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().