tcpip_send_msg_wait_sem() function
Sends a message to TCPIP thread to call a function. Caller thread blocks on on a provided semaphore, which ist NOT automatically signalled by TCPIP thread, this has to be done by the user. It is recommended to use LWIP_TCPIP_CORE_LOCKING since this is the way with least runtime overhead.
Arguments
fn
function to be called from TCPIP thread
apimsg
argument to API function
Return value
ERR_OK if the function was called, another err_t if not