under xterm
               o Handling Multiple Terminal Screens
               o Testing for Terminal Capabilities
               o Tuning for Speed
               o Special Features of ncurses
          + Compatibility with Older Versions
               o Refresh of Overlapping Windows
               o Background Erase
          + XSI Curses Conformance
     * The Panels Library
          + Compiling With the Panels Library
          + Overview of Panels
          + Panels, Input, and the Standard Screen
          + Hiding Panels
          + Miscellaneous Other Facilities
     * The Menu Library
          + Compiling with the menu Library
          + Overview of Menus
          + Selecting items
          + Menu Display
          + Menu Windows
          + Processing Menu Input
          + Miscellaneous Other Features
     * The Forms Library
          + Compiling with the forms Library
          + Overview of Forms
          + Creating and Freeing Fields and Forms
          + Fetching and Changing Field Attributes
               o Fetching Size and Location Data
               o Changing the Field Location
               o The Justification Attribute
               o Field Display Attributes
               o Field Option Bits
               o Field Status
               o Field User Pointer
          + Variable-Sized Fields
          + Field Validation
               o TYPE_ALPHA
               o TYPE_ALNUM
               o TYPE_ENUM
               o TYPE_INTEGER
               o TYPE_NUMERIC
               o TYPE_REGEXP
          + Direct Field Buffer Manipulation
          + Attributes of Forms
          + Control of Form Display
          + Input Processing in the Forms Driver
               o Page Navigation Requests
               o Inter-Field Navigation Requests
               o Intra-Field Navigation Requests
               o Scrolling Requests
               o Field Editing Requests
               o Order Requests
               o Application Commands
          + Field Change Hooks
          + Field Change Commands
          + Form Options
          + Custom Validation Types
               o Union Types
               o New Field Types
               o Validation Function Arguments
               o Order Functions For Custom Types
               o Avoiding Problems
     _________________________________________________________________

                                 Introduction

   This document is an introduction to programming with curses. It is not
   an exhaustive reference for the curses Application Programming
   Interface (API); that role is filled by the curses manual pages.
   Rather, it is intended to help C programmers ease into using the
   package.

   This document is aimed at C applications programmers not yet
   specifically familiar with ncurses. If you are already an experienced
   curses programmer, you should nevertheless read the sections on Mouse
   Interfacing, Debugging, Compatibility with Older Versions, and Hints,
   Tips, and Tricks. These will bring you up to speed on the special
   features and quirks of the ncurses implementation. If you are not so
   experienced, keep reading.

   The curses package is a subroutine library for terminal-independent
   screen-painting and input-event handling which presents a high level
   screen model to the programmer, hiding differences between terminal
   types and doing automatic optimization of output to change one screen
   full of text into another. Curses uses terminfo, which is a database
   format that can describe the capabilities of thousands of different
   terminals.

   The curses API may seem something of an archaism on UNIX desktops
   increasingly dominated by X, Motif, and Tcl/Tk. Nevertheless, UNIX
   still supports tty lines and X supports xterm(1); the curses API has
   the advantage of (a) back-portability to character-cell terminals, and
   (b) simplicity. For an application that does not require bit-mapped
   graphics and multiple fonts, an interface implementation using curses
   will typically be a great deal simpler and less expensive than one
   using an X toolkit.

A Brief History of Curses

   Historically, the first ancestor of curses was the routines written to
   provide screen-handling for the game rogue; these used the
   already-existing termcap database facility for describing terminal
   capabilities. These routines were abstracted into a documented library
   and first released with the early BSD UNIX versions.

   System III UNIX from Bell Labs featured a rewritten and much-improved
   curses library. It introduced the terminfo format. Terminfo is based
   on Berkeley's termcap database, but contains a number of improvements
   and extensions. Parameterized capabilities strings were introduced,
   making it possible to describe multiple video attributes, and colors
   and to handle far more unusual terminals than possible with termcap.
   In the later AT&T System V releases, curses evolved to use more
   facilities and offer more capabilities, going far beyond BSD curses in
   power and flexibility.

Scope of This Document

   This document describes ncurses, a free implementation of the System V
   curses API with some clearly marked extensions. It includes the
   following System V curses features:
     * Support for multiple screen highlights (BSD curses could only
       handle one `standout' highlight, usually reverse-video).
     * Support for line- and box-drawing using forms characters.
     * Recognition of function keys on input.
     * Color support.
     * Support for pads (windows of larger than screen size on which the
       screen or a subwindow defines a viewport).

   Also, this package makes use of the insert and delete line and
   character features of terminals so equipped, and determines how to
   optimally use these features with no help from the programmer. It
   allows arbitrary combinations of video attributes to be displayed,
   even on terminals that leave ``magic cookies'' on the screen to mark
   changes in attributes.

   The ncurses package can also capture and use event reports from a
   mouse in some environments (notably, xterm under the X window system).
   This document includes tips for using the mouse.

   The ncurses package was originated by Pavel Curtis. The original
   maintainer of this package is Zeyd Ben-Halim <zmbenhal@netcom.com>.
   Eric S. Raymond <esr@snark.thyrsus.com> wrote many of the new features
   in versions after 1.8.1 and wrote most of this introduction. J�rgen
   Pfeifer wrote all of the menu and forms code as well as the Ada95
   binding. Ongoing work is being done by Thomas Dickey and J�rgen
   Pfeifer. Florian La Roche acts as the maintainer for the Free Software
   Foundation, which holds the copyright on ncurses. Contact the current
   maintainers at bug-ncurses@gnu.org.

   This document also describes the panels extension library, similarly
   modeled on the SVr4 panels facility. This library allows you to
   associate backing store with each of a stack or deck of overlapping
   windows, and provides operations for moving windows around in the
   stack that change their visibility in the natural way (handling window
   overlaps).

   Finally, this document describes in detail the menus and forms
   extension libraries, also cloned from System V, which support easy
   construction and sequences of menus and fill-in forms.

Terminology

   In this document, the following terminology is used with reasonable
   consistency:

   window
          A data structure describing a sub-rectangle of the screen
          (possibly the entire screen). You can write to a window as
          though it were a miniature screen, scrolling independently of
          other windows on the physical screen.

   screens
          A subset of windows which are as large as the terminal screen,
          i.e., they start at the upper left hand corner and encompass
          the lower right hand corner. One of these, stdscr, is
          automatically provided for the programmer.

   terminal screen
          The package's idea of what the terminal display currently looks
          like, i.e., what the user sees now. This is a special screen.

                              The Curses Library

An Overview of Curses

  Compiling Programs using Curses

   In order to use the library, it is necessary to have certain types and
   variables defined. Therefore, the programmer must have a line:
          #include <curses.h>

   at the top of the program source. The screen package uses the Standard
   I/O library, so <curses.h> includes <stdio.h>. <curses.h> also
   includes <termios.h>, <termio.h>, or <sgtty.h> depending on your
   system. It is redundant (but harmless) for the programmer to do these
   includes, too. In linking with curses you need to have -lncurses in
   your LDFLAGS or on the command line. There is no need for any other
   libraries.

  Updating the Screen

   In order to update the screen optimally, it is necessary for the
   routines to know what the screen currently looks like and what the
   programmer wants it to look like next. For this purpose, a data type
   (structure) named WINDOW is defined which describes a window image to
   the routines, including its starting position on the screen (the (y,
   x) coordinates of the upper left hand corner) and its size. One of
   these (called curscr, for current screen) is a screen image of what
   the terminal currently looks like. Another screen (called stdscr, for
   standard screen) is provided by default to make changes on.

   A window is a purely internal representation. It is used to build and
   store a potential image of a portion of the terminal. It doesn't bear
   any necessary relation to what is really on the terminal screen; it's
   more like a scratchpad or write buffer.

   To make the section of physical screen corresponding to a window
   reflect the contents of the window structure, the routine refresh()
   (or wrefresh() if the window is not stdscr) is called.

   A given physical screen section may be within the scope of any number
   of overlapping windows. Also, changes can be made to windows in any
   order, without regard to motion efficiency. Then, at will, the
   programmer can effectively say ``make it look like this,'' and let the
   package implementation determine the most efficient way to repaint the
   screen.

  Standard Windows and Function Naming Conventions

   As hinted above, the routines can use several windows, but two are
   automatically given: curscr, which knows what the terminal looks like,
   and stdscr, which is what the programmer wants the terminal to look
   like next. The user should never actually access curscr directly.
   Changes should be made to through the API, and then the routine
   refresh() (or wrefresh()) called.

   Many functions are defined to use stdscr as a default screen. For
   example, to add a character to stdscr, one calls addch() with the
   desired character as argument. To write to a different window. use the
   routine waddch() (for `w'indow-specific addch()) is provided. This
   convention of prepending function names with a `w' when they are to be
   applied to specific windows is consistent. The only routines which do
   not follow it are those for which a window must always be specified.

   In order to move the current (y, x) coordinates from one point to
   another, the routines move() and wmove() are provided. However, it is
   often desirable to first move and then perform some I/O operation. In
   order to avoid clumsiness, most I/O routines can be preceded by the
   prefix 'mv' and the desired (y, x) coordinates prepended to the
   arguments to the function. For example, the calls
          move(y, x);
          addch(ch);

   can be replaced by
          mvaddch(y, x, ch);

   and
          wmove(win, y, x);
          waddch(win, ch);

   can be replaced by
          mvwaddch(win, y, x, ch);

   Note that the window description pointer (win) comes before the added
   (y, x) coordinates. If a function requires a window pointer, it is
   always the first parameter passed.

  Variables

   The curses library sets some variables describing the terminal
   capabilities.
      type   name      description
      ------------------------------------------------------------------
      int    LINES     number of lines on the terminal
      int    COLS      number of columns on the terminal

   The curses.h also introduces some #define constants and types of
   general usefulness:

   bool
          boolean type, actually a `char' (e.g., bool doneit;)

   TRUE
          boolean `true' flag (1).

   FALSE
          boolean `false' flag (0).

   ERR
          error flag returned by routines on a failure (-1).

   OK
          error flag returned by routines when things go right.

Using the Library

   Now we describe how to actually use the screen package. In it, we
   assume all updating, reading, etc. is applied to stdscr. These
   instructions will work on any window, providing you change the
   function names and parameters as mentioned above.

