Laboratory Exercise 6

Laboratory Exercises

This is a chance to get some experience with signals and timers.

    1. There’s one task for this week: Write a program, timeit, that acts as a simple count-down timer using the interval timer to keep track of the passage of time.

The timer takes a single command-line argument indicating the number of seconds to count down and counts down to zero in half-second increments printing “Tick...” on odd half-seconds and “Tock\n” on even ones. When time’s up it prints “Time’s up!\n” and ends. Since an example is worth a whole mess of words, consider the following example:

    • ./timeit 2 Tick...Tock Tick...Tock Time’s up!

    • 

Note that the last “Tock” will come immediately before the “Time’s up!”

Details

    • If the argument is missing, not a number, or less than zero, timeit should print a usage message and exit with nonzero status.

    • timeit must use setitimer(2) to generate the SIGALRMs for each clock tick. Because the display requires half-second accuracy, the granularity of the timer must be finer than one tick per second. There’s really no reason to tick faster than that, though.

    • Be a good computing citizen: Don’t just spin while waiting for signals. Use pause(2) or sigsuspend(2).

Tricks and Tools

You should probably look into the functions and system calls in Figure 1.

Debugging a program with asynchronous events can be very tricky. Remember also that while debugging, you can also generate SIGALRMs via kill(1) rather than from the timer.

Caution: If you are working remotely you may not see the behavior you expect because remote operation adds another layer of potential buffering. This is particularly true when using ssh, because ssh uses block-ciphers for its secure link. Plain old-fashioned net-lag can have the same effect, though. If you experience bursty behavior, you may want to go to the lab in person and work at the console to be sure you are seeing what you think you are seeing.

Also, remember that stdio is buffered. If you write something to the console with putchar() or printf(), it may not appear immediately. You should either use unbuffered writes, or flush the output stream after writing.


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getitimer(2)

for getting and setting the system interval timer
setitimer(2)





sigaction(2)

the POSIX reliable interface for signal handling



sigprocmask(2)

for manipulating the blocking or unblocking of particular signals



sigemptyset(3)

for  manipulating  signal  sets  used  by  sigaction(2)  and
sigfillset(3)

sigprocmask(2)
sigaddset(3)


sigdelset(3)


sigismember(3)





strtol(3)

for converting the command-line arguments
sscanf(3)





fflush(3)

to flush a stdio stream



pause(2)

Suspend execution until a signal is delivered



sigsuspend(2)

Temporarioy replace the procmask and suspend execution until a


signal is delivered




Figure 1: Useful system calls and library functions

What to turn in

For the Laboratory Exercises: Submit the program described above to the lab06 direc-tory of pn-cs357 via handin.

A makefile (called Makefile) that will build your program when given either the target “timeit” or just “all”.

A README file that contains:

    • Your name(s). In addition to your names, please include your Cal Poly login names with it, in parentheses. E.g. (pnico)

    • Any special instructions for running your program.

    • Any other thing you want me to know while I am grading it.

The README file should be plain text, i.e, not a Word document, and should be named “README”, all capitals with no extension.

Sample Runs

Since the execution of the program evolves over time it’s difficult to give the full effect. I’ve included some examples below, but I will also place an executable version on the CSL in ~pn-cs357/demos so you can run it yourself.

    • timeit 23skedoo 23skedoo: malformed time. usage: timeit <seconds>
    • timeit one hour

usage: timeit <seconds>

    • timeit 3 Tick...Tock Tick...Tock


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Tick...Tock

Time’s up!

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