#include <stdlib.h>
#include <string.h>
#define GL_GLEXT_PROTOTYPES
#include <GL/glx.h>

int main(int argc, char *argv[])
{
  int width = 300, height = 300;

  Display *dpy = XOpenDisplay(NULL);

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

  int pbattrs[] = {GLX_PBUFFER_WIDTH, width, GLX_PBUFFER_HEIGHT, height, GLX_PRESERVED_CONTENTS, True, None};
  GLXPbuffer pbuffer = glXCreatePbuffer(dpy, dconfigs[0], pbattrs);

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

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

  int size = width*height*4;
  glBufferData(GL_PIXEL_UNPACK_BUFFER_EXT, size, NULL, GL_STREAM_DRAW);
  void *pixeldata = malloc(size);
  memset(pixeldata, 0x80, size);

  glBufferSubData(GL_PIXEL_UNPACK_BUFFER_EXT, 0, size, pixeldata);
  glDrawPixels(width, height, GL_BGRA, GL_UNSIGNED_BYTE, NULL);
  glXSwapBuffers(dpy, pbuffer);

  GLXPbuffer pbuffer2 = glXCreatePbuffer(dpy, dconfigs[0], pbattrs);

  glXMakeCurrent(dpy, pbuffer2, ctx);

  glBufferSubData(GL_PIXEL_UNPACK_BUFFER_EXT, 0, size, pixeldata);
  glDrawPixels(width, height, GL_BGRA, GL_UNSIGNED_BYTE, NULL);
  glXSwapBuffers(dpy, pbuffer2);

  free(pixeldata);
  glDeleteBuffers(1, &pbo);
  glXDestroyPbuffer(dpy, pbuffer);
  glXDestroyPbuffer(dpy, pbuffer2);

  return 0;
}