[FreeRTOS+TCP API Reference]
BaseType_t FreeRTOS_maywrite( Socket_t xSocket );
Returns the number of bytes that can be added to a TCP socket’s Tx stream before the Tx stream is full.
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 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.