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Processes, Signals and Shell Lab

Processes, Signals and Shell Lab. Siddharth Dhulipalla. Recitation 9: Section L (1:30pm - 2:20pm) Monday, October 22, 2012. Outline. Midterm Overview Processes Signals Shell Lab Overview Cheating Policy Reminder. Midterm Overview. Average: 54/71 Pick up from ECE Course Hub in HH 1112

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Processes, Signals and Shell Lab

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  1. Processes, Signals and Shell Lab • Siddharth Dhulipalla Recitation 9: Section L (1:30pm - 2:20pm) Monday, October 22, 2012

  2. Outline • Midterm Overview • Processes • Signals • Shell Lab Overview • Cheating Policy Reminder

  3. Midterm Overview • Average: 54/71 • Pick up from ECE Course Hub in HH 1112 • Alternate solution to assembly question found • Regrade requests accepted until Wednesday • Question?

  4. Processes

  5. Processes • What is a process? • An instance of a program in execution • A Great Idea in Computer Science • Ubiquitous on multitasking systems • Fundamental abstraction provided by the OS • Single thread of execution (control flow) • Full, private memory space and registers • Various other state (file descriptors, etc.)

  6. Processes • Four basic process control functions • fork() • exec() • exit() • wait() • Standard on all Unix systems

  7. fork() • Creates a process (more like mitosis than birth) • Parent and child are exactly alike • Except for return value (%eax) • Equal, but private: • Threads of execution • Registers • Memory • File descriptors (note: files, however, are shared)

  8. exec() • Replaces process • No processes created • New process (mostly) unaware of old state • How programs are run • Replace memory image with new program • Set up stack with arguments • Start execution at the entry point (main) • Actually a family of functions • man 3 exec

  9. exec() Example Code int rc = fork(); if (rc == 0) { printf(“child\n”); execl(“/bin/echo”, “/bin/echo”, “sup!”, NULL); exit(EXIT_FAILURE); }printf(“parent\n”); Output child sup! parent Any other possibilities?

  10. exit() • Terminates a process • OS frees resources used by the process • Tiny leftover data • Zombie state • Exit status for parent • Must be freed (reaped)

  11. wait() • Blocks until child process changes state • Reaps child if it terminated • Frees all remaining resources and gets exit status • Can be used for synchronization • Lots of details • man 2 wait

  12. wait() Example Code int rc = fork(); if (rc == 0) { printf(“child\n”); execl(“/bin/echo”, “/bin/echo”, “sup!”, NULL); exit(EXIT_FAILURE); }int status; waitpid(rc, &status, 0); printf(“child status %d\n”, WEXITSTATUS(status)); Output child sup! child status 0 Any other possibilities? Take a look at: http://csapp.cs.cmu.edu/public/waside/waside-graphs.pdf

  13. States • Running • Executing instructions on the CPU • Number bounded by the number of CPU cores • Runnable • Waiting to be running • Blocked • Waiting for an event • Not runnable • Zombie • Terminated but not yet reaped

  14. Signals

  15. Signals • Primitive form of interprocess communication • Notify process of an event • Asynchronous with normal execution • Several types • man 7 signal • Sent in various ways • ^C, ^Z, ^\ • kill command • kill system call

  16. Signals • What to do when receiving a signal • Ignore • Catch and run signal handler • Terminate • man sigaction • Blocking • man sigprocmask • Can’t modify behavior of SIGKILL and SIGSTOP • Signals don’t queue

  17. Signals • Signal Handlers • Can be installed to run when a signal is received • Type is “void (*sa_handler)(int)” • Separate control flow in the same process • Resumes control flow on return • Signal handlers can be called anytime

  18. Signals • Interesting signals • SIGKILL – force kill a process • SIGINT – kill a process • SIGSTOP and SIGCONT – suspend and resume a process • SIGCHLD – child changed state • SIGSEGV – everyone knows this

  19. Shell Lab

  20. Shell Lab • Write a not-so-basic shell • Fork and execute a new program • Wait for foreground jobs • Support background jobs • React to changes in child state • Input and output redirection • ls > ls_out and wc < filename • Many different designs • Some much better than others

  21. Shell Lab Tips • Read the code we’ve given you. • There is a lot of stuff you don’t need to write for yourself. • It’s a good example of the kind of code we expect from you • If you find yourself using sleep() as a way of avoiding race conditions, you are doing it VERY wrong. We will dock performance points for this. • You should only use it for performance to avoid your code having to execute useless instructions. Your code should still work if we remove calls to sleep. • Read the spec carefully and start early!

  22. Shell Lab Hazards • Race Conditions • Hard to debug so start early • Reaping zombies • Again, race conditions • Fiddle with signals • Waiting for foreground job • One of the only places where sleep is acceptable (though you don’t NEED it)

  23. Cheating Policy Reminder • Don’t do it. • Two students caught by Moss have received R’s and could potentially be kicked out of their program • Two more cases are currently looked into • Never worth it.

  24. Questions?

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