Measure screen lag on rpi

lagtest.c

/*
instructions: hook up one oscilloscope channel to a photodiode circuit and the other to GPIO 21; trigger on GPIO 21
*/

#include <stdio.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <string.h>
#include <sys/mman.h>
#include <linux/fb.h>
#include <pigpio.h>


#define PIN 21

int main() {
  char* fb;
  int handle = open("/dev/fb0", O_RDWR);
  struct fb_var_screeninfo vinfo;
  struct fb_fix_screeninfo finfo;

  ioctl(handle, FBIOGET_VSCREENINFO, &vinfo);
  vinfo.xres_virtual = vinfo.xres;
  vinfo.yres_virtual = vinfo.yres;
  vinfo.xoffset = 0;
  vinfo.bits_per_pixel = 16;
  ioctl(handle, FBIOPUT_VSCREENINFO, &vinfo);

  ioctl(handle, FBIOGET_FSCREENINFO, &finfo);
  printf("%d\n", finfo.smem_len);
  fb = (char*)mmap(0, finfo.smem_len, PROT_READ|PROT_WRITE, MAP_SHARED, handle, 0);
  gpioInitialise();
  gpioSetMode(PIN,PI_OUTPUT);
  while(1) {
    unsigned dummy;
//    ioctl(handle, FBIO_WAITFORVSYNC, &dummy);
    gpioWrite(PIN,PI_HIGH);
    memset(fb, 255, finfo.smem_len);
    sleep(2);
// ioctl(handle, FBIO_WAITFORVSYNC, &dummy);
    gpioWrite(PIN,PI_LOW);
    memset(fb, 0, finfo.smem_len);
    sleep(2);
  }
}