How to tell if your command line tool is being piped-to

pipetest.c

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv) {
  if (!isatty(1)) {
    printf("thanks for piping me, dude\n");
  } else {
    printf("oh good, I'm not piped\n");
  }
  
  return 1;
}