How do I can call initgraph more than one time? - c

Question

Please look at the following code:
#include <stdio.h>
#include <graphics.h>
#include <conio.h>
using namespace std;
void drawrect()
{
int gdriver = IBM8514, gmode;
initgraph(&gdriver, &gmode, "");
rectangle(500, 500, 700, 700);
getch();
cleardevice();
closegraph();
}
int main()
{
int f=1;
while(f)
{
char c;
printf("Press \"e\" to end, and any other character to draw a rectangle");
scanf("%c",&c);
c=='e' ? f=0:f=1;
drawrect();
fflush(stdin);
}
}
at the first time when I run this program, It works correctly and draws a rectangle, but after the first time, the rectangle function doesn't work and the GUI screen is completely blank, While I've cleared and closed previous graphic
So why it doesn't work at second time?

Answer 1

You code has undefined behaviour. The call to initgraph
int gdriver = IBM8514, gmode;
initgraph(&gdriver, &gmode, "");
should pass a pointer to the graphics mode you want to use. This page describes the function and its arguments, and about the mode it says:
*graphmode is an integer that specifies the initial graphics mode (unless *graphdriver equals DETECT; in which case, *graphmode is set
by initgraph to the highest resolution available for the detected
driver). You can give *graphmode a value using a constant of the
graphics_modes enumeration type, which is defined in graphics.h and
listed below.
graphdriver and graphmode must be set to valid values from the
following tables, or you will get unpredictable results. The exception
is graphdriver = DETECT.
But you have not set the mode, and as the second paragraph quoted says, the result is unpredictable. This can be: working how you intended, not working, working strangely, or frying the processor.
So set the graphics mode you want to use with say
int gdriver = IBM8514, gmode = 0;
or whatever mode you need to use. Alternatively you can tell the system to detect for itself, in which case you can use
int gdriver = DETECT, gmode;

Answer 2

Init and close should be called just once and not be called in the drawrect but usually in the main instead ... also having getch in rendering routine makes no sense too...
I will not touch other issues here of your code as I am not coding console stuff for years and BGI even longer but I would start with reordering the code to this:
#include <stdio.h>
#include <graphics.h>
#include <conio.h>
using namespace std;
void drawrect()
{
rectangle(500, 500, 700, 700);
}
int main()
{
int f=1;
int gdriver = IBM8514, gmode;
initgraph(&gdriver, &gmode, "");
while(f)
{
char c;
printf("Press \"e\" to end, and any other character to draw a rectangle");
scanf("%c",&c);
c=='e' ? f=0:f=1;
drawrect();
getch();
fflush(stdin);
}
cleardevice();
closegraph();
}
Also in future address the library by its real name BGI because graphics.h has no meaning as almost all gfx api/libs got a file with that name ...