I'm writing simple http server.
I want to shutdown socket after server send all data.
I considered that compare return byte of write() to socket with actuall content length, but I had read that the return value just means that data moved to send-buffer of the socket. (Im not sure and I don't know how can I check it)
If so, can I shutdown the socket just after check the bytes are same? What if the datas sended need to be retransmitted at TCP level after server send FIN flag?
The OS does not discard data you have written when you call shutdown(SHUT_WR). If the other end already shut down its end (you can tell because you received 0 bytes) then you should be able to close the socket, and the OS will keep it open until it has finished sending everything.
The FIN is treated like part of the data. It has to be retransmitted if the other end doesn't receive it, and it doesn't get processed until everything before it has been received. This is called "graceful shutdown" or "graceful close". This is unlike RST, which signals that the connection should be aborted immediately.