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FreeRTOS+TCP is still in the lab
FreeRTOS+TCP is already used in commercial products and we encourage you to try it yourself. Be aware however that we are still refining its design, and the source code and documentation do not yet meet Real Time Engineers Ltd's strict quality standards.
Please use the forum for support, feedback and ideas, or contact us directly if you have a specific business interest.


[FreeRTOS+TCP API Reference]

BaseType_t FreeRTOS_select( SocketSet_t xSocketSet, TickType_t xBlockTimeTicks );

Block on a "socket set" until an event of interest occurs on a socket within the set. ipconfigSUPPORT_SELECT_FUNCTION must be set to 1 in FreeRTOSIPConfig.h for FreeRTOS_select() (and its associated functions) to be available.

Socket Sets allow an application RTOS task to block on multiple sockets simultaneously.

To use a socket set:

  1. Create a socket set by calling FreeRTOS_CreateSocketSet(). A socket set is equivalent to the Berkeley sockets fd_set type.

  2. Add one or more sockets to the set using calls to FreeRTOS_FD_SET(). FreeRTOS_FD_SET() is equivalent to the Berkeley sockets FD_SET() macro.

  3. Call FreeRTOS_Select() to test the sockets in the set to see if any of the sockets have an event pending.

  4. If FreeRTOS_select() returns a non-zero value then check all sockets in the set using a call to FreeRTOS_FD_ISSET() to determine which events are pending.
A socket can only be a member of one set at any time.

The FreeRTOS_FD_CLR() API function removes a socket from a set.


xSocketSet   The socket set being tested.

xBlockTimeTicks   The maximum time, in ticks, that the calling RTOS task will remain in the Blocked state (with other tasks executing) to wait for a member of the socket set to get an event.


If xBlockTimeTicks expired before a socket in the socket set had an event, then zero is returned. Otherwise a non-zero value is returned. All sockets which belong to the socket set must be checked by calling FreeRTOS_FD_ISSET()

If a socket in the set receives a signal, causing the task that was blocked on the socket to abort its read operation, then the call to FreeRTOS_select() will return -pdFREERTOS_ERRNO_EINTR.

Example usage:

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

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

static void prvMultipleSocketRxTask( void *pvParameters )
SocketSet_t xFD_Set;
BaseType_t xResult;
Socket_t xSockets[2];
struct freertos_sockaddr xAddress;
uint32_t xClientLength = sizeof( struct freertos_sockaddr ), x;
uint32_t ulReceivedValue = 0;
int32_t lBytes;
const TickType_t xRxBlockTime = 0;

    /* Create the set of sockets that will be passed into FreeRTOS_select(). */
    xFD_Set = FreeRTOS_CreateSocketSet();

    /* Create two sockets to add to the set. */
    for( x = 0; x < 2; x++ )
        /* Create the socket. */
        xSockets[x] = FreeRTOS_socket( FREERTOS_AF_INET,
                                   FREERTOS_IPPROTO_UDP );

        /* Bind the socket to a port number 1000, 1001... */
        xAddress.sin_port = FreeRTOS_htons( 1000 + x );
        FreeRTOS_bind( xSockets[x], &xAddress, sizeof( struct freertos_sockaddr ) );

        /* Once it has been tested that a socket has a eSELECT_READ
        event, blocking on a read call is not necessary any more.  Set the
        Rx block time to 0. */
        FreeRTOS_setsockopt( xSockets[x], 0, FREERTOS_SO_RCVTIMEO,
                             &xRxBlockTime, sizeof( xRxBlockTime ) );

        /* Add the created socket to the set for the READ event only. */
        FreeRTOS_FD_SET( xSockets[x], xFD_Set, eSELECT_READ );

    for( ;; )
        /* Wait for any event within the socket set. */
        xResult = FreeRTOS_select( xFD_Set, portMAX_DELAY );
        if( xResult != 0 )
            /* The return value should never be zero because FreeRTOS_select() was called
            with an indefinite delay (assuming INCLUDE_vTaskSuspend is set to 1).
            Now check each socket which belongs to the set if it had an event */

            for( x = 0; x < 2; x++ )
                if( FreeRTOS_FD_ISSET ( xSockets[x], xFD_Set ) )
                    /* Read from the socket. */
                    lBytes = FreeRTOS_recvfrom( xSockets[x], &( ulReceivedValue ),
                                        sizeof( uint32_t ), 0, &xAddress,
                                        &xClientLength );
                    /* Process the received data here. */
Example use of the FreeRTOS_select() API function

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