Prepends len bytes of val to string.
Because len is provided, val may contain
embedded nuls and need not be nul-terminated.
Since this function does not stop at nul bytes,
it is the caller's responsibility to ensure that
val has at least len addressable bytes.
Prepends len bytes of val to string. Because len is provided, val may contain embedded nuls and need not be nul-terminated. Since this function does not stop at nul bytes, it is the caller's responsibility to ensure that val has at least len addressable bytes.