If the socket referenced by the xSocket parameter is not a TCP socket then -pdFREERTOS_ERRNO_EINVAL is returned.
If the socket is not in a state that allows data to be sent (for example it is in the Listening state or is in the process of being shut down) then -1 is returned.
If the socket is in the Established the returned value is the number of bytes that can be added to the socket’s Tx stream.
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