/*usr/bin/env cat >/tmp/r.c <<ZZZ; gcc -O /tmp/r.c -lGL -lX11 -o /tmp/r && /tmp/r # */
#include <stdlib.h>
#include <stdio.h>
#define GL_GLEXT_PROTOTYPES
#include <GL/glx.h>

#define T(c) ({fprintf(stderr, "before %s\n", #c); c; fprintf(stderr, "after %s\n", #c);})

int main(int argc, char *argv[])
{
  int i, n, width = 1024, height = 512;

  Display *dpy = XOpenDisplay(NULL);

  int ncfg, attrs[] = {GLX_DOUBLEBUFFER, GL_TRUE, None};
  GLXFBConfig *dconfigs = glXChooseFBConfig(dpy, 0, attrs, &ncfg);

  GLXContext ctx = glXCreateNewContext(dpy, dconfigs[0], GLX_RGBA_TYPE, NULL, True);

  Window w = XCreateSimpleWindow(dpy, DefaultRootWindow(dpy), 0, 0, width, height, 0, 0, 0);
  XMapWindow(dpy, w);

  glXMakeCurrent(dpy, w, ctx);

  GLuint pbo;
  glGenBuffers(1, &pbo);
  glBindBuffer(GL_PIXEL_UNPACK_BUFFER_EXT, pbo);

  int size = width*height*4;
  T(glBufferData(GL_PIXEL_UNPACK_BUFFER_EXT, size, NULL, GL_STREAM_DRAW));
  int *pixeldata = malloc(size);
  for (i = 0; i < size / 4; i++)
    pixeldata[i] = i | (i << 8);

  n = argc == 2 ? atoi(argv[1]) : 0;
  for (i = 0; i < n; i++)
    {
      T(glBufferSubData(GL_PIXEL_UNPACK_BUFFER_EXT, 0, size - i * 4, pixeldata + i));
      T(glDrawPixels(width, height, GL_BGRA, GL_UNSIGNED_BYTE, NULL));
      T(glXSwapBuffers(dpy, w));
    }

  return 0;
}
#define ZZZ
ZZZ