libX11: XPutImage first call - c

Question

I've created XImage using XCreateImage and use XPutImage to display it on window, but XPutImage shows this picture only on second call of it. Why this happens?
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
void draw(char *rgb_out, int w, int h)
{
int i = 0;
for (i = 0;i < w*h;i += 4) {
rgb_out[i + 1] = 0;
rgb_out[i + 2] = 0;
rgb_out[i + 3] = 0;
}
return;
}
XImage *create_ximage(Display *display, Visual *visual, int width, int height)
{
char *image32 = (char *)malloc(width * height * 4);
draw(image32, width, height);
return XCreateImage(display, visual, 24,
ZPixmap, 0, image32,
width, height, 32, 0);
}
int main(int argc, char **argv)
{
int win_b_color;
int win_w_color;
XEvent xev;
Window window;
GC gc;
Display *display = XOpenDisplay(NULL);
Visual *visual;
XImage *ximage;
win_b_color = BlackPixel(display, DefaultScreen(display));
win_w_color = BlackPixel(display, DefaultScreen(display));
window = XCreateSimpleWindow(display,
DefaultRootWindow(display),
0, 0, 600, 400, 0,
win_b_color, win_w_color);
gc = XCreateGC(display, window, 0, NULL);
visual = DefaultVisual(display, 0);
XMapWindow(display, window);
XFlush(display);
ximage = create_ximage(display, visual, 100, 100);
while (1) {
int r;
r = XPutImage(display, window,
gc, ximage, 0, 0, 0, 0,
100, 100);
printf("RES: %i\n", r);
XSync(display, 1);
XFlush(display);
getchar();
}
return 0;
}

Answer 1

The trick is to wait that the windows is mapped. You can do this by Expose event.
int main(int argc, char **argv)
{
int win_b_color;
int win_w_color;
XEvent xev;
Window window;
GC gc;
Display *display = XOpenDisplay(NULL);
Visual *visual;
XImage *ximage;
win_b_color = BlackPixel(display, DefaultScreen(display));
win_w_color = BlackPixel(display, DefaultScreen(display));
window = XCreateSimpleWindow(display,
DefaultRootWindow(display),
0, 0, 600, 400, 0,
win_b_color, win_w_color);
visual = DefaultVisual(display, 0);
XSelectInput(display, window, ExposureMask | KeyPressMask);
XMapWindow(display, window);
XFlush(display);
gc = XCreateGC(display, window, 0, NULL);
ximage = create_ximage(display, visual, 100, 100);
XEvent event;
bool exit = false;
while (!exit) {
int r;
XNextEvent(display, &event);
if (event.type == Expose)
{
r = XPutImage(display, window,
gc, ximage, 0, 0, 0, 0,
100, 100);
printf("RES: %i\n", r);
}
else if (event.type == KeyPress)
exit = true;
}
return 0;
}