   Here is a sample program to motivate the discussion:
#include <curses.h>
#include <signal.h>

static void finish(int sig);

int
main(int argc, char *argv[])
{
    int num = 0;

    /* initialize your non-curses data structures here */

    (void) signal(SIGINT, finish);      /* arrange interrupts to terminate */

    (void) initscr();      /* initialize the curses library */
    keypad(stdscr, TRUE);  /* enable keyboard mapping */
    (void) nonl();         /* tell curses not to do NL->CR/NL on output */
    (void) cbreak();       /* take input chars one at a time, no wait for \n */
    (void) echo();         /* echo input - in color */

    if (has_colors())
    {
        start_color();

        /*
         * Simple color assignment, often all we need.  Color pair 0 cannot
         * be redefined.  This example uses the same value for the color
         * pair as for the foreground color, though of course that is not
         * necessary:
         */
        init_pair(1, COLOR_RED,     COLOR_BLACK);
        init_pair(2, COLOR_GREEN,   COLOR_BLACK);
        init_pair(3, COLOR_YELLOW,  COLOR_BLACK);
        init_pair(4, COLOR_BLUE,    COLOR_BLACK);
        init_pair(5, COLOR_CYAN,    COLOR_BLACK);
        init_pair(6, COLOR_MAGENTA, COLOR_BLACK);
        init_pair(7, COLOR_WHITE,   COLOR_BLACK);
    }

    for (;;)
    {
        int c = getch();     /* refresh, accept single keystroke of input */
        attrset(COLOR_PAIR(num % 8));
        num++;

        /* process the command keystroke */
    }

    finish(0);               /* we're done */
}

static void finish(int sig)
{
    endwin();

    /* do your non-curses wrapup here */

    exit(0);
}

  Starting up

   In order to use the screen package, the routines must know about
   terminal characteristics, and the space for curscr and stdscr must be
   allocated. These function initscr() does both these things. Since it
   must allocate space for the windows, it can overflow memory when
   attempting to do so. On the rare occasions this happens, initscr()
   will terminate the program with an error message. initscr() must
   always be called before any of the routines which affect windows are
   used. If it is not, the program will core dump as soon as either
   curscr or stdscr are referenced. However, it is usually best to wait
   to call it until after you are sure you will need it, like after
   checking for startup errors. Terminal status changing routines like
   nl() and cbreak() should be called after initscr().

   Once the screen windows have been allocated, you can set them up for
   your program. If you want to, say, allow a screen to scroll, use
   scrollok(). If you want the cursor to be left in place after the last
   change, use leaveok(). If this isn't done, refresh() will move the
   cursor to the window's current (y, x) coordinates after updating it.

   You can create new windows of your own using the functions newwin(),
   derwin(), and subwin(). The routine delwin() will allow you to get rid
   of old windows. All the options described above can be applied to any
   window.

  Output

   Now that we have set things up, we will want to actually update the
   terminal. The basic functions used to change what will go on a window
   are addch() and move(). addch() adds a character at the current (y, x)
   coordinates. move() changes the current (y, x) coordinates to whatever
   you want them to be. It returns ERR if you try to move off the window.
   As mentioned above, you can combine the two into mvaddch() to do both
   things at once.

   The other output functions, such as addstr() and printw(), all call
   addch() to add characters to the window.

   After you have put on the window what you want there, when you want
   the portion of the terminal covered by the window to be made to look
   like it, you must call refresh(). In order to optimize finding
   changes, refresh() assumes that any part of the window not changed
   since the last refresh() of that window has not been changed on the
   terminal, i.e., that you have not refreshed a portion of the terminal
   with an overlapping window. If this is not the case, the routine
   touchwin() is provided to make it look like the entire window has been
   changed, thus making refresh() check the whole subsection of the
   terminal for changes.

   If you call wrefresh() with curscr as its argument, it will make the
   screen look like curscr thinks it looks like. This is useful for
   implementing a command which would redraw the screen in case it get
   messed up.

  Input

   The complementary function to addch() is getch() which, if echo is
   set, will call addch() to echo the character. Since the screen package
   needs to know what is on the terminal at all times, if characters are
   to be echoed, the tty must be in raw or cbreak mode. Since initially
   the terminal has echoing enabled and is in ordinary ``cooked'' mode,
   one or the other has to changed before calling getch(); otherwise, the
   program's output will be unpredictable.

   When you need to accept line-oriented input in a window, the functions
   wgetstr() and friends are available. There is even a wscanw() function
   that can do scanf()(3)-style multi-field parsing on window input.
   These pseudo-line-oriented functions turn on echoing while they
   execute.

   The example code above uses the call keypad(stdscr, TRUE) to enable
   support for function-key mapping. With this feature, the getch() code
   watches the input stream for character sequences that correspond to
   arrow and function keys. These sequences are returned as
   pseudo-character values. The #define values returned are listed in the
   curses.h The mapping from sequences to #define values is determined by
   key_ capabilities in the terminal's terminfo entry.

  Using Forms Characters

   The addch() function (and some others, including box() and border())
   can accept some pseudo-character arguments which are specially defined
   by ncurses. These are #define values set up in the curses.h header;
   see there for a complete list (look for the prefix ACS_).

   The most useful of the ACS defines are the forms-drawing characters.
   You can use these to draw boxes and simple graphs on the screen. If
   the terminal does not have such characters, curses.h will map them to
   a recognizable (though ugly) set of ASCII defaults.

  Character Attributes and Color

   The ncurses package supports screen highlights including standout,
   reverse-video, underline, and blink. It also supports color, which is
   treated as another kind of highlight.

   Highlights are encoded, internally, as high bits of the
   pseudo-character type (chtype) that curses.h uses to represent the
   contents of a screen cell. See the curses.h header file for a complete
   list of highlight mask values (look for the prefix A_).

   There are two ways to make highlights. One is to logical-or the value
   of the highlights you want into the character argument of an addch()
   call, or any other output call that takes a chtype argument.

   The other is to set the current-highlight value. This is logical-or'ed
   with any highlight you specify the first way. You do this with the
   functions attron(), attroff(), and attrset(); see the manual pages for
   details. Color is a special kind of highlight. The package actually
   thinks in terms of color pairs, combinations of foreground and
   background colors. The sample code above sets up eight color pairs,
   all of the guaranteed-available colors on black. Note that each color
   pair is, in effect, given the name of its foreground color. Any other
   range of eight non-conflicting values could have been used as the
   first arguments of the init_pair() values.

   Once you've done an init_pair() that creates color-pair N, you can use
   COLOR_PAIR(N) as a highlight that invokes that particular color
   combination. Note that COLOR_PAIR(N), for constant N, is itself a
   compile-time constant and can be used in initializers.

  Mouse Interfacing

   The ncurses library also provides a mouse interface.

     NOTE: this facility is specific to ncurses, it is not part of
     either the XSI Curses standard, nor of System V Release 4, nor BSD
     curses. System V Release 4 curses contains code with similar
     interface definitions, however it is not documented. Other than by
     disassembling the library, we have no way to determine exactly how
     that mouse code works. Thus, we recommend that you wrap
     mouse-related code in an #ifdef using the feature macro
     NCURSES_MOUSE_VERSION so it will not be compiled and linked on
     non-ncurses systems.

   Presently, mouse event reporting works in the following environments:
     * xterm and similar programs such as rxvt.
     * Linux console, when configured with gpm(1), Alessandro Rubini's
       mouse server.
     * OS/2 EMX

   The mouse interface is very simple. To activate it, you use the
   function mousemask(), passing it as first argument a bit-mask that
   specifies what kinds of events you want your program to be able to
   see. It will return the bit-mask of events that actually become
   visible, which may differ from the argument if the mouse device is not
   capable of reporting some of the event types you specify.

   Once the mouse is active, your application's command loop should watch
   for a return value of KEY_MOUSE from wgetch(). When you see this, a
   mouse event report has been queued. To pick it off the queue, use the
   function getmouse() (you must do this before the next wgetch(),
   otherwise another mouse event might come in and make the first one
   inaccessible).

   Each call to getmouse() fills a structure (the address of which you'll
   pass it) with mouse event data. The event data includes zero-origin,
   screen-relative character-cell coordinates of the mouse pointer. It
   also includes an event mask. Bits in this mask will be set,
   corresponding to the event type being reported.

   The mouse structure contains two additional fields which may be
   significant in the future as ncurses interfaces to new kinds of
   pointing device. In addition to x and y coordinates, there is a slot
   for a z coordinate; this might be useful with touch-screens that can
   return a pressure or duration parameter. There is also a device ID
   field, which could be used to distinguish between multiple pointing
   devices.

   The class of visible events may be changed at any time via
   mousemask(). Events that can be reported include presses, releases,
   single-, double- and triple-clicks (you can set the maximum
   button-down time for clicks). If you don't make clicks visible, they
   will be reported as press-release pairs. In some environments, the
   event mask may include bits reporting the state of shift, alt, and
   ctrl keys on the keyboard during the event.

   A function to check whether a mouse event fell within a given window
   is also supplied. You can use this to see whether a given window
   should consider a mouse event relevant to it.

   Because mouse event reporting will not be available in all
   environments, it would be unwise to build ncurses applications that
   require the use of a mouse. Rather, you should use the mouse as a
   shortcut for point-and-shoot commands your application would normally
   accept from the keyboard. Two of the test games in the ncurses
   distribution (bs and knight) contain code that illustrates how this
   can be done.

   See the manual page curs_mouse(3X) for full details of the
   mouse-interface functions.

  Finishing Up

   In order to clean up after the ncurses routines, the routine endwin()
   is provided. It restores tty modes to what they were when initscr()
   was first called, and moves the cursor down to the lower-left corner.
   Thus, anytime after the call to initscr, endwin() should be called
   before exiting.

Function Descriptions

   We describe the detailed behavior of some important curses functions
   here, as a supplement to the manual page descriptions.

  Initialization and Wrapup

   initscr()
          The first function called should almost always be initscr().
          This will determine the terminal type and initialize curses
          data structures. initscr() also arranges that the first call to
          refresh() will clear the screen. If an error occurs a message
          is written to standard error and the program exits. Otherwise
          it returns a pointer to stdscr. A few functions may be called
          before initscr (slk_init(), filter(), ripofflines(), use_env(),
          and, if you are using multiple terminals, newterm().)

   endwin()
          Your program should always call endwin() before exiting or
          shelling out of the program. This function will restore tty
          modes, move the cursor to the lower left corner of the screen,
          reset the terminal into the proper non-visual mode. Calling
          refresh() or doupdate() after a temporary escape from the
          program will restore the ncurses screen from before the escape.

   newterm(type, ofp, ifp)
          A program which outputs to more than one terminal should use
          newterm() instead of initscr(). newterm() should be called once
          for each terminal. It returns a variable of type SCREEN * which
          should be saved as a reference to that terminal. The arguments
          are the type of the terminal (a string) and FILE pointers for
          the output and input of the terminal. If type is NULL then the
          environment variable $TERM is used. endwin() should called once
          at wrapup time for each terminal opened using this function.

   set_term(new)
          This function is used to switch to a different terminal
          previously opened by newterm(). The screen reference for the
          new terminal is passed as the parameter. The previous terminal
          is returned by the function. All other calls affect only the
          current terminal.

   delscreen(sp)
          The inverse of newterm(); deallocates the data structures
          associated with a given SCREEN reference.

