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FreeRTOS+TCP V2 is now included in the main FreeRTOS kernel download under the MIT license.


[FreeRTOS+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.


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+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,
                                                            configECHO_SERVER_ADDR3 );

    /* 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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