April 10, 2006

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Discussed Solutions to Exam 3

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Basic Shell Setup

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(courtesy [ATT])

see [USP] §11.1-§11.3 for more details

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Signal Handling in the Foreground

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• a <ctrl-c> should not cause the shell to exit
  • shell should ignore SIGINT and SIGQUIT and block them
• since child should be able to be terminated, child must set the handlers for SIGINT and SIGQUIT back to the default before execing
• ush3.c
  • signalsetup.c
• improvement in ush4.c
  • shell should not just ignore SIGINT
    • it should deal with it
  • furthermore, shell should only block SIGINT and SIGQUIT when absolutely necessary
  • if shell (parent) receives a SIGINT, parent clears the input line and returns to prompt through siglongjump
  • child (command) should not do this
  • child should take the default action, which is to exit
    • parent blocks the signal before the fork
    • child restores the default action prior to unblocking
      • done by default with execvp
    • child cannot wait for this feature in execvp
      • a signal may come in between unblocking the signal and the call to execvp
  • William Kimball asks `why not just remove the if statement entirely from the child?'
  • why is wait now at the top of the loop?

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References

[USP] K. A. Robbins and S. Robbins. UNIX Systems Programming: Concurrency, Communication, and Threads. Prentice Hall, 2003