  Causing Output to the Terminal

   refresh() and wrefresh(win)
          These functions must be called to actually get any output on
          the terminal, as other routines merely manipulate data
          structures. wrefresh() copies the named window to the physical
          terminal screen, taking into account what is already there in
          order to do optimizations. refresh() does a refresh of
          stdscr(). Unless leaveok() has been enabled, the physical
          cursor of the terminal is left at the location of the window's
          cursor.

   doupdate() and wnoutrefresh(win)
          These two functions allow multiple updates with more efficiency
          than wrefresh. To use them, it is important to understand how
          curses works. In addition to all the window structures, curses
          keeps two data structures representing the terminal screen: a
          physical screen, describing what is actually on the screen, and
          a virtual screen, describing what the programmer wants to have
          on the screen. wrefresh works by first copying the named window
          to the virtual screen (wnoutrefresh()), and then calling the
          routine to update the screen (doupdate()). If the programmer
          wishes to output several windows at once, a series of calls to
          wrefresh will result in alternating calls to wnoutrefresh() and
          doupdate(), causing several bursts of output to the screen. By
          calling wnoutrefresh() for each window, it is then possible to
          call doupdate() once, resulting in only one burst of output,
          with fewer total characters transmitted (this also avoids a
          visually annoying flicker at each update).

  Low-Level Capability Access

   setupterm(term, filenum, errret)
          This routine is called to initialize a terminal's description,
          without setting up the curses screen structures or changing the
          tty-driver mode bits. term is the character string representing
          the name of the terminal being used. filenum is the UNIX file
          descriptor of the terminal to be used for output. errret is a
          pointer to an integer, in which a success or failure indication
          is returned. The values returned can be 1 (all is well), 0 (no
          such terminal), or -1 (some problem locating the terminfo
          database).

          The value of term can be given as NULL, which will cause the
          value of TERM in the environment to be used. The errret pointer
          can also be given as NULL, meaning no error code is wanted. If
          errret is defaulted, and something goes wrong, setupterm() will
          print an appropriate error message and exit, rather than
          returning. Thus, a simple program can call setupterm(0, 1, 0)
          and not worry about initialization errors.

          After the call to setupterm(), the global variable cur_term is
          set to point to the current structure of terminal capabilities.
          By calling setupterm() for each terminal, and saving and
          restoring cur_term, it is possible for a program to use two or
          more terminals at once. Setupterm() also stores the names
          section of the terminal description in the global character
          array ttytype[]. Subsequent calls to setupterm() will overwrite
          this array, so you'll have to save it yourself if need be.

  Debugging

     NOTE: These functions are not part of the standard curses API!

   trace()
          This function can be used to explicitly set a trace level. If
          the trace level is nonzero, execution of your program will
          generate a file called `trace' in the current working directory
          containing a report on the library's actions. Higher trace
          levels enable more detailed (and verbose) reporting -- see
          comments attached to TRACE_ defines in the curses.h file for
          details. (It is also possible to set a trace level by assigning
          a trace level value to the environment variable NCURSES_TRACE).

   _tracef()
          This function can be used to output your own debugging
          information. It is only available only if you link with
          -lncurses_g. It can be used the same way as printf(), only it
          outputs a newline after the end of arguments. The output goes
          to a file called trace in the current directory.

   Trace logs can be difficult to interpret due to the sheer volume of
   data dumped in them. There is a script called tracemunch included with
   the ncurses distribution that can alleviate this problem somewhat; it
   compacts long sequences of similar operations into more succinct
   single-line pseudo-operations. These pseudo-ops can be distinguished
   by the fact that they are named in capital letters.

Hints, Tips, and Tricks

   The ncurses manual pages are a complete reference for this library. In
   the remainder of this document, we discuss various useful methods that
   may not be obvious from the manual page descriptions.

  Some Notes of Caution

   If you find yourself thinking you need to use noraw() or nocbreak(),
   think again and move carefully. It's probably better design to use
   getstr() or one of its relatives to simulate cooked mode. The noraw()
   and nocbreak() functions try to restore cooked mode, but they may end
   up clobbering some control bits set before you started your
   application. Also, they have always been poorly documented, and are
   likely to hurt your application's usability with other curses
   libraries.

   Bear in mind that refresh() is a synonym for wrefresh(stdscr). Don't
   try to mix use of stdscr with use of windows declared by newwin(); a
   refresh() call will blow them off the screen. The right way to handle
   this is to use subwin(), or not touch stdscr at all and tile your
   screen with declared windows which you then wnoutrefresh() somewhere
   in your program event loop, with a single doupdate() call to trigger
   actual repainting.

   You are much less likely to run into problems if you design your
   screen layouts to use tiled rather than overlapping windows.
   Historically, curses support for overlapping windows has been weak,
   fragile, and poorly documented. The ncurses library is not yet an
   exception to this rule.

   There is a panels library included in the ncurses distribution that
   does a pretty good job of strengthening the overlapping-windows
   facilities.

   Try to avoid using the global variables LINES and COLS. Use getmaxyx()
   on the stdscr context instead. Reason: your code may be ported to run
   in an environment with window resizes, in which case several screens
   could be open with different sizes.

  Temporarily Leaving NCURSES Mode

   Sometimes you will want to write a program that spends most of its
   time in screen mode, but occasionally returns to ordinary `cooked'
   mode. A common reason for this is to support shell-out. This behavior
   is simple to arrange in ncurses.

   To leave ncurses mode, call endwin() as you would if you were
   intending to terminate the program. This will take the screen back to
   cooked mode; you can do your shell-out. When you want to return to
   ncurses mode, simply call refresh() or doupdate(). This will repaint
   the screen.

   There is a boolean function, isendwin(), which code can use to test
   whether ncurses screen mode is active. It returns TRUE in the interval
   between an endwin() call and the following refresh(), FALSE otherwise.

   Here is some sample code for shellout:
    addstr("Shelling out...");
    def_prog_mode();           /* save current tty modes */
    endwin();                  /* restore original tty modes */
    system("sh");              /* run shell */
    addstr("returned.\n");     /* prepare return message */
    refresh();                 /* restore save modes, repaint screen */

  Using NCURSES under XTERM

   A resize operation in X sends SIGWINCH to the application running
   under xterm. The ncurses library provides an experimental signal
   handler, but in general does not catch this signal, because it cannot
   know how you want the screen re-painted. You will usually have to
   write the SIGWINCH handler yourself. Ncurses can give you some help.

   The easiest way to code your SIGWINCH handler is to have it do an
   endwin, followed by an refresh and a screen repaint you code yourself.
   The refresh will pick up the new screen size from the xterm's
   environment.

   That is the standard way, of course (it even works with some vendor's
   curses implementations). Its drawback is that it clears the screen to
   reinitialize the display, and does not resize subwindows which must be
   shrunk. Ncurses provides an extension which works better, the
   resizeterm function. That function ensures that all windows are
   limited to the new screen dimensions, and pads stdscr with blanks if
   the screen is larger.

   Finally, ncurses can be configured to provide its own SIGWINCH
   handler, based on resizeterm.

  Handling Multiple Terminal Screens

   The initscr() function actually calls a function named newterm() to do
   most of its work. If you are writing a program that opens multiple
   terminals, use newterm() directly.

   For each call, you will have to specify a terminal type and a pair of
   file pointers; each call will return a screen reference, and stdscr
   will be set to the last one allocated. You will switch between screens
   with the set_term call. Note that you will also have to call
   def_shell_mode and def_prog_mode on each tty yourself.

  Testing for Terminal Capabilities

   Sometimes you may want to write programs that test for the presence of
   various capabilities before deciding whether to go into ncurses mode.
   An easy way to do this is to call setupterm(), then use the functions
   tigetflag(), tigetnum(), and tigetstr() to do your testing.

   A particularly useful case of this often comes up when you want to
   test whether a given terminal type should be treated as `smart'
   (cursor-addressable) or `stupid'. The right way to test this is to see
   if the return value of tigetstr("cup") is non-NULL. Alternatively, you
   can include the term.h file and test the value of the macro
   cursor_address.

  Tuning for Speed

   Use the addchstr() family of functions for fast screen-painting of
   text when you know the text doesn't contain any control characters.
   Try to make attribute changes infrequent on your screens. Don't use
   the immedok() option!

  Special Features of NCURSES

   The wresize() function allows you to resize a window in place. The
   associated resizeterm() function simplifies the construction of
   SIGWINCH handlers, for resizing all windows.

   The define_key() function allows you to define at runtime function-key
   control sequences which are not in the terminal description. The
   keyok() function allows you to temporarily enable or disable
   interpretation of any function-key control sequence.

   The use_default_colors() function allows you to construct applications
   which can use the terminal's default foreground and background colors
   as an additional "default" color. Several terminal emulators support
   this feature, which is based on ISO 6429.

   Ncurses supports up 16 colors, unlike SVr4 curses which defines only
   8. While most terminals which provide color allow only 8 colors, about
   a quarter (including XFree86 xterm) support 16 colors.

Compatibility with Older Versions

   Despite our best efforts, there are some differences between ncurses
   and the (undocumented!) behavior of older curses implementations.
   These arise from ambiguities or omissions in the documentation of the
   API.

  Refresh of Overlapping Windows

   If you define two windows A and B that overlap, and then alternately
   scribble on and refresh them, the changes made to the overlapping
   region under historic curses versions were often not documented
   precisely.

   To understand why this is a problem, remember that screen updates are
   calculated between two representations of the entire display. The
   documentation says that when you refresh a window, it is first copied
   to to the virtual screen, and then changes are calculated to update
   the physical screen (and applied to the terminal). But "copied to" is
   not very specific, and subtle differences in how copying works can
   produce different behaviors in the case where two overlapping windows
   are each being refreshed at unpredictable intervals.

   What happens to the overlapping region depends on what wnoutrefresh()
   does with its argument -- what portions of the argument window it
   copies to the virtual screen. Some implementations do "change copy",
   copying down only locations in the window that have changed (or been
   marked changed with wtouchln() and friends). Some implementations do
   "entire copy", copying all window locations to the virtual screen
   whether or not they have changed.

   The ncurses library itself has not always been consistent on this
   score. Due to a bug, versions 1.8.7 to 1.9.8a did entire copy.
   Versions 1.8.6 and older, and versions 1.9.9 and newer, do change
   copy.

