xlib C program doesn´t work every time

[bitbird]

Hello,
I tried to draw two lines with xlib.h in C.
If I start this compiled program it show me in most cases a window with two lines as it should be.
But sometimes the two lines are not drawn.If I insert XFlush() before the second XSync() it worked better but not every time.Why?How can I solve the problem?How works the X-Server buffer in detail?

#include <X11/Xlib.h>
 #include <stdio.h>
 #include <unistd.h>
 
 int main(void) {
   Display *d;
   Window w;
   int s;
 
                        /* open connection with the server */
   d = XOpenDisplay(NULL);
   if (d == NULL) {
     fprintf(stderr, "Cannot open display\n");
     return 1;
   }
 
   s = DefaultScreen(d);
 
                        /* create window */
   w = XCreateSimpleWindow(d, RootWindow(d, s), 10, 10, 200, 200, 1,
                           BlackPixel(d, s), WhitePixel(d, s));
 
                      
                        /* map (show) the window */
   XMapWindow(d, w);
 
   GC gc=DefaultGC(d, s);
   XSync(d, False); //the mistake seems to depend on this function
                        
  
   XDrawLine(d, w, gc,100, 0, 100, 200);
   XDrawLine(d, w, gc, 0, 100, 200, 100);
   
   XSync(d, False); //

   sleep(1);
 
                        /* close connection to server */
   XCloseDisplay(d);
 
   return 0;
 }

Thanks for all answers

[Rubberman]

Look at the default graphics context you are using which will specify the line width to use. It may be that it is sometimes drawing it where it cannot be displayed by your hardware. You might want to set up a custom context to use and try a wider line width for your XDrawLine() calls. Adding the XFlush() shouldn't be needed with XSync(), but I'm not 100% sure about that.