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When I create a TCP socket in blocking mode and use the send (or sendto) functions, when the will the function call return?
Will it have to wait till the other side of the socket has received the data? In that case, if there is traffic jam on the internet, could it block for a long time?
Both the sender and the receiver (and possibly intermediaries) will buffer the data.
Sending data successfully is no guarantee that the receiving end has received it.
Normally writes to a blocking socket, won’t block as long as there is space in the sending-side buffer.
Once the sender’s buffer is full, then the write WILL block, until there is space for the entire write in it.
If the write is partially successful (the receiver closed the socket, shut it down or an error occurred), then the write might return fewer bytes than you had intended. A subsequent write should give an error or return 0 – such conditions are irreversible on TCP sockets.
Note that if a subsequent send() or write() gives an error, then some previously written data could be lost forever. I don’t think there is a real way of knowing how much data actually arrived (or was acknowledged, anyway).