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                        Using Video Input and Output
 
Since output for Presentation Manager applications is provided by using
windows and the Gpi functions, only non-Presentation Manager programs use
the MS OS/2 Vio functions. Typically, you use these functions in
character-based programs. The following sections explain how to use some of
the Vio functions.
 
Displaying a Character
 
The KbdCharIn function does not echo a keystroke as it reads it; that is,
the function does not write the corresponding character to the screen. If
you want to display the character, you can do so by using the VioWrtTTY
function. The following code fragment writes the letter A to the screen:
 
CHAR ch = 'A';
 
VioWrtTTY(&ch, 1, 0);
 
Video functions, like keyboard functions, require a handle to identify the
screen to be accessed. In the preceding example, the third argument, 0, is
the default video handle and identifies the system screen. The system screen
is always available to programs and does not need be opened to be used.
Video handle 0 and standard-output handle 1 are not the same. The
standard-output file can be closed or redefined, but the system-screen
handle cannot.
 
If the standard-output handle has not been redirected, you can also write a
character to the screen by using the DosWrite function. DosWrite calls the
VioWrtTTY function to write the character, so when writing to the screen,
these functions are identical.
 
The VioWrtTTY function always writes the character to the current position
of the cursor and then advances the cursor one position. If the cursor
reaches the end of a line, it wraps to the beginning of the next line. If
the cursor reaches the end of the screen, VioWrtTTY scrolls the screen
contents up one line.
 
Writing a Character to a Specific Location
 
You can write a character to a specific location on the screen by using the
VioWrtNChar function. This function lets you specify the row and column at
which to place the character. The following code fragment writes the letter
A to row 10, column 15:
 
CHAR ch = 'A';
 
VioWrtNChar(&ch, 1, 10, 15, 0);
 
The VioWrtNChar function uses the coordinate system of the screen to
determine where to place the character. Such a coordinate system typically
divides the screen into rows and columns. Each coordinate represents a
character cell, a rectangular area of the screen large enough to display one
character. When you write a character to a particular coordinate, it
overwrites the character that was previously there.
 
In MS OS/2, the upper-left corner of the screen is at position (0,0) and,
for most screen modes, the lower-right corner is at position (24,79). If you
attempt to write a character outside these boundaries, the function returns
an error.
 
You can write the same character to the screen repeatedly by specifying a
value greater than 1 as the second argument. For example, the following code
fragment clears a 25 by 80 screen:
 
CHAR ch = ' ';
 
VioWrtNChar(&ch, 2000, 0, 0, 0);
 
If the VioWrtNChar function reaches the end of a line, it automatically
wraps to the beginning of the next line. However, it does not scroll the
screen contents when it reaches the bottom of the screen, and it does not
update the cursor position.
 
You can write a single attribute to the screen a specified number of times
by using the VioWrtNAttr function.
 
Writing a String of Characters to the Screen
 
You can write a string of characters directly to the screen by using the
VioWrtCharStr function. When you write characters to the screen in this way,
you can specify the row and column at which the characters are to start.
This function is similar to the VioWrtNChar function, except that instead of
a single character, you specify an array of characters. The following code
fragment writes the string "Hello, world" to the middle of the screen:
 
VioWrtCharStr("Hello, world",  12, 13, 34, 0 );
 
You can write a string of characters to the screen with a specific attribute
by using the VioWrtCharStrAtt function.
 
Writing Character Cells to the Screen
 
You can write character cells to the screen by using the VioWrtNCell or
VioWrtCellStr function. A character cell is a 2-byte value that specifies a
character and its attribute. A character attribute defines the color,
intensity, and appearance of the character to be written. The following code
fragment writes a red letter A to the middle of a color screen:
 
BYTE abCell[2] = { 'A', 0x04 };
 
VioWrtNCell(abCell, 1, 13, 40, 0);
 
Character cells are useful in programs that take full advantage of the
text-mode capabilities of the display adapter. However, the meaning and
range of values for attributes depend on the device, so it is important that
you check the display-adapter type. You can do this by using the
VioGetConfig function, as shown in the following code fragment:
 
VIOCONFIGINFO vioin;
vioin.cb = sizeof(vioin);
 
