Hello everyone, Sorry about the delay in getting this week's notes to you... Here are the tutorial notes for this week. Remember, feel free to contact me if you have any questions about the material. (You don't have to email me after your tutorial this week, but if there are any problems please let me know about them.) Reminders ========= - Eron is away this week and I've asked his students to attend one of the other tutorials. As a result, you may each have a few more students tonight. Tutorial 11 Notes -- August 3 ============================= There are only two tutorials remaining this term, including this tutorial. This week we're going to cover client-server communication with sockets, and select(). Next week there will be a couple small examples to present, but I'd like to reserve most of the time for exam review. (1) Assignment 4 Assignment 4 has been posted and is due Friday, August 12. You should probably take a look at the assignment before class so you can answer any questions. The most difficult part of the assignment is understanding the message passing protocol. Some sample code is given to establish a simple client and server (which you'll briefly go over tonight), so most of the work is actually deciding what to send between the client and server. If you have any questions about the assignment, please feel free to ask me about it. (2) select() One of the problems we have when we want to read from multiple sources is that the read operation will block if the sending source isn't ready (at least according to the model we've been using). So, if a process has to make a choice as to which source to read, it's possible that we could choose wrong and the process may be blocked indefinitely. One solution to this problem is to use select(). select() monitors a set of file descriptors and returns with information about which file descriptors are ready to be read (there could be more than 1). In this example (which was given as sample code for A4), we demonstrate the use of select() with a collection of pipes. Here's the code, slightly shortened and with error checking removed. If you want to try out the program yourself, grab the "pipeselect.c" file from the A4 specification page. int i, len, r, pid, maxfd, pipefd[5][2]; char buf[256]; fd_set allset, readset; for (i = 0 ; i < 5 ; i++) { /* (A) create pipes and 5 children */ pipe(pipefd[i]); pid = fork(); if (pid == 0) { /* child code */ close(fd[0]); for ( ; ; ) { r = rand() % 5 + 1; sleep(r); /* sleep for a while */ len = sprintf(buf, "Child %d, PID %d, slept for %d sec.", i, getpid(), r); write(pipefd[i][1], buf, len); } } } if (pid != 0) { /* (B) parent code */ FD_ZERO(&allset); maxfd = pipefd[0][0]; for (i = 0 ; i < 5 ; i++) { /* (C) build set of fds */ close(pipefd[i][1]); FD_SET(pipefd[i][0], &allset); if (pipefd[i][0] > maxfd) maxfd = pipefd[i][0]; } for ( ; ; ) { /* (D) wait for fds to become ready */ readset = allset; select(maxfd + 1, &readset, NULL, NULL, NULL); for (i = 0 ; i < 5 ; i++) { if (FD_ISSET(pipefd[i][0], &readset)) { len = read(pipefd[i][0], buf, 256); buf[len] = '\0'; printf("Parent read from pipe %d: \"%s\"\n", num, buf); } } } } Running this program will produce output of the form: Parent read from pipe 0: "Child 0, PID 3593, slept for 3 sec." Parent read from pipe 1: "Child 1, PID 3594, slept for 3 sec." Parent read from pipe 3: "Child 3, PID 3596, slept for 3 sec." Parent read from pipe 3: "Child 3, PID 3596, slept for 1 sec." Parent read from pipe 2: "Child 2, PID 3595, slept for 5 sec." Parent read from pipe 4: "Child 4, PID 3597, slept for 5 sec." Parent read from pipe 1: "Child 1, PID 3594, slept for 2 sec." Parent read from pipe 3: "Child 3, PID 3596, slept for 1 sec." Parent read from pipe 3: "Child 3, PID 3596, slept for 1 sec." ...the program keeps running until stopped... Here are some points you can make: - In part (A), 5 child processes are created. A pipe is created for communication between the parent and each child. - Once a child has been created it enters a loop. The child sleeps for a random length of time, then writes a message to its pipe that identifies the child and the length of time it slept. - Once the parent has created all the pipes and child processes, it executes the code at (B). The code at (C) is used to create the set of file descriptors that will be monitored by select(). A number of operations are defined to manipulate the file descriptor sets (fd_set): FD_ZERO(&set); /* create empty set */ FD_SET(fd, &set); /* add fd to set */ FD_ISSET(fd, &set); /* test if fd is in set */ The code at (C) also keeps track of the value of the maximum file descriptor, also needed by select(). - At (D), the parent enters a loop. The select() call is made using the complete set of file descriptors. select() will block until at least one file descriptor in this set becomes ready (i.e., there is information to read on a pipe). - select() modifies the set of file descriptors passed to it (readset) so that when select() returns, those file descriptors that are ready will be left in the set. - At this point the program simply checks to test and see if a file descriptor is in the set (using FD_ISSET). If it is, the program reads from the appropriate pipe and displays the message that was read. Since more than one file descriptor could be ready, we need to test each file descriptor. It isn't good enough to process the first ready file descriptor and quit. (Note: the return value of select() is the number of file descriptors that are ready so we could write more efficient code, i.e., if it returns N then after we have processed N pipes we could quit.) - The program then beings the next loop by restoring the set of file descriptors to be monitored, calling select(), and processing the ready file descriptors again. - In general, select() can be used with any file descriptors. (For instance, A4 requires sockets, not pipes.) (3) Client-server communication with sockets We've been talking about client-server communication using sockets for the last couple of weeks. The sample code "client.c" and "server.c" demonstrates a very simple client and server that exchange messages with each other. You probably won't have time to present these programs, but instead you can just go over the basics. On the server end, the server has to make 4 function calls to establish a connection with a client: socket() - create the socket bind() - bind address/port information to socket listen() - setup queue for connecting clients accept() - wait for a connection from a client On the client side there are only two steps: socket() - create the socket connect() - connect to the server (using approprirate address info.) (The basic client-server code that establishes a communication channel is pretty much always the same as it is in client.c and server.c, so for A4 it'll basically be cut-and-paste.) The first thing to note on the server side is that when accept() succeeds, it returns a file descriptor that can be used for communication with the connected client. At this point you can use this file descriptor as you would any other file descriptor, i.e., with read(), write(), select(), etc. The other important thing to note on the server side is that accept() blocks. As a result, a server that wants to be able to accept new connections, as well as communicate with existing clients that are already connected, should use select() with the file descriptor of its socket. If select() returns that this file descriptor is ready, the server should then accept() a connection (to actually complete the communication channel with the client). Note: in terms of A4, they have to figure out how to set up select() so that it works correctly with accept(). When a new client connects, the file descriptor of the new client needs to be added to the set of file descriptors that will be monitored on the next select() call. On the client side, provided connect() succeeds, the client will be able to communicate with the server using the file descriptor of its socket. Again, at this point the file descriptor is no different than an ordinary file descriptor or a pipe's file descriptor. Note: you may not have time to cover everything this week, and that's okay. I've had some questions about select() so it's probably important to get through that part, but it's okay if you don't have time for client-server communication.