FreeRTOS_select()

[FreeRTOS+TCP API Reference]

FreeRTOS_sockets.h 
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.

Parameters:

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.

Returns:

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_SOCK_DGRAM, 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