   For most commercial curses implementations, it is not documented and
   not known for sure (at least not to the ncurses maintainers) whether
   they do change copy or entire copy. We know that System V release 3
   curses has logic in it that looks like an attempt to do change copy,
   but the surrounding logic and data representations are sufficiently
   complex, and our knowledge sufficiently indirect, that it's hard to
   know whether this is reliable. It is not clear what the SVr4
   documentation and XSI standard intend. The XSI Curses standard barely
   mentions wnoutrefresh(); the SVr4 documents seem to be describing
   entire-copy, but it is possible with some effort and straining to read
   them the other way.

   It might therefore be unwise to rely on either behavior in programs
   that might have to be linked with other curses implementations.
   Instead, you can do an explicit touchwin() before the wnoutrefresh()
   call to guarantee an entire-contents copy anywhere.

   The really clean way to handle this is to use the panels library. If,
   when you want a screen update, you do update_panels(), it will do all
   the necessary wnoutrfresh() calls for whatever panel stacking order
   you have defined. Then you can do one doupdate() and there will be a
   single burst of physical I/O that will do all your updates.

  Background Erase

   If you have been using a very old versions of ncurses (1.8.7 or older)
   you may be surprised by the behavior of the erase functions. In older
   versions, erased areas of a window were filled with a blank modified
   by the window's current attribute (as set by wattrset(), wattron(),
   wattroff() and friends).

   In newer versions, this is not so. Instead, the attribute of erased
   blanks is normal unless and until it is modified by the functions
   bkgdset() or wbkgdset().

   This change in behavior conforms ncurses to System V Release 4 and the
   XSI Curses standard.

XSI Curses Conformance

   The ncurses library is intended to be base-level conformant with the
   XSI Curses standard from X/Open. Many extended-level features (in
   fact, almost all features not directly concerned with wide characters
   and internationalization) are also supported.

   One effect of XSI conformance is the change in behavior described
   under "Background Erase -- Compatibility with Old Versions".

   Also, ncurses meets the XSI requirement that every macro entry point
   have a corresponding function which may be linked (and will be
   prototype-checked) if the macro definition is disabled with #undef.

                              The Panels Library

   The ncurses library by itself provides good support for screen
   displays in which the windows are tiled (non-overlapping). In the more
   general case that windows may overlap, you have to use a series of
   wnoutrefresh() calls followed by a doupdate(), and be careful about
   the order you do the window refreshes in. It has to be bottom-upwards,
   otherwise parts of windows that should be obscured will show through.

   When your interface design is such that windows may dive deeper into
   the visibility stack or pop to the top at runtime, the resulting
   book-keeping can be tedious and difficult to get right. Hence the
   panels library.

   The panel library first appeared in AT&T System V. The version
   documented here is the panel code distributed with ncurses.

Compiling With the Panels Library

   Your panels-using modules must import the panels library declarations
   with
          #include <panel.h>

   and must be linked explicitly with the panels library using an -lpanel
   argument. Note that they must also link the ncurses library with
   -lncurses. Many linkers are two-pass and will accept either order, but
   it is still good practice to put -lpanel first and -lncurses second.

Overview of Panels

   A panel object is a window that is implicitly treated as part of a
   deck including all other panel objects. The deck has an implicit
   bottom-to-top visibility order. The panels library includes an update
   function (analogous to refresh()) that displays all panels in the deck
   in the proper order to resolve overlaps. The standard window, stdscr,
   is considered below all panels.

   Details on the panels functions are available in the man pages. We'll
   just hit the highlights here.

   You create a panel from a window by calling new_panel() on a window
   pointer. It then becomes the top of the deck. The panel's window is
   available as the value of panel_window() called with the panel pointer
   as argument.

   You can delete a panel (removing it from the deck) with del_panel.
   This will not deallocate the associated window; you have to do that
   yourself. You can replace a panel's window with a different window by
   calling replace_window. The new window may be of different size; the
   panel code will re-compute all overlaps. This operation doesn't change
   the panel's position in the deck.

   To move a panel's window, use move_panel(). The mvwin() function on
   the panel's window isn't sufficient because it doesn't update the
   panels library's representation of where the windows are. This
   operation leaves the panel's depth, contents, and size unchanged.

   Two functions (top_panel(), bottom_panel()) are provided for
   rearranging the deck. The first pops its argument window to the top of
   the deck; the second sends it to the bottom. Either operation leaves
   the panel's screen location, contents, and size unchanged.

   The function update_panels() does all the wnoutrefresh() calls needed
   to prepare for doupdate() (which you must call yourself, afterwards).

   Typically, you will want to call update_panels() and doupdate() just
   before accepting command input, once in each cycle of interaction with
   the user. If you call update_panels() after each and every panel
   write, you'll generate a lot of unnecessary refresh activity and
   screen flicker.

Panels, Input, and the Standard Screen

   You shouldn't mix wnoutrefresh() or wrefresh() operations with panels
   code; this will work only if the argument window is either in the top
   panel or unobscured by any other panels.

   The stsdcr window is a special case. It is considered below all
   panels. Because changes to panels may obscure parts of stdscr, though,
   you should call update_panels() before doupdate() even when you only
   change stdscr.

   Note that wgetch automatically calls wrefresh. Therefore, before
   requesting input from a panel window, you need to be sure that the
   panel is totally unobscured.

   There is presently no way to display changes to one obscured panel
   without repainting all panels.

Hiding Panels

   It's possible to remove a panel from the deck temporarily; use
   hide_panel for this. Use show_panel() to render it visible again. The
   predicate function panel_hidden tests whether or not a panel is
   hidden.

   The panel_update code ignores hidden panels. You cannot do top_panel()
   or bottom_panel on a hidden panel(). Other panels operations are
   applicable.

Miscellaneous Other Facilities

   It's possible to navigate the deck using the functions panel_above()
   and panel_below. Handed a panel pointer, they return the panel above
   or below that panel. Handed NULL, they return the bottom-most or
   top-most panel.

   Every panel has an associated user pointer, not used by the panel
   code, to which you can attach application data. See the man page
   documentation of set_panel_userptr() and panel_userptr for details.

                               The Menu Library

   A menu is a screen display that assists the user to choose some subset
   of a given set of items. The menu library is a curses extension that
   supports easy programming of menu hierarchies with a uniform but
   flexible interface.

   The menu library first appeared in AT&T System V. The version
   documented here is the menu code distributed with ncurses.

Compiling With the menu Library

   Your menu-using modules must import the menu library declarations with
          #include <menu.h>

   and must be linked explicitly with the menus library using an -lmenu
   argument. Note that they must also link the ncurses library with
   -lncurses. Many linkers are two-pass and will accept either order, but
   it is still good practice to put -lmenu first and -lncurses second.

Overview of Menus

   The menus created by this library consist of collections of items
   including a name string part and a description string part. To make
   menus, you create groups of these items and connect them with menu
   frame objects.

   The menu can then by posted, that is written to an associated window.
   Actually, each menu has two associated windows; a containing window in
   which the programmer can scribble titles or borders, and a subwindow
   in which the menu items proper are displayed. If this subwindow is too
   small to display all the items, it will be a scrollable viewport on
   the collection of items.

   A menu may also be unposted (that is, undisplayed), and finally freed
   to make the storage associated with it and its items available for
   re-use.

   The general flow of control of a menu program looks like this:
    1. Initialize curses.
    2. Create the menu items, using new_item().
    3. Create the menu using new_menu().
    4. Post the menu using menu_post().
    5. Refresh the screen.
    6. Process user requests via an input loop.
    7. Unpost the menu using menu_unpost().
    8. Free the menu, using free_menu().
    9. Free the items using free_item().
   10. Terminate curses.

Selecting items

   Menus may be multi-valued or (the default) single-valued (see the
   manual page menu_opts(3x) to see how to change the default). Both
   types always have a current item.

   From a single-valued menu you can read the selected value simply by
   looking at the current item. From a multi-valued menu, you get the
   selected set by looping through the items applying the item_value()
   predicate function. Your menu-processing code can use the function
   set_item_value() to flag the items in the select set.

   Menu items can be made unselectable using set_item_opts() or
   item_opts_off() with the O_SELECTABLE argument. This is the only
   option so far defined for menus, but it is good practice to code as
   though other option bits might be on.

Menu Display

   The menu library calculates a minimum display size for your window,
   based on the following variables:
     * The number and maximum length of the menu items
     * Whether the O_ROWMAJOR option is enabled
     * Whether display of descriptions is enabled
     * Whatever menu format may have been set by the programmer
     * The length of the menu mark string used for highlighting selected
       items

   The function set_menu_format() allows you to set the maximum size of
   the viewport or menu page that will be used to display menu items. You
   can retrieve any format associated with a menu with menu_format(). The
   default format is rows=16, columns=1.

   The actual menu page may be smaller than the format size. This depends
   on the item number and size and whether O_ROWMAJOR is on. This option
   (on by default) causes menu items to be displayed in a `raster-scan'
   pattern, so that if more than one item will fit horizontally the first
   couple of items are side-by-side in the top row. The alternative is
   column-major display, which tries to put the first several items in
   the first column.

   As mentioned above, a menu format not large enough to allow all items
   to fit on-screen will result in a menu display that is vertically
   scrollable.

   You can scroll it with requests to the menu driver, which will be
   described in the section on menu input handling.

   Each menu has a mark string used to visually tag selected items; see
   the menu_mark(3x) manual page for details. The mark string length also
   influences the menu page size.

   The function scale_menu() returns the minimum display size that the
   menu code computes from all these factors. There are other menu
   display attributes including a select attribute, an attribute for
   selectable items, an attribute for unselectable items, and a pad
   character used to separate item name text from description text. These
   have reasonable defaults which the library allows you to change (see
   the menu_attribs(3x) manual page.

Menu Windows

   Each menu has, as mentioned previously, a pair of associated windows.
   Both these windows are painted when the menu is posted and erased when
   the menu is unposted.

   The outer or frame window is not otherwise touched by the menu
   routines. It exists so the programmer can associate a title, a border,
   or perhaps help text with the menu and have it properly refreshed or
   erased at post/unpost time. The inner window or subwindow is where the
   current menu page is displayed.

   By default, both windows are stdscr. You can set them with the
   functions in menu_win(3x).

   When you call menu_post(), you write the menu to its subwindow. When
   you call menu_unpost(), you erase the subwindow, However, neither of
   these actually modifies the screen. To do that, call wrefresh() or
   some equivalent.

Processing Menu Input

   The main loop of your menu-processing code should call menu_driver()
   repeatedly. The first argument of this routine is a menu pointer; the
   second is a menu command code. You should write an input-fetching
   routine that maps input characters to menu command codes, and pass its
   output to menu_driver(). The menu command codes are fully documented
   in menu_driver(3x).

