Sockets in a socket set have a number of associated event bits of interest.
An event bit of interest becoming set will cause the socket to unblock a
call to FreeRTOS_select(). Event bits of interest are set using the
API function, and cleared using the FreeRTOS_FD_CLR() API function. If
all the event bits are clear then the socket will be removed from the
ipconfigSUPPORT_SELECT_FUNCTION must be set to 1 in
for FreeRTOS_FD_CLR() to be available.
Each socket member has its own set of event bits, which can be a bitwise
OR combination of the following values:
For a socket that is reading data, the eSELECT_READ event
will be pending in a socket as long as the socket contains
For a socket that is listening for new connections, the
eSELECT_READ event will be pended each time a new connection
The eSELECT_WRITE event will remain pending as long as the
socket has space for writing.
If a TCP socket is actively connecting to a pear the
eSELECT_WRITE event will be triggered as soon as the connection
One the eSELECT_WRITE event has been pended it should either
be disabled, or the caller should write enough data to the
socket so as to completely fill up the transmit buffer -
otherwise the pending eSELECT_WRITE event will not be cleared.
The eSELECT_EXCEPT event will become pending if the socket
The socket having a bit of interest cleared or being
removed from the socket set.
The socket set the socket is a member of.
The bits which should be cleared, use 'eSELECT_ALL' to clear all
bits and remove the socket from the set.
/* FreeRTOS includes. */
/* FreeRTOS+TCP includes. */
void vConnectExample( )
struct freertos_sockaddr xEchoServerAddress;
const TickType_t xZeroTimeOut = 0;
/* Create a TCP socket. */
xSocket = FreeRTOS_socket( FREERTOS_AF_INET, FREERTOS_SOCK_STREAM, FREERTOS_IPPROTO_TCP );
/* Create a socket set. */
xSocketSet = FreeRTOS_CreateSocketSet()( );
/* Make the socket a member of the set.
Only the WRITE event can unblock a call to select() */
FreeRTOS_FD_SET( xSocket, xSocketSet, eSELECT_WRITE );
/* When working with select(), time-outs on API's aren't necessary */
FreeRTOS_setsockopt( xSocket, 0, FREERTOS_SO_RCVTIMEO, &xZeroTimeOut, sizeof( xZeroTimeOut ) );
FreeRTOS_setsockopt( xSocket, 0, FREERTOS_SO_SNDTIMEO, &xZeroTimeOut, sizeof( xZeroTimeOut ) );
/* Fill in the peer's address */
xEchoServerAddress.sin_port = FreeRTOS_htons( echoECHO_PORT );
xEchoServerAddress.sin_addr = FreeRTOS_inet_addr_quick( configECHO_SERVER_ADDR0,
/* Now initiate an active connect procedure to a peer. This call is non-blocking */
FreeRTOS_connect( xSocket, &xEchoServerAddress, sizeof( xEchoServerAddress ) );
/* Now block for at most 30 seconds. A successful connection will unblock
select() with a eSELECT_WRITE event */
if( FreeRTOS_select( xSocketSet, 30000 ) != 0 )
BaseType_t xMask = FreeRTOS_FD_ISSET ( xSocket, xSocketSet );
if( xMask != 0 )
/* Clear the WRITE event bit, it is not interesting any more */
FreeRTOS_FD_CLR( xSocket, xSocketSet, eSELECT_WRITE );
/* Set the READ event bit */
FreeRTOS_FD_SET( xSocket, xSocketSet, eSELECT_READ );
Example use of the FreeRTOS_FD_SET / FD_CLR / FD_ISSET() API functions