`echo "potato" | ./not` after you compile

not.c

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

int main(void) {
  int inp;
  
  fprintf(stdout, "not ");

  while (1) {
    inp = getchar();
    if (inp == EOF) break;
    fprintf(stdout, "%c", inp);
  }

  return EXIT_SUCCESS;
}