   The simplest group of command codes is REQ_NEXT_ITEM, REQ_PREV_ITEM,
   REQ_FIRST_ITEM, REQ_LAST_ITEM, REQ_UP_ITEM, REQ_DOWN_ITEM,
   REQ_LEFT_ITEM, REQ_RIGHT_ITEM. These change the currently selected
   item. These requests may cause scrolling of the menu page if it only
   partially displayed.

   There are explicit requests for scrolling which also change the
   current item (because the select location does not change, but the
   item there does). These are REQ_SCR_DLINE, REQ_SCR_ULINE,
   REQ_SCR_DPAGE, and REQ_SCR_UPAGE.

   The REQ_TOGGLE_ITEM selects or deselects the current item. It is for
   use in multi-valued menus; if you use it with O_ONEVALUE on, you'll
   get an error return (E_REQUEST_DENIED).

   Each menu has an associated pattern buffer. The menu_driver() logic
   tries to accumulate printable ASCII characters passed in in that
   buffer; when it matches a prefix of an item name, that item (or the
   next matching item) is selected. If appending a character yields no
   new match, that character is deleted from the pattern buffer, and
   menu_driver() returns E_NO_MATCH.

   Some requests change the pattern buffer directly: REQ_CLEAR_PATTERN,
   REQ_BACK_PATTERN, REQ_NEXT_MATCH, REQ_PREV_MATCH. The latter two are
   useful when pattern buffer input matches more than one item in a
   multi-valued menu.

   Each successful scroll or item navigation request clears the pattern
   buffer. It is also possible to set the pattern buffer explicitly with
   set_menu_pattern().

   Finally, menu driver requests above the constant MAX_COMMAND are
   considered application-specific commands. The menu_driver() code
   ignores them and returns E_UNKNOWN_COMMAND.

Miscellaneous Other Features

   Various menu options can affect the processing and visual appearance
   and input processing of menus. See menu_opts(3x) for details.

   It is possible to change the current item from application code; this
   is useful if you want to write your own navigation requests. It is
   also possible to explicitly set the top row of the menu display. See
   mitem_current(3x). If your application needs to change the menu
   subwindow cursor for any reason, pos_menu_cursor() will restore it to
   the correct location for continuing menu driver processing.

   It is possible to set hooks to be called at menu initialization and
   wrapup time, and whenever the selected item changes. See
   menu_hook(3x).

   Each item, and each menu, has an associated user pointer on which you
   can hang application data. See mitem_userptr(3x) and menu_userptr(3x).

                               The Forms Library

   The form library is a curses extension that supports easy programming
   of on-screen forms for data entry and program control.

   The form library first appeared in AT&T System V. The version
   documented here is the form code distributed with ncurses.

Compiling With the form Library

   Your form-using modules must import the form library declarations with
          #include <form.h>

   and must be linked explicitly with the forms library using an -lform
   argument. Note that they must also link the ncurses library with
   -lncurses. Many linkers are two-pass and will accept either order, but
   it is still good practice to put -lform first and -lncurses second.

Overview of Forms

   A form is a collection of fields; each field may be either a label
   (explanatory text) or a data-entry location. Long forms may be
   segmented into pages; each entry to a new page clears the screen.

   To make forms, you create groups of fields and connect them with form
   frame objects; the form library makes this relatively simple.

   Once defined, a form can be posted, that is written to an associated
   window. Actually, each form has two associated windows; a containing
   window in which the programmer can scribble titles or borders, and a
   subwindow in which the form fields proper are displayed.

   As the form user fills out the posted form, navigation and editing
   keys support movement between fields, editing keys support modifying
   field, and plain text adds to or changes data in a current field. The
   form library allows you (the forms designer) to bind each navigation
   and editing key to any keystroke accepted by curses Fields may have
   validation conditions on them, so that they check input data for type
   and value. The form library supplies a rich set of pre-defined field
   types, and makes it relatively easy to define new ones.

   Once its transaction is completed (or aborted), a form may be unposted
   (that is, undisplayed), and finally freed to make the storage
   associated with it and its items available for re-use.

   The general flow of control of a form program looks like this:
    1. Initialize curses.
    2. Create the form fields, using new_field().
    3. Create the form using new_form().
    4. Post the form using form_post().
    5. Refresh the screen.
    6. Process user requests via an input loop.
    7. Unpost the form using form_unpost().
    8. Free the form, using free_form().
    9. Free the fields using free_field().
   10. Terminate curses.

   Note that this looks much like a menu program; the form library
   handles tasks which are in many ways similar, and its interface was
   obviously designed to resemble that of the menu library wherever
   possible.

   In forms programs, however, the `process user requests' is somewhat
   more complicated than for menus. Besides menu-like navigation
   operations, the menu driver loop has to support field editing and data
   validation.

Creating and Freeing Fields and Forms

   The basic function for creating fields is new_field():
FIELD *new_field(int height, int width,   /* new field size */
                 int top, int left,       /* upper left corner */
                 int offscreen,           /* number of offscreen rows */
                 int nbuf);               /* number of working buffers */

   Menu items always occupy a single row, but forms fields may have
   multiple rows. So new_field() requires you to specify a width and
   height (the first two arguments, which mist both be greater than
   zero).

   You must also specify the location of the field's upper left corner on
   the screen (the third and fourth arguments, which must be zero or
   greater). Note that these coordinates are relative to the form
   subwindow, which will coincide with stdscr by default but need not be
   stdscr if you've done an explicit set_form_window() call.

   The fifth argument allows you to specify a number of off-screen rows.
   If this is zero, the entire field will always be displayed. If it is
   nonzero, the form will be scrollable, with only one screen-full
   (initially the top part) displayed at any given time. If you make a
   field dynamic and grow it so it will no longer fit on the screen, the
   form will become scrollable even if the offscreen argument was
   initially zero.

   The forms library allocates one working buffer per field; the size of
   each buffer is ((height + offscreen)*width + 1, one character for each
   position in the field plus a NUL terminator. The sixth argument is the
   number of additional data buffers to allocate for the field; your
   application can use them for its own purposes.
FIELD *dup_field(FIELD *field,            /* field to copy */
                 int top, int left);      /* location of new copy */

   The function dup_field() duplicates an existing field at a new
   location. Size and buffering information are copied; some attribute
   flags and status bits are not (see the form_field_new(3X) for
   details).
FIELD *link_field(FIELD *field,           /* field to copy */
                  int top, int left);     /* location of new copy */

   The function link_field() also duplicates an existing field at a new
   location. The difference from dup_field() is that it arranges for the
   new field's buffer to be shared with the old one.

   Besides the obvious use in making a field editable from two different
   form pages, linked fields give you a way to hack in dynamic labels. If
   you declare several fields linked to an original, and then make them
   inactive, changes from the original will still be propagated to the
   linked fields.

   As with duplicated fields, linked fields have attribute bits separate
   from the original.

   As you might guess, all these field-allocations return NULL if the
   field allocation is not possible due to an out-of-memory error or
   out-of-bounds arguments.

   To connect fields to a form, use
FORM *new_form(FIELD **fields);

   This function expects to see a NULL-terminated array of field
   pointers. Said fields are connected to a newly-allocated form object;
   its address is returned (or else NULL if the allocation fails).

   Note that new_field() does not copy the pointer array into private
   storage; if you modify the contents of the pointer array during forms
   processing, all manner of bizarre things might happen. Also note that
   any given field may only be connected to one form.

   The functions free_field() and free_form are available to free field
   and form objects. It is an error to attempt to free a field connected
   to a form, but not vice-versa; thus, you will generally free your form
   objects first.

Fetching and Changing Field Attributes

   Each form field has a number of location and size attributes
   associated with it. There are other field attributes used to control
   display and editing of the field. Some (for example, the O_STATIC bit)
   involve sufficient complications to be covered in sections of their
   own later on. We cover the functions used to get and set several basic
   attributes here.

   When a field is created, the attributes not specified by the new_field
   function are copied from an invisible system default field. In
   attribute-setting and -fetching functions, the argument NULL is taken
   to mean this field. Changes to it persist as defaults until your forms
   application terminates.

  Fetching Size and Location Data

   You can retrieve field sizes and locations through:
int field_info(FIELD *field,              /* field from which to fetch */
               int *height, *int width,   /* field size */
               int *top, int *left,       /* upper left corner */
               int *offscreen,            /* number of offscreen rows */
               int *nbuf);                /* number of working buffers */

   This function is a sort of inverse of new_field(); instead of setting
   size and location attributes of a new field, it fetches them from an
   existing one.

  Changing the Field Location

   It is possible to move a field's location on the screen:
int move_field(FIELD *field,              /* field to alter */
               int top, int left);        /* new upper-left corner */

   You can, of course. query the current location through field_info().

  The Justification Attribute

   One-line fields may be unjustified, justified right, justified left,
   or centered. Here is how you manipulate this attribute:
int set_field_just(FIELD *field,          /* field to alter */
                   int justmode);         /* mode to set */

int field_just(FIELD *field);             /* fetch mode of field */

   The mode values accepted and returned by this functions are
   preprocessor macros NO_JUSTIFICATION, JUSTIFY_RIGHT, JUSTIFY_LEFT, or
   JUSTIFY_CENTER.

  Field Display Attributes

   For each field, you can set a foreground attribute for entered
   characters, a background attribute for the entire field, and a pad
   character for the unfilled portion of the field. You can also control
   pagination of the form.

   This group of four field attributes controls the visual appearance of
   the field on the screen, without affecting in any way the data in the
   field buffer.
int set_field_fore(FIELD *field,          /* field to alter */
                   chtype attr);          /* attribute to set */

chtype field_fore(FIELD *field);          /* field to query */

int set_field_back(FIELD *field,          /* field to alter */
                   chtype attr);          /* attribute to set */

chtype field_back(FIELD *field);          /* field to query */

int set_field_pad(FIELD *field,           /* field to alter */
                 int pad);                /* pad character to set */

chtype field_pad(FIELD *field);

int set_new_page(FIELD *field,            /* field to alter */
                 int flag);               /* TRUE to force new page */

chtype new_page(FIELD *field);            /* field to query */

   The attributes set and returned by the first four functions are normal
   curses(3x) display attribute values (A_STANDOUT, A_BOLD, A_REVERSE
   etc). The page bit of a field controls whether it is displayed at the
   start of a new form screen.

  Field Option Bits

   There is also a large collection of field option bits you can set to
   control various aspects of forms processing. You can manipulate them
   with these functions:
int set_field_opts(FIELD *field,          /* field to alter */
                   int attr);             /* attribute to set */

int field_opts_on(FIELD *field,           /* field to alter */
                  int attr);              /* attributes to turn on */

int field_opts_off(FIELD *field,          /* field to alter */
                   int attr);             /* attributes to turn off */

int field_opts(FIELD *field);             /* field to query */

   By default, all options are on. Here are the available option bits:

   O_VISIBLE
          Controls whether the field is visible on the screen. Can be
          used during form processing to hide or pop up fields depending
          on the value of parent fields.

