recvfrom api in socket program for linux

Question

int acceptSocket;
char buf[100];
long sentbytes;
socklen_t len;
int port = 18227;

int CreateSocket()
{
    long rc;
    struct sockaddr_in addr, client;

       // Socket creation for UDP
       acceptSocket=socket(AF_INET,SOCK_DGRAM,0);

       if(acceptSocket==-1)
       {
         printf("Failure: socket creation is failed, failure code\n");
         return 1;
       }
       else
       {
         printf("Socket started!\n");
       }

     memset(&addr, 0, sizeof(struct sockaddr_in));
     addr.sin_family=AF_INET;
     addr.sin_port=htons(port);
     addr.sin_addr.s_addr=htonl(INADDR_ANY);
     rc=bind(acceptSocket,(struct sockaddr*)&addr,sizeof(addr));

     if(rc== -1)
     {
       printf("Failure: listen, failure code:\n");
       return 1;
     }
     else
     {
       printf("Socket an port %d \n",port);
     }

     while(rc!=-1)
         {
         len =sizeof(client);
         rc=recvfrom(acceptSocket,buf, sizeof(buf), 0, (struct sockaddr*) &client, &len);
         if(rc==0)
         {
           printf("Server has no connection..\n");
           break;
         }
         if(rc==-1)
         {
             printf("something went wrong with data %s", strerror(errno));
           break;
         }

         XcpIp_RxCallback( (uint16) rc, (uint8*) buf, (uint16) port );

            makeTimer("First Timer", &firstTimerID, 2, 2);   //2ms
                makeTimer("Second Timer", &secondTimerID, 10, 10);    //10ms
                makeTimer("Third Timer", &thirdTimerID, 100, 100);  //100ms
          }

       close(acceptSocket);
       return 0;
     }

int main()
{
     Xcp_Initialize();
     CreateSocket();
     return 0;
}

//API for sending the data to the client.
void XcpApp_IpTransmit( uint16 XcpPort,  Xcp_StatePtr8 pBytes, uint16 numBytes )
{
    struct sockaddr_in client;

        if ((long)XcpPort==port){
                sentbytes = sendto(acceptSocket,(char*)pBytes,(long)numBytes,0, (struct sockaddr*)&client, sizeof(client));
        }
        XcpIp_TxCallback(port,(uint16)sentbytes);
}

I created a server side program for receiving data from the client and sending the response back to the client. I don't know where is the mistake in the above program, I am able to receive data from the client and the rc value is zer0 in that case. Then I am checking the value of rc and it is coming out of the program. later There is no response back to the client. Client is a tool for sending the data to the specified port and ip address.

what could be the problem and why rc value is zero after the recvfrom api ?? NOTE: Client will be keep on sending the data. Could anyone help me in this ?

Answer 1

According to recvfrom man page

Upon successful completion, recvfrom() shall return the length of the message in bytes. If no messages are available to be received and the peer has performed an orderly shutdown, recvfrom() shall return 0. Otherwise, the function shall return -1 and set errno to indicate the error.

So i think in your case You get a return value of 0 for recvfrom() when the connection was closed by the other host (wether this disconnection was graceful or not doesn't matter)

Also i have updated your code please try it on your end

#define BUFLEN 100  //Max length of buffer
#define PORT 18227   //The port on which to listen for incoming data

void die(char *s)
{
    perror(s);
exit(1);
}

int acceptSocket;
char buf[BUFLEN];
long sentbytes;

int CreateSocket()
{
    struct sockaddr_in si_me, si_other, si_other2;

    int s, i, slen = sizeof(si_other) , recv_len;

    //create a UDP socket
    if ((acceptSocket=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1)
    {
        die("socket");
    }

    // zero out the structure
    memset((char *) &si_me, 0, sizeof(si_me));

    si_me.sin_family = AF_INET;
    si_me.sin_port = htons(PORT);
    si_me.sin_addr.s_addr = htonl(INADDR_ANY);

    //bind socket to port
    if( bind(acceptSocket , (struct sockaddr*)&si_me, sizeof(si_me) ) == -1)
    {
        die("bind");
    }

    //keep listening for data
    while(1)
    {
        printf("Waiting for data...");
        fflush(stdout);

        //try to receive some data, this is a blocking call
        if ((recv_len = recvfrom(acceptSocket, buf, BUFLEN, 0, (struct sockaddr *) &si_other, &slen)) == -1)   // read datagram from server socket
        {
            die("recvfrom()");
        }

        //print details of the client/peer and the data received
        printf("Received packet from %s:%d\n", inet_ntoa(si_other.sin_addr), ntohs(si_other.sin_port));         printf("Data: %s\n" , buf);

        XcpIp_RxCallback( (uint16) rc, (uint8*) buf, (uint16) port );

        makeTimer("First Timer", &firstTimerID, 2, 2);   //2ms
        makeTimer("Second Timer", &secondTimerID, 10, 10);    //10ms
        makeTimer("Third Timer", &thirdTimerID, 100, 100);  //100ms
    }

    close(s);
 }