читаю:
http://www.cplusplus.com/reference/clibrary/cstring/strncmp/

Compares up to num characters of the C string str1 to those of the C string str2.
This function starts comparing the first character of each string. If they are equal to each other, it continues with the following pairs until the characters differ, until a terminating null-character is reached, or until num characters match in both strings, whichever happens first.

То либо первая либо вторая строка заканчивается 0

но смотрю исходники от IAR и там проверка только первой строки, да и порядок проверки не тот. Где истина ?

      int strncmp(const char *s1, const char *s2, size_t n)
      {       /* compare unsigned char s1[max n], s2[max n] */
        for (; 0 < n; ++s1, ++s2, --n)
          if (*s1 != *s2)
            return (  *(unsigned char *)s1
                    < *(unsigned char *)s2 ? -1 : +1);
          else if (*s1 == '\0')
            return (0);
        return (0);
      }