A simple function to case insensitively compare 2 strings, up to the n chars. This function follows the C standard of returning 0 if the strings are equal, a negative number if the first character that does not match has a lower value in str1 than in str2, or a positive number if the first character that does not match has a greater value in str1 than in str2.

Note that the type of n is ptrdiff_t in this function. This is simply because in my current project, an ll(1) parser for SQL statements, my n value is of this type. Typically this would be a type of size_t instead. ptrdiff_t cannot blindly be converted to a size_t as the latter is unsigned while the former is not. However, I am not using in a way where I’ll have a negative ptrdiff_t so I could safely convert this to a size_t and not cause issues.

int strncmpci(char const *a, char const *b, ptrdiff_t n) {
  while (n && *a &&
         (tolower(*(unsigned char *)a) == tolower(*(unsigned char *)b))) {
    a++;
    b++;
    n--;
  }
  if (n == 0) {
    return 0;
  } else {
    return (tolower(*(unsigned char *)a) - tolower(*(unsigned char *)b));
  }
}