If the socket referenced by the xSocket parameter is not a TCP socket then -pdFREERTOS_ERRNO_EINVAL is returned.
If the socket referenced by the xSocket parameter does not yet have an Rx stream then 0 is returned (the Rx stream is not created until it is required).
In all other cases the returned value is the number of bytes that remain in the socket’s Rx stream.
Copyright (C) Amazon Web Services, Inc. or its affiliates. All rights reserved.