Updated Jul 2025
FreeRTOS_FD_CLR()
[FreeRTOS-Plus-TCP API Reference]
FreeRTOS_sockets.h
1void 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:
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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.
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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.
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eSELECT_EXCEPT
The eSELECT_EXCEPT event will become pending if the socket gets disconnected.
Parameters:
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xSocket
The socket having a bit of interest cleared or being removed from the socket set.
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xSocketSet
The socket set the socket is a member of.
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xBitsToClear
The bits which should be cleared, use 'eSELECT_ALL' to clear all bits and remove the socket from the set.
Returns:
void
1/* FreeRTOS includes. */2#include "FreeRTOS.h"3#include "task.h"4#include "queue.h"56/* FreeRTOS-Plus-TCP includes. */7#include "FreeRTOS_IP.h"8#include "FreeRTOS_Sockets.h"910void vConnectExample( )11{12Socket_t xSocket;13struct freertos_sockaddr xEchoServerAddress;14const TickType_t xZeroTimeOut = 0;15SocketSet_t xSocketSet;1617 /* Create a TCP socket. */18 xSocket = FreeRTOS_socket( FREERTOS_AF_INET, FREERTOS_SOCK_STREAM, FREERTOS_IPPROTO_TCP );1920 /* Create a socket set. */21 xSocketSet = FreeRTOS_CreateSocketSet()( );2223 /* Make the socket a member of the set.24 Only the WRITE event can unblock a call to select() */25 FreeRTOS_FD_SET( xSocket, xSocketSet, eSELECT_WRITE );2627 /* When working with select(), time-outs on API's aren't necessary */28 FreeRTOS_setsockopt( xSocket, 0, FREERTOS_SO_RCVTIMEO, &xZeroTimeOut, sizeof( xZeroTimeOut ) );29 FreeRTOS_setsockopt( xSocket, 0, FREERTOS_SO_SNDTIMEO, &xZeroTimeOut, sizeof( xZeroTimeOut ) );3031 /* Fill in the peer's address */32 xEchoServerAddress.sin_port = FreeRTOS_htons( echoECHO_PORT );33 xEchoServerAddress.sin_addr = FreeRTOS_inet_addr_quick( configECHO_SERVER_ADDR0,34 configECHO_SERVER_ADDR1,35 configECHO_SERVER_ADDR2,36 configECHO_SERVER_ADDR3 );3738 /* Now initiate an active connect procedure to a peer. This call is non-blocking */39 FreeRTOS_connect( xSocket, &xEchoServerAddress, sizeof( xEchoServerAddress ) );4041 /* Now block for at most 30 seconds. A successful connection will unblock42 select() with a eSELECT\_WRITE event */43 if( FreeRTOS_select( xSocketSet, 30000 ) != 0 )44 {45 BaseType_t xMask = FreeRTOS_FD_ISSET ( xSocket, xSocketSet );46 if( xMask != 0 )47 {48 /* Clear the WRITE event bit, it is not interesting any more */49 FreeRTOS_FD_CLR( xSocket, xSocketSet, eSELECT_WRITE );5051 /* Set the READ event bit */52 FreeRTOS_FD_SET( xSocket, xSocketSet, eSELECT_READ );53 }54 }55}
Example use of the FreeRTOS_FD_SET / FD_CLR / FD_ISSET() API functions