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I understand from this discussion that boost::asio::async_write writes data to the kernel buffers only. It does not mean that the peer has received the data. But if I am sending big packets of size let's say 200000 bytes each, and then I pull the network cable to kill the connection abruptly. Will it still keep reporting on and on saying 200000 bytes written into kernel buffers for each async_write? My testing says that it doesn't. It gives up with a large buffer like 200000 bytes and does not report all bytes sent. But if its a small buffer like 30-40 bytes, it keeps reporting okay?

Question:
The primary point of raising this question is: Is there an underlying buffer size which gets filled up at one point for async_write to say that now its not able to write anymore because the earlier scheduled data has not gone out? If yes then what is the size of this underlying buffer? Can I query it from the boost::asio::ip::tcp::socket?

TheWaterProgrammer
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    It will transfer to the buffer until it fills, then it will cease until space becomes available,mwhuch can only happen if the cable is restored and the receiver starts receiving again. – user207421 Feb 13 '20 at 23:13

1 Answers1

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You can query/change the underlying system socket buffer size with send_buffer_size socket option.

The operating system though can dynamically adjust the socket buffer size and limit its maximum size:

  tcp_wmem (since Linux 2.4)
          This is a vector of 3 integers: [min, default, max].  These
          parameters are used by TCP to regulate send buffer sizes.  TCP
          dynamically adjusts the size of the send buffer from the
          default values listed below, in the range of these values,
          depending on memory available.
dewaffled
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