sebastiannielsen2 has asked for the wisdom of the Perl Monks concerning the following question:

I have a device which sends chunks of data each 30 seconds. No delimiter, no end of line and no way ever to see if the device is finished sending data.

The device sends its data through STDIN. (actually, its xinetd that receives data through a socket and then sends it to STDIN of the program).

What I need, is a function, that Waits indefinitely until SOMETHING is available on STDIN, and once SOMETHING is available on STDIN, it will "read" in 7 seconds, and then continue.

Amother way to do it could be to wait until SOMETHING is abailable to STDIN, and then "read" until the device has been silent for 5 seconds, then I can consider the device finished with its data block.

Any way to accomplish this in perl?

  • Comment on How I unconditionally wait for data in 7 seconds?

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Re: How I unconditionally wait for data in 7 seconds?
by BrowserUk (Patriarch) on Feb 11, 2015 at 06:08 UTC

    Try this: 

    #! perl -slw
use strict;
use Time::HiRes qw[ time ];


binmode STDIN;

my $buf = '';
while( 1 ) {
    sysread( STDIN, $buf, 1 );
    my $end = time() + 7;
    sleep 0.1 while time() < $end;
    sysread( STDIN, $buf, 1024, length( $buf ) );
    printf "Got: '%f'\n", unpack 'd', $buf;
}

    [download]

    The idea is that instead of trying to interrupt or timeout a read after 7 seconds; you read 1 byte; blocking until it arrives; you then wait 7 seconds and then read the rest of whatever has arrived in the interim.

    When combined with this:

    #! perl -sw
use strict;
use Time::HiRes qw[ time ];

binmode STDOUT;

while( 1 ) {
    my $packet = pack 'd', time;
    syswrite( STDOUT, $packet );
    sleep 30;
}

    [download]

    It produces this:

    C:\test>sender | listener
Got: '1423634772.508475'
Got: '1423634802.506555'
Got: '1423634832.504635'
Terminating on signal SIGINT(2)
Terminating on signal SIGINT(2)

    [download]
    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority". I'm with torvalds on this
    In the absence of evidence, opinion is indistinguishable from prejudice. Agile (and TDD) debunked
[reply]
[d/l]
[select]
      What is the Point of executing "sleep 0.1" about 70 times, instead of just executing "sleep 7"?
[reply]
        What is the Point of executing "sleep 0.1" about 70 times, instead of just executing "sleep 7"?

        Resolution. sleep isn't guaranteed to end after exactly the specified time.

        For example, your thread or process could be sitting in the run queue awaiting a cpu to come free when the actual sleep time runs out; so by the time control is returned to your thread, a few 10s, or 100s of microseconds have elapsed beyond the specified time. If you consistently sleep for 7 seconds, and the sleep consistently takes 7.001 seconds before returning control, your time-slots progressively slip a little more each time.

        By looping over a smaller sleep and comparing against real-time; you may be a fraction out each time, but those fractions don't accumulate.

        And you can tailor the sleep time used to balance cpu usage against accuracy.

        Maybe you don't need the accuracy. But whenever you are attempting to time synchronise between systems; you should allow for discrepancies like these.

        With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority". I'm with torvalds on this
        In the absence of evidence, opinion is indistinguishable from prejudice. Agile (and TDD) debunked
[reply]
      The code did not work. Tried it now and it requires a newline for it to accept the input. It wont even detect the first char until I send a newline.
[reply]
        Your sender might not be sending until its got a newline to send.
[reply]

        Only if you didn't use the code I posted.

        So, post your short, runnable example that demonstrates the problem and I'll point out what you did wrong.

        With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority". I'm with torvalds on this
        In the absence of evidence, opinion is indistinguishable from prejudice. Agile (and TDD) debunked
[reply]

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