March 29, 2006

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Sophisticated Program Control: sigsetjmp and siglongjmp

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• consider the following control flow problems:
  • if blocking a ctrl-c during a complex calculation, where to transfer program control when the calculation is complete?
  • how to restart a dialog when the user enters an erroneous response somewhere several nested-menus deep?
• C programs use signals (indirectly or directly) to address these situations
• resembles exception-handling in OO languages and systems
• also similar to the call/cc construct of Scheme
• indirect approach
  • signal handler sets a flag
  • program tests that flag in strategic places
  • complex: program might have to return through several layers of functions
• direct approach: use sigsetjmp and siglongjmp
  • sigsetjump is like a statement label
  • siglongjmp is like a goto
  • difference is that this pair of functions cleans up (unravels) the program stack and signal states in addition to transferring control
  • sigsetjump takes an env parameter of type sigjmp_buf to store the context
  • siglongjump takes env to restore the context and an int val to simulate the return of sigsetjump

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Overview of Using Asynchronous I/O

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• recall system calls block the current process
  • e.g., read and write usually block a process until the i/o completes (synchronous)
• what if we want to continue executing?
  • use POSIX:AIO Asynchronous I/O Extension
• relevant functions
  • aio_read
  • aio_write
  • aio_return
  • aio_error
  • aio_cancel
• main idea is that a signal notifies the process when the i/o is complete
• struct aiocb (asynchronous i/o control block) contains
  • fields which specify how to read/write (these are the same as the parameters to read and write)
    • int aio_fildes
    • volatile void *aio_buf
    • size_t aio_nbytes
  • a field (struct sigevent aio_sigevent) to specify the signal number and handler used during the notification of i/o completion
    • aio_sigevent.sigev_notify
      • if this variable contains SIGEV_NONE, then the OS does not generate a signal when i/o completes (use polling instead; see below)
    • aio_sigevent.sigev_signo
• example: consider a program that reads from a pipe and writes to an ordinary file
  • may want to use asynchronous i/o for the pipe since since it might block for a long time
  • may want to use regular i/o (with blocking) for the write to the file
  • see asynsignalmain.c ([USP] Program 8.13) and asyncmonitorsignal.c ([USP] Program 8.14)
• another example (courtesy [USP] §8.10, pp. 299-300): reading from a disk file and writing to a slow device
  • main calls aio_read
  • read signal handler calls aio_write
  • write signal handler calls aio_read
• can we do asynchronous i/o without signals?
  • sure, use what is called polling
  • break up other work into small pieces
  • call aio_error after each piece to check if the i/o operation completed
• what should we do if the i/o is still not finished, yet we have no more work to do?
  • there are several options (see bottom of [USP] p. 297)
    • involving functions like select, pause, sigsuspend, and sigwait
  • use aio_suspend

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Introduction to POSIX Times

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• Epoch: 00:00 (midnight), January 1, 1970, Coordinated Universal Time
  • also called UTC, Greenwich Mean Time or GMT
• time: returns time (in seconds) since Epoch
• difftime
• other functions for manipulating times
  • localtime
  • asctime
    • e.g., asctime (localtime (time (NULL))
  • ctime
    • e.g., ctime (time (NULL))
      • same as asctime (localtime (time (NULL))
  • be careful, ctime uses static storage to hold the string
• struct tm

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References

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