1. CS 241 Section Week #10 (04/01/10) ‏

2. Topics This Section  MP5 Overview  Signals

3. MP5 Overview MP5 looks at a modern programming paradigm that was popularized by Google called “MapReduce”. Specifically, you’ll be gathering statistics on newsgroup files via multiple processes and delivering them back to the MONITOR, a piece of code we provide for you.

4. MP5 Overview You have three key resources to help you understand and program MP5: The README.txt A “high-level” README (PDF, found in the announcement newsgroup) A presentation entirely focused on MP5 details (PPT/Powerpoint, found in the announcement newsgroup)

5. MP5 Overview

6. MP5: Warnings! Closing your shared memory is IMPORTANT! Shared memory is a system-wide resource, not a user- wide resource. Excessive open shared memory resources can cause problems in other programs – don’t do it! When you’re done programming for the night, run:./cleanmemory.sh and make sure your program is running: shmctl(shmid, IPC_RMID, &buf);

7. MP5: Warnings! Ensuring you’re not fork-bombing is CRITICAL! If you’re running fork() inside a loop, or if your fork()’d program ended up inside an infinite loop, MAKE SURE your loops terminates. (Test your program with printf() calls before you add fork() calls.) Before running your program, you can run: bash (unless you’re already using bash) ulimit -u 64 (set your session max processes)

8. MP5: Warnings! If you have excessive processes after your program exits: ps -af | grep NetID: wfagen2 19283 19281 0 21:37 pts/117 00:00:00./mapreduce wfagen2 20324 20322 0 21:41 pts/117 00:00:00./mapreduce wfagen2 22687 22685 0 21:51 pts/117 00:00:00./mapreduce kill -9 the processes you want to kill: kill -9 19283 kill -9 20324 kill -9 22687

9. Signals As you learned in lecture, signals are simply asynchronous notifications delivered to your program. Every signal that is delivered must be acted upon! By default, every signal has a “default action” As a programmer, you can redefine what action some of the signals perform when they’re delivered.

10. Example Signals You already use signals all the time: SIGINT: SIGnal to INTerrupt execution. (Ctrl+C will sent this signal to the currently running process in a shell window.) Default Action: Terminate Process SIGSEGV: Signal of invalid memory reference. (This is the signal the kernel sends to your program when a segmentation fault occurs.) Default Action: Terminate Process

11. Signals There are two primary ways of changing the default behavior of signals: Signal Handler: Run a specific piece of code when a specific signal is delivered. Function Calls: sigaction(), signal() Block Signal: Acknowledges the signal, possibly blocking until it’s received. Function Calls: sigwait(), sigsuspend(), sigprocmask()

12. Signal Sets Many of the signal calls require the use of a “signal set” (sigset_t *set parameter). This “signal set” works like a mathematical set, where you can either start with: a set of all signals (sigfullset()) OR a set of no signals (sigemptyset()) And you can: add signals to the set (sigaddset()) OR remove signals (sigremoveset())

13. Signal Magic You have everything you need to program a signal!

14. Signal Example Goal: Run a function upon receiving SIGINT. struct sigaction action; action.sa_handler = my_function; sigemptyset (&action.sa_mask); action.sa_flags = 0; sigaction(SIGINT, &action, NULL);

15. Signal Example Goal: Block SIGINT from terminating the program (but don’t run any function). sigset_t set, oldset; sigemptyset(&set); sigaddset(&set, SIGINT); sigprocmask(SIG_BLOCK, &set, &oldset);

16. Signal Magic Like many things in C, there are multiple ways to accomplish the same end result. In MP5, you’ll need to both send signals and catch signals. You’ll find the provided MONITOR will help you out with some signal code. Lecture and these slides also provide great resources.