strpos() not working when strstr() works

Dear Board

I’m using STM32F103 controller, when I use strstr function for finding a word in a string the compiler throws a warning. No issue for me but the code works fine

But when I use strops(), the compiler throws error. It says this is used for C199 etc.. where can I see the string functions supported for MDK-IDE??

Below is the code im trying to execute for which IDE throws error

This code is extracted from keil reference book and applied for MDK according to my usage

#include <string.h>
#include <stdio.h> /* for printf */

void tst_strcat (void) {

  unsigned char buf [21];
  unsigned char s [] = "Test String";

  strcpy (buf, s);
  strcat (buf, " #2");

  printf ("new string is %s\n", buf);
}