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5. Screen Management

The S-Lang library provides two interfaces to terminal independent routines for manipulating the display on a terminal. The highest level interface, known as the SLsmg interface is discussed in this section. It provides high level screen management functions more manipulating the display in an optimal manner and is similar in spirit to the curses library. The lowest level interface, or the SLtt interface, is used by the SLsmg routines to actually perform the task of writing to the display. This interface is discussed in another section. Like the keyboard routines, the SLsmg routines are platform independent and work the same on MSDOS, OS/2, Unix, and VMS.

The screen management, or SLsmg, routines are initialized by function SLsmg_init_smg. Once initialized, the application uses various SLsmg functions to write to a virtual display. This does not cause the physical terminal display to be updated immediately. The physical display is updated to look like the virtual display only after a call to the function SLsmg_refresh. Before exiting, the application using these routines is required to call SLsmg_reset_smg to reset the display system.

The following subsections explore S-Lang's screen management system in greater detail.

5.1 Initialization

The function SLsmg_init_smg must be called before any other SLsmg function can be used. It has the simple prototype:

      int SLsmg_init_smg (void);
It returns non-zero if successful or zero if it cannot allocate space for the virtual display.

For this routine to properly initialize the virtual display, the capabilities of the terminal must be known as well as the size of the physical display. For these reasons, the lower level SLtt routines come into play. In particular, before the first call to SLsmg_init_smg, the application is required to call the function SLtt_get_terminfo before calling SLsmg_init_smg.

The SLtt_get_terminfo function sets the global variables SLtt_Screen_Rows and SLtt_Screen_Cols to the values appropriate for the terminal. It does this by calling the SLtt_get_screen_size function to query the terminal driver for the appropriate values for these variables. From this point on, it is up to the application to maintain the correct values for these variables by calling the SLtt_get_screen_size function whenever the display size changes, e.g., in response to a SIGWINCH signal. If the screen size changes, the application must also re-initialize the SLsmg routines by calling SLsmg_init_tty again so that the size of the virtual screen can be adjusted accordingly.

Finally, if the application is going to read characters from the keyboard, it is also a good idea to initialize the keyboard routines at this point as well.

5.2 Resetting SLsmg

Before the program exits or suspends, the function SLsmg_reset_tty should be called to shutdown the display system. This function has the prototype

      void SLsmg_reset_smg (void);
This will deallocate any memory allocated for the virtual screen and reset the terminal's display.

Basically, a program that uses the SLsmg screen management functions and S-Lang's keyboard interface will look something like:

      #include "slang.h"
      int main ()
         SLtt_get_terminfo ();
         SLang_init_tty (-1, 0, 0);
         SLsmg_init_smg ();
         /* do stuff .... */
         SLsmg_reset_smg ();
         SLang_reset_tty ();
         return 0;
If this program is compiled and run, all it will do is clear the screen and position the cursor at the bottom of the display. In the following sections, other SLsmg functions will be introduced which may be used to make this simple program do much more.

5.3 SLsmg Functions

In the previous sections, functions for initializing and shutting down the SLsmg routines were discussed. In this section, the rest of the SLsmg functions are presented. These functions act only on the virtual display. The physical display is updated when the SLsmg_refresh function is called and not until that time. This function has the simple prototype:

     void SLsmg_refresh (void);

Positioning the cursor

The SLsmg_gotorc function is used to position the cursor at a given row and column. The prototype for this function is:

      void SLsmg_gotorc (int row, int col);
The origin of the screen is at the top left corner and is given the coordinate (0, 0), i.e., the top row of the screen corresponds to row = 0 and the first column corresponds to col = 0. The last row of the screen is given by row = SLtt_Screen_Rows - 1.

It is possible to change the origin of the coordinate system by using the function SLsmg_set_screen_start with prototype:

     void SLsmg_set_screen_start (int *r, int *c);
This function takes pointers to the new values of the first row and first column. It returns the previous values by modifying the values of the integers at the addresses specified by the parameter list. A NULL pointer may be passed to indicate that the origin is to be set to its initial value of 0. For example,
      int r = 10;
      SLsmg_set_screen_start (&r, NULL);
sets the origin to (10, 0) and after the function returns, the variable r will have the value of the previous row origin.

