/// that at least `n` bytes are available.
  ///
  /// Returns number of bytes read or -1 in case of error. Because of this
  /// constraint and the fact that `ssize_t` is not defined in `stdint.h`/C++
  /// standard, the return type is `int64_t`.
  ///
  /// This is thread safe.
  ///
  /// Note: the `buffer` argument is NOT a null terminated string!
  ///