toggle GPIO17 camera schedule every Nth frame, triggeded by Raspberry hardware camera sync pulses on GPIO18 (https://www.raspberrypi.org/forums/viewtopic.php?f=43&t=267761)

toggle.c

/*
   $ gcc -O6 -Wall -pedantic -Wextra -o toggle toggle.c -lpigpio -lrt -lpthread
   $

   Usage:
   $ sudo killall pigpiod
   $ sudo ./toggle 30 &
   $ raspivid -md 1 -w 1920 -h 1080 -p 22,50,960,540 -o tst.h264 -fps 30 -t 8000 -pts tst.pts
   $ ./ptsanalyze tst.pts 0
   $ # https://github.com/Hermann-SW/userland/blob/master/tools/ptsanalyze
   $
   Captures 8s 1920x1080@30fps video, toggles camera 30th frame (every second),
   then does frame delay and skip analysis.
*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <assert.h>

#include <pigpio.h>

#define gpioCAMsel 17
#define gpioHWsync 18

int main(int argc, char *argv[])
{
   int N,i=0,sel=0;
   assert(argc == 1+1);
   assert(0 < (N = atoi(argv[1])));
       
   assert(gpioInitialise()>=0);

   gpioSetMode(gpioHWsync, PI_OUTPUT);  // needs to be OUTPUT although read 
   gpioSetMode(gpioCAMsel, PI_OUTPUT);
   gpioWrite(gpioCAMsel, sel);

   while (1)
   {   
      while (gpioRead(gpioHWsync) ==0) {}
      while (gpioRead(gpioHWsync) ==1) {}
      if (++i == N)
      {
         i = 0;
         sel = 1-sel;
         gpioWrite(gpioCAMsel, sel);
      }
   }

   // never reached
   gpioTerminate();

   return 0;
}