   O_ACTIVE
          Controls whether the field is active during forms processing
          (i.e. visited by form navigation keys). Can be used to make
          labels or derived fields with buffer values alterable by the
          forms application, not the user.

   O_PUBLIC
          Controls whether data is displayed during field entry. If this
          option is turned off on a field, the library will accept and
          edit data in that field, but it will not be displayed and the
          visible field cursor will not move. You can turn off the
          O_PUBLIC bit to define password fields.

   O_EDIT
          Controls whether the field's data can be modified. When this
          option is off, all editing requests except REQ_PREV_CHOICE and
          REQ_NEXT_CHOICE will fail. Such read-only fields may be useful
          for help messages.

   O_WRAP
          Controls word-wrapping in multi-line fields. Normally, when any
          character of a (blank-separated) word reaches the end of the
          current line, the entire word is wrapped to the next line
          (assuming there is one). When this option is off, the word will
          be split across the line break.

   O_BLANK
          Controls field blanking. When this option is on, entering a
          character at the first field position erases the entire field
          (except for the just-entered character).

   O_AUTOSKIP
          Controls automatic skip to next field when this one fills.
          Normally, when the forms user tries to type more data into a
          field than will fit, the editing location jumps to next field.
          When this option is off, the user's cursor will hang at the end
          of the field. This option is ignored in dynamic fields that
          have not reached their size limit.

   O_NULLOK
          Controls whether validation is applied to blank fields.
          Normally, it is not; the user can leave a field blank without
          invoking the usual validation check on exit. If this option is
          off on a field, exit from it will invoke a validation check.

   O_PASSOK
          Controls whether validation occurs on every exit, or only after
          the field is modified. Normally the latter is true. Setting
          O_PASSOK may be useful if your field's validation function may
          change during forms processing.

   O_STATIC
          Controls whether the field is fixed to its initial dimensions.
          If you turn this off, the field becomes dynamic and will
          stretch to fit entered data.

   A field's options cannot be changed while the field is currently
   selected. However, options may be changed on posted fields that are
   not current.

   The option values are bit-masks and can be composed with logical-or in
   the obvious way.

Field Status

   Every field has a status flag, which is set to FALSE when the field is
   created and TRUE when the value in field buffer 0 changes. This flag
   can be queried and set directly:
int set_field_status(FIELD *field,      /* field to alter */
                   int status);         /* mode to set */

int field_status(FIELD *field);         /* fetch mode of field */

   Setting this flag under program control can be useful if you use the
   same form repeatedly, looking for modified fields each time.

   Calling field_status() on a field not currently selected for input
   will return a correct value. Calling field_status() on a field that is
   currently selected for input may not necessarily give a correct field
   status value, because entered data isn't necessarily copied to buffer
   zero before the exit validation check. To guarantee that the returned
   status value reflects reality, call field_status() either (1) in the
   field's exit validation check routine, (2) from the field's or form's
   initialization or termination hooks, or (3) just after a
   REQ_VALIDATION request has been processed by the forms driver.

Field User Pointer

   Each field structure contains one character pointer slot that is not
   used by the forms library. It is intended to be used by applications
   to store private per-field data. You can manipulate it with:
int set_field_userptr(FIELD *field,       /* field to alter */
                   char *userptr);        /* mode to set */

char *field_userptr(FIELD *field);        /* fetch mode of field */

   (Properly, this user pointer field ought to have (void *) type. The
   (char *) type is retained for System V compatibility.)

   It is valid to set the user pointer of the default field (with a
   set_field_userptr() call passed a NULL field pointer.) When a new
   field is created, the default-field user pointer is copied to
   initialize the new field's user pointer.

Variable-Sized Fields

   Normally, a field is fixed at the size specified for it at creation
   time. If, however, you turn off its O_STATIC bit, it becomes dynamic
   and will automatically resize itself to accommodate data as it is
   entered. If the field has extra buffers associated with it, they will
   grow right along with the main input buffer.

   A one-line dynamic field will have a fixed height (1) but variable
   width, scrolling horizontally to display data within the field area as
   originally dimensioned and located. A multi-line dynamic field will
   have a fixed width, but variable height (number of rows), scrolling
   vertically to display data within the field area as originally
   dimensioned and located.

   Normally, a dynamic field is allowed to grow without limit. But it is
   possible to set an upper limit on the size of a dynamic field. You do
   it with this function:
int set_max_field(FIELD *field,     /* field to alter (may not be NULL) */
                   int max_size);   /* upper limit on field size */

   If the field is one-line, max_size is taken to be a column size limit;
   if it is multi-line, it is taken to be a line size limit. To disable
   any limit, use an argument of zero. The growth limit can be changed
   whether or not the O_STATIC bit is on, but has no effect until it is.

   The following properties of a field change when it becomes dynamic:
     * If there is no growth limit, there is no final position of the
       field; therefore O_AUTOSKIP and O_NL_OVERLOAD are ignored.
     * Field justification will be ignored (though whatever justification
       is set up will be retained internally and can be queried).
     * The dup_field() and link_field() calls copy dynamic-buffer sizes.
       If the O_STATIC option is set on one of a collection of links,
       buffer resizing will occur only when the field is edited through
       that link.
     * The call field_info() will retrieve the original static size of
       the field; use dynamic_field_info() to get the actual dynamic
       size.

Field Validation

   By default, a field will accept any data that will fit in its input
   buffer. However, it is possible to attach a validation type to a
   field. If you do this, any attempt to leave the field while it
   contains data that doesn't match the validation type will fail. Some
   validation types also have a character-validity check for each time a
   character is entered in the field.

   A field's validation check (if any) is not called when
   set_field_buffer() modifies the input buffer, nor when that buffer is
   changed through a linked field.

   The form library provides a rich set of pre-defined validation types,
   and gives you the capability to define custom ones of your own. You
   can examine and change field validation attributes with the following
   functions:
int set_field_type(FIELD *field,          /* field to alter */
                   FIELDTYPE *ftype,      /* type to associate */
                   ...);                  /* additional arguments*/

FIELDTYPE *field_type(FIELD *field);      /* field to query */

   The validation type of a field is considered an attribute of the
   field. As with other field attributes, Also, doing set_field_type()
   with a NULL field default will change the system default for
   validation of newly-created fields.

   Here are the pre-defined validation types:

  TYPE_ALPHA

   This field type accepts alphabetic data; no blanks, no digits, no
   special characters (this is checked at character-entry time). It is
   set up with:
int set_field_type(FIELD *field,          /* field to alter */
                   TYPE_ALPHA,            /* type to associate */
                   int width);            /* maximum width of field */

   The width argument sets a minimum width of data. Typically you'll want
   to set this to the field width; if it's greater than the field width,
   the validation check will always fail. A minimum width of zero makes
   field completion optional.

  TYPE_ALNUM

   This field type accepts alphabetic data and digits; no blanks, no
   special characters (this is checked at character-entry time). It is
   set up with:
int set_field_type(FIELD *field,          /* field to alter */
                   TYPE_ALNUM,            /* type to associate */
                   int width);            /* maximum width of field */

   The width argument sets a minimum width of data. As with TYPE_ALPHA,
   typically you'll want to set this to the field width; if it's greater
   than the field width, the validation check will always fail. A minimum
   width of zero makes field completion optional.

  TYPE_ENUM

   This type allows you to restrict a field's values to be among a
   specified set of string values (for example, the two-letter postal
   codes for U.S. states). It is set up with:
int set_field_type(FIELD *field,          /* field to alter */
                   TYPE_ENUM,             /* type to associate */
                   char **valuelist;      /* list of possible values */
                   int checkcase;         /* case-sensitive? */
                   int checkunique);      /* must specify uniquely? */

   The valuelist parameter must point at a NULL-terminated list of valid
   strings. The checkcase argument, if true, makes comparison with the
   string case-sensitive.

   When the user exits a TYPE_ENUM field, the validation procedure tries
   to complete the data in the buffer to a valid entry. If a complete
   choice string has been entered, it is of course valid. But it is also
   possible to enter a prefix of a valid string and have it completed for
   you.

   By default, if you enter such a prefix and it matches more than one
   value in the string list, the prefix will be completed to the first
   matching value. But the checkunique argument, if true, requires prefix
   matches to be unique in order to be valid.

   The REQ_NEXT_CHOICE and REQ_PREV_CHOICE input requests can be
   particularly useful with these fields.

  TYPE_INTEGER

   This field type accepts an integer. It is set up as follows:
int set_field_type(FIELD *field,          /* field to alter */
                   TYPE_INTEGER,          /* type to associate */
                   int padding,           /* # places to zero-pad to */
                   int vmin, int vmax);   /* valid range */

   Valid characters consist of an optional leading minus and digits. The
   range check is performed on exit. If the range maximum is less than or
   equal to the minimum, the range is ignored.

   If the value passes its range check, it is padded with as many leading
   zero digits as necessary to meet the padding argument.

   A TYPE_INTEGER value buffer can conveniently be interpreted with the C
   library function atoi(3).

  TYPE_NUMERIC

   This field type accepts a decimal number. It is set up as follows:
int set_field_type(FIELD *field,              /* field to alter */
                   TYPE_NUMERIC,              /* type to associate */
                   int padding,               /* # places of precision */
                   double vmin, double vmax); /* valid range */

   Valid characters consist of an optional leading minus and digits.
   possibly including a decimal point. If your system supports locale's,
   the decimal point character used must be the one defined by your
   locale. The range check is performed on exit. If the range maximum is
   less than or equal to the minimum, the range is ignored.

   If the value passes its range check, it is padded with as many
   trailing zero digits as necessary to meet the padding argument.

   A TYPE_NUMERIC value buffer can conveniently be interpreted with the C
   library function atof(3).

  TYPE_REGEXP

   This field type accepts data matching a regular expression. It is set
   up as follows:
int set_field_type(FIELD *field,          /* field to alter */
                   TYPE_REGEXP,           /* type to associate */
                   char *regexp);         /* expression to match */

   The syntax for regular expressions is that of regcomp(3). The check
   for regular-expression match is performed on exit.

Direct Field Buffer Manipulation

   The chief attribute of a field is its buffer contents. When a form has
   been completed, your application usually needs to know the state of
   each field buffer. You can find this out with:
char *field_buffer(FIELD *field,          /* field to query */
                   int bufindex);         /* number of buffer to query */

   Normally, the state of the zero-numbered buffer for each field is set
   by the user's editing actions on that field. It's sometimes useful to
   be able to set the value of the zero-numbered (or some other) buffer
   from your application:
int set_field_buffer(FIELD *field,        /* field to alter */
                   int bufindex,          /* number of buffer to alter */
                   char *value);          /* string value to set */

   If the field is not large enough and cannot be resized to a
   sufficiently large size to contain the specified value, the value will
   be truncated to fit.

