Perl Print function does not work properly when Sleep() is used

Question

I have the following code to print a "." each second to simulate a progress bar.

$num = 15;
while($num--){
    sleep(1);
    print ".";
}

The problem I'm having now is the "." character is not printed after each loop. Instead, all 15 "." are printed at once after the loop exits. However if I print ".\n", it works fine. But the "." will be printed on a new line every time which is not what I want.

It seems quite weird and could not figure out why. Could anyone provide some help? Thank you.

Regards, Allen

score 21 · Accepted Answer · answered Apr 17 '11 at 11:51

21

That's probably because I/O is being buffered. Try disabling buffering (autoflush):

$| = 1;

before your loop.

For a more complete explanation, refer to How do I flush/unbuffer an output filehandle? Why must I do this?

answered Apr 17 '11 at 11:51

Mat

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Thank you Mat, this works great! I will have a read of the article you recommended. I'm amazed by the rapidity of the answering of my question. I was expecting to get an answer before I go to bed tonight. – Allen Qin Apr 17 '11 at 12:05

score 4 · Answer 2 · answered Apr 17 '11 at 11:55

4

You need to disable buffering on STDOUT. See $| in perlfaq

answered Apr 17 '11 at 11:55

aschepler

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Thank you @aschepler , after disabling buggering, it works fine. – Allen Qin Apr 17 '11 at 12:07
1

One should always disable buggering at the top of every Perl script. – mob Apr 18 '11 at 03:34

score 1 · Answer 3 · answered Apr 17 '11 at 11:51

1

The standard output stream is usually buffered. Try flushing the output stream after each print statement. See the Perl Cookbook, §7.13 for a detailed explanation and solution.

answered Apr 17 '11 at 11:51

Philip

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Thanks for your explanation Philip. Buffering is indeed the problem. – Allen Qin Apr 17 '11 at 12:08
1

Please don't link to pirated content – ysth Apr 18 '11 at 04:51

Perl Print function does not work properly when Sleep() is used

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