Weirdly, this seems to be a classic.

fizzbuzz.h

// run it online:
// https://repl.it/repls/SquigglyPessimisticCron
// gberthiaume 2020
//
#include <stdio.h>
#include <string.h>
#include <stdint.h>

char* fizzbuzz(uint8_t const n)
{
  static char response[sizeof "fizzbuzz"] = {0};
  char const*  fizz = "";
  char const* buzz = "";
  
  
  if (n % 3 == 0) {
    fizz = "fizz";
  }
  if (n % 5 == 0) {
    buzz = "buzz";
  }

  if(*buzz != '\0' || *fizz != '\0' ) {
    snprintf(response, sizeof("fizzbuzz"),"%s%s", fizz, buzz);
  } else {
    snprintf(response, sizeof("255"),"%d", n);
  }
  return response;
}

// Let's try it.
int main(void) {
  for(size_t i = 1; i<=100; i++){
    printf("%d -> %s\n", i, fizzbuzz(i));
  }
  return 0;
}