   Calling field_buffer() with a null field pointer will raise an error.
   Calling field_buffer() on a field not currently selected for input
   will return a correct value. Calling field_buffer() on a field that is
   currently selected for input may not necessarily give a correct field
   buffer value, because entered data isn't necessarily copied to buffer
   zero before the exit validation check. To guarantee that the returned
   buffer value reflects on-screen reality, call field_buffer() either
   (1) in the field's exit validation check routine, (2) from the field's
   or form's initialization or termination hooks, or (3) just after a
   REQ_VALIDATION request has been processed by the forms driver.

Attributes of Forms

   As with field attributes, form attributes inherit a default from a
   system default form structure. These defaults can be queried or set by
   of these functions using a form-pointer argument of NULL.

   The principal attribute of a form is its field list. You can query and
   change this list with:
int set_form_fields(FORM *form,           /* form to alter */
                    FIELD **fields);      /* fields to connect */

char *form_fields(FORM *form);            /* fetch fields of form */

int field_count(FORM *form);              /* count connect fields */

   The second argument of set_form_fields() may be a NULL-terminated
   field pointer array like the one required by new_form(). In that case,
   the old fields of the form are disconnected but not freed (and
   eligible to be connected to other forms), then the new fields are
   connected.

   It may also be null, in which case the old fields are disconnected
   (and not freed) but no new ones are connected.

   The field_count() function simply counts the number of fields
   connected to a given from. It returns -1 if the form-pointer argument
   is NULL.

Control of Form Display

   In the overview section, you saw that to display a form you normally
   start by defining its size (and fields), posting it, and refreshing
   the screen. There is an hidden step before posting, which is the
   association of the form with a frame window (actually, a pair of
   windows) within which it will be displayed. By default, the forms
   library associates every form with the full-screen window stdscr.

   By making this step explicit, you can associate a form with a declared
   frame window on your screen display. This can be useful if you want to
   adapt the form display to different screen sizes, dynamically tile
   forms on the screen, or use a form as part of an interface layout
   managed by panels.

   The two windows associated with each form have the same functions as
   their analogues in the menu library. Both these windows are painted
   when the form is posted and erased when the form is unposted.

   The outer or frame window is not otherwise touched by the form
   routines. It exists so the programmer can associate a title, a border,
   or perhaps help text with the form and have it properly refreshed or
   erased at post/unpost time. The inner window or subwindow is where the
   current form page is actually displayed.

   In order to declare your own frame window for a form, you'll need to
   know the size of the form's bounding rectangle. You can get this
   information with:
int scale_form(FORM *form,                /* form to query */
               int *rows,                 /* form rows */
               int *cols);                /* form cols */

   The form dimensions are passed back in the locations pointed to by the
   arguments. Once you have this information, you can use it to declare
   of windows, then use one of these functions:
int set_form_win(FORM *form,              /* form to alter */
                 WINDOW *win);            /* frame window to connect */

WINDOW *form_win(FORM *form);             /* fetch frame window of form */

int set_form_sub(FORM *form,              /* form to alter */
                 WINDOW *win);            /* form subwindow to connect */

WINDOW *form_sub(FORM *form);             /* fetch form subwindow of form */

   Note that curses operations, including refresh(), on the form, should
   be done on the frame window, not the form subwindow.

   It is possible to check from your application whether all of a
   scrollable field is actually displayed within the menu subwindow. Use
   these functions:
int data_ahead(FORM *form);               /* form to be queried */

int data_behind(FORM *form);              /* form to be queried */

   The function data_ahead() returns TRUE if (a) the current field is
   one-line and has undisplayed data off to the right, (b) the current
   field is multi-line and there is data off-screen below it.

   The function data_behind() returns TRUE if the first (upper left hand)
   character position is off-screen (not being displayed).

   Finally, there is a function to restore the form window's cursor to
   the value expected by the forms driver:
int pos_form_cursor(FORM *)               /* form to be queried */

   If your application changes the form window cursor, call this function
   before handing control back to the forms driver in order to
   re-synchronize it.

Input Processing in the Forms Driver

   The function form_driver() handles virtualized input requests for form
   navigation, editing, and validation requests, just as menu_driver does
   for menus (see the section on menu input handling).
int form_driver(FORM *form,               /* form to pass input to */
                int request);             /* form request code */

   Your input virtualization function needs to take input and then
   convert it to either an alphanumeric character (which is treated as
   data to be entered in the currently-selected field), or a forms
   processing request.

   The forms driver provides hooks (through input-validation and
   field-termination functions) with which your application code can
   check that the input taken by the driver matched what was expected.

  Page Navigation Requests

   These requests cause page-level moves through the form, triggering
   display of a new form screen.

   REQ_NEXT_PAGE
          Move to the next form page.

   REQ_PREV_PAGE
          Move to the previous form page.

   REQ_FIRST_PAGE
          Move to the first form page.

   REQ_LAST_PAGE
          Move to the last form page.

   These requests treat the list as cyclic; that is, REQ_NEXT_PAGE from
   the last page goes to the first, and REQ_PREV_PAGE from the first page
   goes to the last.

  Inter-Field Navigation Requests

   These requests handle navigation between fields on the same page.

   REQ_NEXT_FIELD
          Move to next field.

   REQ_PREV_FIELD
          Move to previous field.

   REQ_FIRST_FIELD
          Move to the first field.

   REQ_LAST_FIELD
          Move to the last field.

   REQ_SNEXT_FIELD
          Move to sorted next field.

   REQ_SPREV_FIELD
          Move to sorted previous field.

   REQ_SFIRST_FIELD
          Move to the sorted first field.

   REQ_SLAST_FIELD
          Move to the sorted last field.

   REQ_LEFT_FIELD
          Move left to field.

   REQ_RIGHT_FIELD
          Move right to field.

   REQ_UP_FIELD
          Move up to field.

   REQ_DOWN_FIELD
          Move down to field.

   These requests treat the list of fields on a page as cyclic; that is,
   REQ_NEXT_FIELD from the last field goes to the first, and
   REQ_PREV_FIELD from the first field goes to the last. The order of the
   fields for these (and the REQ_FIRST_FIELD and REQ_LAST_FIELD requests)
   is simply the order of the field pointers in the form array (as set up
   by new_form() or set_form_fields()

   It is also possible to traverse the fields as if they had been sorted
   in screen-position order, so the sequence goes left-to-right and
   top-to-bottom. To do this, use the second group of four
   sorted-movement requests.

   Finally, it is possible to move between fields using visual directions
   up, down, right, and left. To accomplish this, use the third group of
   four requests. Note, however, that the position of a form for purposes
   of these requests is its upper-left corner.

   For example, suppose you have a multi-line field B, and two
   single-line fields A and C on the same line with B, with A to the left
   of B and C to the right of B. A REQ_MOVE_RIGHT from A will go to B
   only if A, B, and C all share the same first line; otherwise it will
   skip over B to C.

  Intra-Field Navigation Requests

   These requests drive movement of the edit cursor within the currently
   selected field.

   REQ_NEXT_CHAR
          Move to next character.

   REQ_PREV_CHAR
          Move to previous character.

   REQ_NEXT_LINE
          Move to next line.

   REQ_PREV_LINE
          Move to previous line.

   REQ_NEXT_WORD
          Move to next word.

   REQ_PREV_WORD
          Move to previous word.

   REQ_BEG_FIELD
          Move to beginning of field.

   REQ_END_FIELD
          Move to end of field.

   REQ_BEG_LINE
          Move to beginning of line.

   REQ_END_LINE
          Move to end of line.

   REQ_LEFT_CHAR
          Move left in field.

   REQ_RIGHT_CHAR
          Move right in field.

   REQ_UP_CHAR
          Move up in field.

   REQ_DOWN_CHAR
          Move down in field.

   Each word is separated from the previous and next characters by
   whitespace. The commands to move to beginning and end of line or field
   look for the first or last non-pad character in their ranges.

  Scrolling Requests

   Fields that are dynamic and have grown and fields explicitly created
   with offscreen rows are scrollable. One-line fields scroll
   horizontally; multi-line fields scroll vertically. Most scrolling is
   triggered by editing and intra-field movement (the library scrolls the
   field to keep the cursor visible). It is possible to explicitly
   request scrolling with the following requests:

   REQ_SCR_FLINE
          Scroll vertically forward a line.

   REQ_SCR_BLINE
          Scroll vertically backward a line.

   REQ_SCR_FPAGE
          Scroll vertically forward a page.

   REQ_SCR_BPAGE
          Scroll vertically backward a page.

   REQ_SCR_FHPAGE
          Scroll vertically forward half a page.

   REQ_SCR_BHPAGE
          Scroll vertically backward half a page.

   REQ_SCR_FCHAR
          Scroll horizontally forward a character.

   REQ_SCR_BCHAR
          Scroll horizontally backward a character.

   REQ_SCR_HFLINE
          Scroll horizontally one field width forward.

   REQ_SCR_HBLINE
          Scroll horizontally one field width backward.

   REQ_SCR_HFHALF
          Scroll horizontally one half field width forward.

   REQ_SCR_HBHALF
          Scroll horizontally one half field width backward.

   For scrolling purposes, a page of a field is the height of its visible
   part.

  Editing Requests

   When you pass the forms driver an ASCII character, it is treated as a
   request to add the character to the field's data buffer. Whether this
   is an insertion or a replacement depends on the field's edit mode
   (insertion is the default.

   The following requests support editing the field and changing the edit
   mode:

   REQ_INS_MODE
          Set insertion mode.

   REQ_OVL_MODE
          Set overlay mode.

   REQ_NEW_LINE
          New line request (see below for explanation).

   REQ_INS_CHAR
          Insert space at character location.

   REQ_INS_LINE
          Insert blank line at character location.

   REQ_DEL_CHAR
          Delete character at cursor.

   REQ_DEL_PREV
          Delete previous word at cursor.

   REQ_DEL_LINE
          Delete line at cursor.

   REQ_DEL_WORD
          Delete word at cursor.

   REQ_CLR_EOL
          Clear to end of line.

   REQ_CLR_EOF
          Clear to end of field.

   REQ_CLEAR_FIELD
          Clear entire field.

   The behavior of the REQ_NEW_LINE and REQ_DEL_PREV requests is
   complicated and partly controlled by a pair of forms options. The
   special cases are triggered when the cursor is at the beginning of a
   field, or on the last line of the field.

