[FreeRTOS+TCP API Reference]
BaseType_t FreeRTOS_outstanding( Socket_t xSocket );
Returns the number of bytes in a TCP socket’s Tx stream that are yet to be transmitted.
The socket being queried.
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 a Tx stream then 0 is returned (the Tx 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 Tx stream.