I want write next line with Async WriteFileEx

i want to write file(text) with WriteFileEx. but i don't know... how to offset the next line.

It is pointless adding the length of string. I'm so curious about how to get offset on the next line, not behind the written.

I'll ask for your advice.

ex) abcde, fghij <- i want write next line this string (fghij)

OVERLAPPED  ov { 0 };
LARGE_INTEGER   nLarge { 0 };
WriteFileEx(m_handle, str.c_str(), str.lenght(), &ov, nullptr);

nLarge.QuadPart += ???(How can I get the location of the next line?)
ov.Offset = nLarge.LowPart; 
ov.OffsetHigh = nLarge.HighPart;