Q46217: QuickC Does Not Always Deallocate Memory Given to Video Buffer

Article: Q46217
Product(s): See article
Version(s): 2.00
Operating System(s): MS-DOS
Keyword(s): ENDUSER | S_QuickAsm | mspl13_c
Last Modified: 18-SEP-1989

When a graphics mode program is run from within the QuickC
environment, screen swapping is used as the method of screen exchange.
Thus, a buffer is dynamically allocated for holding the contents of
the graphics-mode output screen. If the video mode is not reset to a
text mode at the end of the program [i.e.,
_setvideomode(_DEFAULTMODE);],  this buffer will not be deallocated
properly, and the available memory will be decreased accordingly. A
program that is constructed in this manner will eventually fail due to
lack of memory, either at start-up or in the body of the code, when an
attempt is made to allocate memory for use by the program.

The following program demonstrates the problem. If the
_setvideomode(_DEFAULTMODE); line is uncommented at the end of the
program, it will run properly inside of the QuickC environment an
unlimited number of times. However, if the program is allowed to
terminate without resetting the video mode, it will fail the second,
third, or fourth try, depending upon the initial amount of free memory
in the system. This program will run properly from a DOS prompt any
number of times. Another solution is simply to shell out of QuickC and
reset the mode to a text mode. The program will then run one more
time. However, the best solution is to change the program.

Sample Program
--------------

#include <stddef.h>
#include <malloc.h>
#include <process.h>
#include <graph.h>
#include <conio.h>

void main (void)
 { long size;
   char huge *image;

   /* Put the display in a EGA mode and allocate a buffer. */
   if (_setvideomode(_ERESCOLOR)==0)
       { _outtext ("Video mode not supported!\n");
         exit (0);
       }

   /* In _VRES16COLOR, size is 153604. */
   size = _imagesize(0,0,639,349);

   if ( (image=(char huge *)halloc(size,1)) == NULL)
        {
         _outtext ("Not enough memory for buffer!\n");
         exit (1);
       }
   /* Draw a couple of shapes and capture the screen. */
   _ellipse (_GBORDER,0,0,639,349);
   _rectangle (_GBORDER,0,0,639,349);
   _getimage (0,0,639,349,image);

   /* Clear the screen and redraw the image. */
   _clearscreen (_GCLEARSCREEN);
   _putimage (0,0,image,_GOR);
   getch();
   hfree ((char huge *)image);       /* carefully free the image buffer */
   /* _setvideomode (_DEFAULTMODE); */
   /* uncomment the line above, and the program works every time */
}

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