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[FreeRTOS-Plus-TCP API Reference]

void FreeRTOS_FD_CLR( Socket_t xSocket, SocketSet_t xSocketSet, BaseType_t xBitsToClear );

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 FreeRTOS_FD_SET() 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 socket set.

ipconfigSUPPORT_SELECT_FUNCTION must be set to 1 in FreeRTOSIPConfig.h 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:

eSELECT_READ For a socket that is reading data, the eSELECT_READ event will be pending in a socket as long as the socket contains unread data.

For a socket that is listening for new connections, the eSELECT_READ event will be pended each time a new connection is received.

eSELECT_WRITE 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 is established.

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.

eSELECT_EXCEPT The eSELECT_EXCEPT event will become pending if the socket gets disconnected.

eSELECT_INTR If any socket is signaled during the call, using FreeRTOS_SignalSocket() or FreeRTOS_SignalSocketFromISR(), then eSELECT_INTR is returned. Note that it's triggered only when ipconfigSUPPORT_SIGNALS is defined.


xSocket   The socket having a bit of interest cleared or being removed from the socket set.

xSocketSet   The socket set the socket is a member of.

xBitsToClear   The bits which should be cleared, use 'eSELECT_ALL' to clear all bits and remove the socket from the set.



/* FreeRTOS includes. */
#include "FreeRTOS.h"
#include "task.h"
#include "queue.h"

/* FreeRTOS-Plus-TCP includes. */
#include "FreeRTOS_IP.h"
#include "FreeRTOS_Sockets.h"

void vConnectExample( )
Socket_t xSocket;
struct freertos_sockaddr xEchoServerAddress;
const TickType_t xZeroTimeOut = 0;
SocketSet_t xSocketSet;

/* Create a TCP socket. */

/* 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
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