Writing to the Display

SLsmg has several routines for outputting text to the virtual display. The following points should be understood:

Although the some of the above items might appear to be too restrictive, in practice this is not seem to be the case. In fact, the design of the output routines was influenced by their actual use and modified to simplify the code of the application utilizing them.

void SLsmg_write_char (char ch);\ Write a single character to the virtual display.

void SLsmg_write_nchars (char *str, int len);\ Write len characters pointed to by str to the virtual display.

void SLsmg_write_string (char *str);\ Write the null terminated string given by pointer str to the virtual display. This function is a wrapper around SLsmg_write_nchars.

void SLsmg_write_nstring (char *str, int n);\ Write the null terminated string given by pointer str to the virtual display. At most, only n characters are written. If the length of the string is less than n, then the string will be padded with blanks. This function is a wrapper around SLsmg_write_nchars.

void SLsmg_printf (char *fmt, ...);\ This function is similar to printf except that it writes to the SLsmg virtual display.

void SLsmg_vprintf (char *, va_list);\ Like SLsmg_printf but uses a variable argument list.

Erasing the Display

The following functions may be used to fill portions of the display with blank characters. The attributes of blank character are the current attributes. (See below for a discussion of character attributes)

void SLsmg_erase_eol (void);\ Erase line from current position to the end of the line.

void SLsmg_erase_eos (void);\ Erase from the current position to the end of the screen.

void SLsmg_cls (void);\ Clear the entire virtual display.

Setting Character Attributes

Character attributes define the visual characteristics the character possesses when it is displayed. Visual characteristics include the foreground and background colors as well as other attributes such as blinking, bold, and so on. Since SLsmg takes a different approach to this problem than other screen management libraries an explanation of this approach is given here. This approach has been motivated by experience with programs that require some sort of screen management.

Most programs that use SLsmg are composed of specific textual objects or objects made up of line drawing characters. For example, consider an application with a menu bar with drop down menus. The menus might be enclosed by some sort of frame or perhaps a shadow. The basic idea is to associate an integer to each of the objects (e.g., menu bar, shadow, current menu item, etc.) and create a mapping from the integer to the set of attributes. In the terminology of SLsmg, the integer is simply called an object.

For example, the menu bar might be associated with the object 1, the drop down menu could be object 2, the shadow could be object 3, and so on.

The range of values for the object integer is restricted from 0 up to and including 255 on all systems except MSDOS where the maximum allowed integer is 15

This difference is due to memory constraints imposed by MSDOS. This restriction might be removed in a future version of the library.
. The object numbered zero should not be regarding as an object at all. Rather it should be regarded as all other objects that have not explicitly been given an object number. SLsmg, or more precisely SLtt, refers to the attributes of this special object as the default or normal attributes.

The SLsmg routines know nothing about the mapping of the color to the attributes associated with the color. The actual mapping takes place at a lower level in the SLtt routines. Hence, to map an object to the actual set of attributes requires a call to any of the following SLtt routines:

     void SLtt_set_color (int obj, char *name, char *fg, char *bg);
     void SLtt_set_color_object (int obj, SLtt_Char_Type attr);
     void SLtt_set_mono (int obj, char *, SLtt_Char_Type attr);
Only the first of these routines will be discussed briefly here. The latter two functions allow more fine control over the object to attribute mapping (such as assigning a ``blink'' attribute to the object). For a more full explanation on all of these routines see the section about the SLtt interface.

The SLtt_set_color function takes four parameters. The first parameter, obj, is simply the integer of the object for which attributes are to be assigned. The second parameter is currently unused by these routines. The third and forth parameters, fg and bg, are the names of the foreground and background color to be used associated with the object. The strings that one can use for the third and fourth parameters can be any one of the 16 colors:

     "black"                "gray"
     "red"                  "brightred"
     "green"                "brightgreen"
     "brown"                "yellow"
     "blue"                 "brightblue"
     "magenta"              "brightmagenta"
     "cyan"                 "brightcyan"
     "lightgray"            "white"
The value of the foreground parameter fg can be anyone of these sixteen colors. However, on most terminals, the background color will can only be one of the colors listed in the first column
This is also true on the Linux console. However, it need not be the case and hopefully the designers of Linux will someday remove this restriction.

