Code review comment for ~dirk.zimoch/epics-base:iocLogClientFixes

The send() trick with 0 bytes does not work on all systems. It seems to depend on the Linux kernel version as well.

I am testing now a different solution. On Linux the call ioctl(sock, TIOCOUTQ, &unsent) gives the number of unsent bytes in the TCP output queue. This allows to keep the unsent bytes in the buffer in case it turns out later that the connection is broken.

On macOS there seems to be a similar functionality with getsockopt(sock, SOL_SOCKET, SO_NWRITE, &unsent), but I cannot test because I have no mac.

Other Unix flavors may or may not have such a feature.

For vxWorks I could not find a way to get the number of unsent bytes, neither for version 5.5 nor for 6.7.

I don't know about RTEMS.

On Windows 10 the number of unsent bytes should be available with WSAIoctl(sock, SIO_TCP_INFO, ..._). I spent most of last week to find out why it is not working before finding out that the Win 10 version I got was too old. I am upgrading now. Older Windows versions seem to have no way to find out the number of unsent bytes.

So I can mitigate the problem of message loss for some clients but not for all. Depending on the OS.