If len is positive, val may contain embedded nuls and need
not be nul-terminated. It is the caller's responsibility to
ensure that val has at least len addressable bytes.
If len is negative, val must be nul-terminated and len
is considered to request the entire string length. This
makes g_string_prepend_len() equivalent to g_string_prepend().
Prepends len bytes of val to string.
If len is positive, val may contain embedded nuls and need not be nul-terminated. It is the caller's responsibility to ensure that val has at least len addressable bytes.
If len is negative, val must be nul-terminated and len is considered to request the entire string length. This makes g_string_prepend_len() equivalent to g_string_prepend().