Discussion about select() - select

Question

There are some points that i cant understand about select() and i wish your guide. As i read about this function, i've found that
The select() function gives you a way to simultaneously check
multiple sockets to see if they have data waiting to be recv()d, or if
you can send() data to them without blocking, or if some exception has
occurred.
1) The first thing that i understood was that this function can check the sockets in parallel. now imagine the sock1 and sock2 receives packets exactly in the same time (packet1 from sock1 and packet2 from sock2) and there are some process that have to done over each packet. is the processing of packets in the parallel? or the packet1 will process then packet 2 will process? (for example in the following code)
int rv = select(maxSd, &readfds, NULL, NULL, NULL);
if (rv == -1) {
perror("select"); // error occurred in select()
} else if (rv == 0) {
printf("Timeout occurred! No data after 10.5 seconds.\n");
} else {
// one or both of the descriptors have data
if (FD_ISSET(sock1, &readfds)) {
printf("socket %i RECEIVED A PACKET \n", sock1);
recvlen = recvfrom(sock1, buf, BUFSIZE, 0, (struct sockaddr *)&remaddr1, &addrlen1);
if (recvlen > 0) {
buf[recvlen] = 0;
printf("received message: \"%s\" (%d bytes)\n", buf, recvlen);
Packet mp;
mp.de_packet((unsigned char *)buf,recvlen);
}
else {
printf("uh oh - something went wrong!\n");
}
}
if (FD_ISSET(sock2, &readfds)) {
printf("socket %i RECEIVED A PACKET \n", sock2);
recvlen2 = recvfrom(sock2, buf2, BUFSIZE, 0, (struct sockaddr *)&remaddr2, &addrlen2);
if (recvlen2 > 0) {
buf[recvlen2] = 0;
printf("received message2: \"%s\" (%d bytes)\n", buf2, recvlen2);
Packet mp;
mp.de_packet((unsigned char *)buf,recvlen);
}
else
printf("uh oh - something went wrong2!\n");
}
}
2) The other doubt about select that i have is related to blocking and non blocking.
What is exactly the meaning of blocking? Does it mean that the program stops on this line till an event occur?
I think that to avoid blocking it is possible to use timeval tv or fcntl(). Is there any better way too?
Thanks in advance

Answer 1

Upon return of the select, provided it didn't return 0 or -1, your program needs to loop on all elements of readfds and evaluate if ISSET, it is set the corresponding socket must be processed. So, your code is also correct supposing only sock1 and sock2 were set in readfds. The evaluation of the sockets in readfds is usually done sequentially by the same thread. Then the packets in each socket can be processed sequentially or in parallel. It must be clear that two sockets are totally independent of each other, there is no possibility of race condition. All this depends on how you program it. For example for each socket that ISSET returns true you can spawn a thread that processes it or you can pass it to a work queue for a set of worker threads to process each one in parallel. There is no limitation of any kind. You could even check readfs in parallel, for example you could have a thread checking the lower half of the set and another thread checking the upper half. This is just an example. Again, there is no limitation providing you program it well without generating any race conditions in your application.
Regarding the concept of blocking or non-blocking, select will always block until a socket in the sets has an event to process (read, write, exception) or there is a timeout (if you set the timeout value).
You could also be talking about blocking and non-blocking sockets, which is different. Blocking sockets are those that can be blocked in a read or write operation. A blocking socket will block in a read operation until there is a Byte ready to be read and it will block in a write operation if the send buffer is full and it cannot write the bytes in the buffer (this may happen in STREAM sockets). It will block until it can write its Bytes. A non-blocking socket will not block in a read operation if there is nothing to read, function read will return -1 and errno will be set to EAGAIN or EWOULDBLOCK (see: http://man7.org/linux/man-pages/man2/read.2.html).
select is usually used with non-blocking sockets so that a thread just blocks there until there is a socket ready to be processed. This is good because otherwise your application would need to be polling the non-blocking sockets all the time, which is not efficient.
select will handle all you sockets in parallel but just to check if there is any event. select does not process any packet, if you pay attention to your example, after select returns your application will read the data from the sockets and this can be done sequentially or in parallel.
I hope this explanation helps you understand the concept.