   First, we consider REQ_NEW_LINE:

   The normal behavior of REQ_NEW_LINE in insert mode is to break the
   current line at the position of the edit cursor, inserting the portion
   of the current line after the cursor as a new line following the
   current and moving the cursor to the beginning of that new line (you
   may think of this as inserting a newline in the field buffer).

   The normal behavior of REQ_NEW_LINE in overlay mode is to clear the
   current line from the position of the edit cursor to end of line. The
   cursor is then moved to the beginning of the next line.

   However, REQ_NEW_LINE at the beginning of a field, or on the last line
   of a field, instead does a REQ_NEXT_FIELD. O_NL_OVERLOAD option is
   off, this special action is disabled.

   Now, let us consider REQ_DEL_PREV:

   The normal behavior of REQ_DEL_PREV is to delete the previous
   character. If insert mode is on, and the cursor is at the start of a
   line, and the text on that line will fit on the previous one, it
   instead appends the contents of the current line to the previous one
   and deletes the current line (you may think of this as deleting a
   newline from the field buffer).

   However, REQ_DEL_PREV at the beginning of a field is instead treated
   as a REQ_PREV_FIELD.

   If the O_BS_OVERLOAD option is off, this special action is disabled
   and the forms driver just returns E_REQUEST_DENIED.

   See Form Options for discussion of how to set and clear the overload
   options.

  Order Requests

   If the type of your field is ordered, and has associated functions for
   getting the next and previous values of the type from a given value,
   there are requests that can fetch that value into the field buffer:

   REQ_NEXT_CHOICE
          Place the successor value of the current value in the buffer.

   REQ_PREV_CHOICE
          Place the predecessor value of the current value in the buffer.

   Of the built-in field types, only TYPE_ENUM has built-in successor and
   predecessor functions. When you define a field type of your own (see
   Custom Validation Types), you can associate our own ordering
   functions.

  Application Commands

   Form requests are represented as integers above the curses value
   greater than KEY_MAX and less than or equal to the constant
   MAX_COMMAND. If your input-virtualization routine returns a value
   above MAX_COMMAND, the forms driver will ignore it.

Field Change Hooks

   It is possible to set function hooks to be executed whenever the
   current field or form changes. Here are the functions that support
   this:
typedef void    (*HOOK)();       /* pointer to function returning void */

int set_form_init(FORM *form,    /* form to alter */
                  HOOK hook);    /* initialization hook */

HOOK form_init(FORM *form);      /* form to query */

int set_form_term(FORM *form,    /* form to alter */
                  HOOK hook);    /* termination hook */

HOOK form_term(FORM *form);      /* form to query */

int set_field_init(FORM *form,   /* form to alter */
                  HOOK hook);    /* initialization hook */

HOOK field_init(FORM *form);     /* form to query */

int set_field_term(FORM *form,   /* form to alter */
                  HOOK hook);    /* termination hook */

HOOK field_term(FORM *form);     /* form to query */

   These functions allow you to either set or query four different hooks.
   In each of the set functions, the second argument should be the
   address of a hook function. These functions differ only in the timing
   of the hook call.

   form_init
          This hook is called when the form is posted; also, just after
          each page change operation.

   field_init
          This hook is called when the form is posted; also, just after
          each field change

   field_term
          This hook is called just after field validation; that is, just
          before the field is altered. It is also called when the form is
          unposted.

   form_term
          This hook is called when the form is unposted; also, just
          before each page change operation.

   Calls to these hooks may be triggered
    1. When user editing requests are processed by the forms driver
    2. When the current page is changed by set_current_field() call
    3. When the current field is changed by a set_form_page() call

   See Field Change Commands for discussion of the latter two cases.

   You can set a default hook for all fields by passing one of the set
   functions a NULL first argument.

   You can disable any of these hooks by (re)setting them to NULL, the
   default value.

Field Change Commands

   Normally, navigation through the form will be driven by the user's
   input requests. But sometimes it is useful to be able to move the
   focus for editing and viewing under control of your application, or
   ask which field it currently is in. The following functions help you
   accomplish this:
int set_current_field(FORM *form,         /* form to alter */
                      FIELD *field);      /* field to shift to */

FIELD *current_field(FORM *form);         /* form to query */

int field_index(FORM *form,               /* form to query */
                FIELD *field);            /* field to get index of */

   The function field_index() returns the index of the given field in the
   given form's field array (the array passed to new_form() or
   set_form_fields()).

   The initial current field of a form is the first active field on the
   first page. The function set_form_fields() resets this.

   It is also possible to move around by pages.
int set_form_page(FORM *form,             /* form to alter */
                  int page);              /* page to go to (0-origin) */

int form_page(FORM *form);                /* return form's current page */

   The initial page of a newly-created form is 0. The function
   set_form_fields() resets this.

Form Options

   Like fields, forms may have control option bits. They can be changed
   or queried with these functions:
int set_form_opts(FORM *form,             /* form to alter */
                  int attr);              /* attribute to set */

int form_opts_on(FORM *form,              /* form to alter */
                 int attr);               /* attributes to turn on */

int form_opts_off(FORM *form,             /* form to alter */
                  int attr);              /* attributes to turn off */

int form_opts(FORM *form);                /* form to query */

   By default, all options are on. Here are the available option bits:

   O_NL_OVERLOAD
          Enable overloading of REQ_NEW_LINE as described in Editing
          Requests. The value of this option is ignored on dynamic fields
          that have not reached their size limit; these have no last
          line, so the circumstances for triggering a REQ_NEXT_FIELD
          never arise.

   O_BS_OVERLOAD
          Enable overloading of REQ_DEL_PREV as described in Editing
          Requests.

   The option values are bit-masks and can be composed with logical-or in
   the obvious way.

Custom Validation Types

   The form library gives you the capability to define custom validation
   types of your own. Further, the optional additional arguments of
   set_field_type effectively allow you to parameterize validation types.
   Most of the complications in the validation-type interface have to do
   with the handling of the additional arguments within custom validation
   functions.

  Union Types

   The simplest way to create a custom data type is to compose it from
   two preexisting ones:
FIELD *link_fieldtype(FIELDTYPE *type1,
                      FIELDTYPE *type2);

   This function creates a field type that will accept any of the values
   legal for either of its argument field types (which may be either
   predefined or programmer-defined). If a set_field_type() call later
   requires arguments, the new composite type expects all arguments for
   the first type, than all arguments for the second. Order functions
   (see Order Requests) associated with the component types will work on
   the composite; what it does is check the validation function for the
   first type, then for the second, to figure what type the buffer
   contents should be treated as.

  New Field Types

   To create a field type from scratch, you need to specify one or both
   of the following things:
     * A character-validation function, to check each character as it is
       entered.
     * A field-validation function to be applied on exit from the field.

   Here's how you do that:
typedef int     (*HOOK)();       /* pointer to function returning int */

FIELDTYPE *new_fieldtype(HOOK f_validate, /* field validator */
                         HOOK c_validate) /* character validator */


int free_fieldtype(FIELDTYPE *ftype);     /* type to free */

   At least one of the arguments of new_fieldtype() must be non-NULL. The
   forms driver will automatically call the new type's validation
   functions at appropriate points in processing a field of the new type.

   The function free_fieldtype() deallocates the argument fieldtype,
   freeing all storage associated with it.

   Normally, a field validator is called when the user attempts to leave
   the field. Its first argument is a field pointer, from which it can
   get to field buffer 0 and test it. If the function returns TRUE, the
   operation succeeds; if it returns FALSE, the edit cursor stays in the
   field.

   A character validator gets the character passed in as a first
   argument. It too should return TRUE if the character is valid, FALSE
   otherwise.

  Validation Function Arguments

   Your field- and character- validation functions will be passed a
   second argument as well. This second argument is the address of a
   structure (which we'll call a pile) built from any of the
   field-type-specific arguments passed to set_field_type(). If no such
   arguments are defined for the field type, this pile pointer argument
   will be NULL.

   In order to arrange for such arguments to be passed to your validation
   functions, you must associate a small set of storage-management
   functions with the type. The forms driver will use these to synthesize
   a pile from the trailing arguments of each set_field_type() argument,
   and a pointer to the pile will be passed to the validation functions.

   Here is how you make the association:
typedef char    *(*PTRHOOK)();    /* pointer to function returning (char *) */
typedef void    (*VOIDHOOK)();    /* pointer to function returning void */

int set_fieldtype_arg(FIELDTYPE *type,    /* type to alter */
                      PTRHOOK make_str,   /* make structure from args */
                      PTRHOOK copy_str,   /* make copy of structure */
                      VOIDHOOK free_str); /* free structure storage */

   Here is how the storage-management hooks are used:

   make_str
          This function is called by set_field_type(). It gets one
          argument, a va_list of the type-specific arguments passed to
          set_field_type(). It is expected to return a pile pointer to a
          data structure that encapsulates those arguments.

   copy_str
          This function is called by form library functions that allocate
          new field instances. It is expected to take a pile pointer,
          copy the pile to allocated storage, and return the address of
          the pile copy.

   free_str
          This function is called by field- and type-deallocation
          routines in the library. It takes a pile pointer argument, and
          is expected to free the storage of that pile.

   The make_str and copy_str functions may return NULL to signal
   allocation failure. The library routines will that call them will
   return error indication when this happens. Thus, your validation
   functions should never see a NULL file pointer and need not check
   specially for it.

  Order Functions For Custom Types

   Some custom field types are simply ordered in the same well-defined
   way that TYPE_ENUM is. For such types, it is possible to define
   successor and predecessor functions to support the REQ_NEXT_CHOICE and
   REQ_PREV_CHOICE requests. Here's how:
typedef int     (*INTHOOK)();     /* pointer to function returning int */

int set_fieldtype_arg(FIELDTYPE *type,    /* type to alter */
                      INTHOOK succ,       /* get successor value */
                      INTHOOK pred);      /* get predecessor value */

   The successor and predecessor arguments will each be passed two
   arguments; a field pointer, and a pile pointer (as for the validation
   functions). They are expected to use the function field_buffer() to
   read the current value, and set_field_buffer() on buffer 0 to set the
   next or previous value. Either hook may return TRUE to indicate
   success (a legal next or previous value was set) or FALSE to indicate
   failure.

  Avoiding Problems

   The interface for defining custom types is complicated and tricky.
   Rather than attempting to create a custom type entirely from scratch,
   you should start by studying the library source code for whichever of
   the pre-defined types seems to be closest to what you want.

   Use that code as a model, and evolve it towards what you really want.
   You will avoid many problems and annoyances that way. The code in the
   ncurses library has been specifically exempted from the package
   copyright to support this.

   If your custom type defines order functions, have do something
   intuitive with a blank field. A useful convention is to make the
   successor of a blank field the types minimum value, and its
   predecessor the maximum.