Of course not all terminals are color terminals. If the S-Lang global variable SLtt_Use_Ansi_Colors is non-zero, the terminal is assumed to be a color terminal. The SLtt_get_terminfo will try to determine whether or not the terminal supports colors and set this variable accordingly. It does this by looking for the capability in the terminfo/termcap database. Unfortunately many Unix databases lack this information and so the SLtt_get_terminfo routine will check whether or not the environment variable COLORTERM exists. If it exists, the terminal will be assumed to support ANSI colors and SLtt_Use_Ansi_Colors will be set to one. Nevertheless, the application should provide some other mechanism to set this variable, e.g., via a command line parameter.

When the SLtt_Use_Ansi_Colors variable is zero, all objects with numbers greater than one will be displayed in inverse video

This behavior can be modified by using the SLtt_set_mono function call.

With this background, the SLsmg functions for setting the character attributes can now be defined. These functions simply set the object attributes that are to be assigned to subsequent characters written to the virtual display. For this reason, the new attribute is called the current attribute.

void SLsmg_set_color (int obj);\ Set the current attribute to those of object obj.

void SLsmg_normal_video (void);\ This function is equivalent to SLsmg_set_color (0).

void SLsmg_reverse_video (void);\ This function is equivalent to SLsmg_set_color (1). On monochrome terminals, it is equivalent to setting the subsequent character attributes to inverse video.

Unfortunately there does not seem to be a standard way for the application or, in particular, the library to determine which color will be used by the terminal for the default background. Such information would be useful in initializing the foreground and background colors associated with the default color object (0). FOr this reason, it is up to the application to provide some means for the user to indicate what these colors are for the particular terminal setup. To facilitate this, the SLtt_get_terminfo function checks for the existence of the COLORFGBG environment variable. If this variable exists, its value will be used to initialize the colors associated with the default color object. Specifically, the value is assumed to consist of a foreground color name and a background color name separated by a semicolon. For example, if the value of COLORTERM is lightgray;blue, the default color object will be initialized to represent a lightgray foreground upon a blue background.

Lines and Alternate Character Sets

The S-Lang screen management library also includes routines for turning on and turning off alternate character sets. This is especially useful for drawing horizontal and vertical lines.

void SLsmg_set_char_set (int flag);\ If flag is non-zero, subsequent write functions will use characters from the alternate character set. If flag is zero, the default, or, ordinary character set will be used.

void SLsmg_draw_hline (int len);\ Draw a horizontal line from the current position to the column that is len characters to the right.

void SLsmg_draw_vline (int len);\ Draw a horizontal line from the current position to the row that is len rows below.

void SLsmg_draw_box (int r, int c, int dr, int dc);\ Draw a box whose upper right corner is at row r and column c. The box spans dr rows and dc columns. The current position will be left at row r and column c.

Miscellaneous Functions

void SLsmg_touch_lines (int r, int n);\ Mark screen rows numbered r, r + 1, ... r + (n - 1) as modified. When SLsmg_refresh is called, these rows will be completely redrawn.

unsigned short SLsmg_char_at(void);\ Returns the character and its attributes object number at the current cursor position. The character itself occupies the lower byte and the object attributes number forms the upper byte. The object returned by this function call should not be written back out via any of the functions that write characters or character strings.

5.4 Variables

The following S-Lang global variables are used by the SLsmg interface. Some of these have been previously discussed.

int SLtt_Screen_Rows;\ int SLtt_Screen_Cols;\ The number of rows and columns of the physical display. If either of these numbers changes, the functions SLsmg_reset_smg and SLsmg_init_smg should be called again so that the SLsmg routines can re-adjust to the new size.

int SLsmg_Tab_Width;\ Set this variable to the tab width that will be used when expanding tab characters. The default is 8.

int SLsmg_Display_Eight_Bit\ This variable determines how characters with the high bit set are to be output. Specifically, a character with the high bit set with a value greater than or equal to this value is output as is; otherwise, it will be output in a 7-bit representation. The default value for this variable is 128 for MSDOS and 160 for other systems (ISO-Latin).

int SLtt_Use_Ansi_Colors;\ If this value is non-zero, the terminal is assumed to support ANSI colors otherwise it is assumed to be monochrome. The default is 0.

int SLtt_Term_Cannot_Scroll;\ If this value is zero, the SLsmg will attempt to scroll the physical display to optimize the update. If it is non-zero, the screen management routines will not perform this optimization. For some applications, this variable should be set to zero. The default value is set by the SLtt_get_terminfo function.

5.5 Hints for using SLsmg

This section discusses some general design issues that one must face when writing an application that requires some sort of screen management.

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