how to use select api in udp for linux - c

Question

int CreateSocket()
{
socklen_t len;
// 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(addr));
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("Oh dear, something went wrong with bind()! %s\n", strerror(errno));
return 1;
}
else
{
printf("Socket an port %d \n",port);
}
while(rc!=-1)
{
fd_set master;
fd_set read_fds;
int retval;
FD_ZERO(&master);
FD_ZERO(&read_fds);
FD_SET(acceptSocket, &master);
FD_SET(acceptSocket, &read_fds);
retval =select(2, &master, NULL, NULL, NULL);
len = sizeof(client);
if(retval == -1)
{
printf("error\n");
}
else if(FD_ISSET (acceptSocket, &master))
{
rc=recvfrom(acceptSocket,buf, 256, 0, (struct sockaddr*) &client, &len);
if(rc==0)
{
printf("Server has no connection..\n");
break;
}
if(rc==-1)
{
printf("Oh dear, something went wrong with read()! %s\n", strerror(errno));
break;
}
XcpIp_RxCallback( (uint16) rc, (uint8*) buf, (uint16) port );
}
else
{
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;
}
The above is a server code for udp layer to recieve data from the client via the ip address and port number. I am using select api to check if there is data via the port then recieving the data else call the timer fucntion. I want to achieve of recieving the data from the client and after recieving I have to call the timer. But the above code is not calling the timer task. What is the mistake in the above code ?? is it efficient to use select api ??

Answer 1

The first parameter to select() is 1 + highest-numbered-FD-in-the-three-sets, so should be acceptSocket + 1.
(I'm assuming that your real problem is more complicated than the above code, because you could just make a blocking recvfrom() call and not bother with select(). You need select() if you want to handle multiple sockets in a single thread, and/or to wake up the blocking call after a timeout - although there are other ways of doing the latter.)

Answer 2

The way you are calling select() with timeout == NULL it will block until there is data to be read for one of the file descripors in master. In your case, with master only containing acceptSocket, the else where you call makeTimer() will never be reached.
Do
struct timeval timeout;
timeout.tv_sec = 0l;
timeout.tv_usec = 0l;
retval = select( acceptSocket+1, master, NULL, NULL, &timeout );
to just check without blocking
Note that select() returns 0 if it returns because of timeout and the number of file descriptors contained in the returned descriptor set otherwise. So you also have to change your condition:
else if(FD_ISSET (acceptSocket, &master))
to
else if(retval > 0 && FD_ISSET (acceptSocket, &master))
because otherwise you would call recvfrom() after a timout as well and in that case it would block