VioGetConfig(0, &vioin, 0);
switch (vioin.adapter) {
    case DISPLAY_MONOCHROME:   /* monochrome adapter              */
        break;
    case DISPLAY_CGA:          /* color graphics adapter (CGA)    */
        break;
    case DISPLAY_EGA:          /* enhanced graphics adapter (EGA) */
        break;
}
 
Moving and Hiding the Cursor
 
If you choose to use the VioWrtTTY or DosWrite function to write text to the
screen, you can control the placement of that text on the screen by using
the VioSetCurPos and VioGetCurPos functions to set and get the position of
the cursor. The cursor is the flashing underscore or block on the screen
that marks the location that will receive the next character written to the
screen. The following code fragment moves the cursor to the middle of the
screen and writes the string "Hello, world":
 
VioSetCurPos(13, 34, 0);
VioWrtTTY("Hello, world", 12, 0);
 
If you choose not to use the cursor, you can remove it from the screen by
using the VioSetCurType function, which requires a VIOCURSORINFO structure.
If you set the attr field in the VIOCURSORINFO structure to 0xFFFF, as shown
in the following code fragment, the function hides the cursor:
 
VIOCURSORINFO vioci;
 
VioGetCurType(&vioci, 0);    /* retrieve current cursor type */
vioci.attr = 0xFFFF;         /* hide the cursor              */
VioSetCurType(&vioci, 0);    /* set new cursor type          */
 
You can restore the cursor by setting the attr field to its original value.
In the preceding example, the VioGetCurType function was used to fill the
VIOCURSORINFO structure with the current information before the structure
was modified to hide the cursor. In general, whenever you use an MS OS/2
function that takes values from a structure, you should be sure that all
fields contain valid values. In this case, the way to ensure valid values is
to fill the structure first by using the VioGetCurType function.
 
You can also use the VioSetCurType function to change the shape of the
cursor. For example, you can change the shape from an underscore to a block
by setting the yStart and cEnd fields to appropriate values. The following
code fragment creates a block cursor:
 
VioGetCurType(&vioci, 0);    /* retrieve current cursor type */
vioci.yStart = 10;           /* start of cursor              */
vioci.cEnd = 0;              /* end of cursor                */
VioSetCurType(&vioci, 0);    /* set new cursor type          */
 
Reading Characters from the Screen
 
You can read characters from the screen by using the VioReadCellStr or
VioReadCharStr function. Reading characters is an easy way to determine the
content of the screen. Some programs use this as a method of input. For
example, a "help" program might check the location of the cursor and read
the line at that point to provide context-sensitive help.
 
The following code fragment reads a character string at the current cursor
position:
 
CHAR achBuffer[80];
USHORT cchBuffer = 80;
USHORT usRow, usCol;
 
VioGetCurPos(&usRow, &usCol, 0);
VioReadCharStr(achBuffer, &cchBuffer, usRow, usCol, 0 );
 
Scrolling the Screen Contents
 
You can scroll all or part of the screen contents by using the VioScrollDn,
VioScrollUp, VioScrollLf, and VioScrollRt functions.
 
The following code fragment scrolls the screen contents up three lines,
leaving three blank lines at the bottom of the screen:
 
BYTE abCell[2] = { ' ', 0x07 };
 
VioScrollUp(3, 0, 24, 79, 3, abCell,  0);
 
You can also use the scroll functions to clear the screen. Whenever the
rectangle that you specify has the same dimensions as the screen, the entire
screen is cleared. The following code fragment clears a 25 by 80 screen:
 
BYTE abCell[2] = { ' ', 0x07 };
 
VioScrollUp(0, 0, 24, 79, 0xFFFF, abCell, 0);
 
Using the ANSI Display Mode
 
You can set the video display to ANSI mode by using the VioSetAnsi function.
When in ANSI mode, the video display checks for and carries out the actions
specified by any ANSI escape sequences that are written to the screen by
such functions as VioWrtTTY.
 
An ANSI escape sequence is a combination of characters, starting with the
escape character (27), that specifies a particular action to be taken by the
video display, such as moving the cursor or displaying subsequent characters
in a new display mode.
 
Note:  The ANSI escape sequences that change color may behave differently
       when used in a VIO window instead of a full-screen session.
 
 
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