Printing a dot in Perl won't come on the screen till I print something else

Question

This sounds crazy but nothing shows up on the screen when I use the line printf(".") until I print something else. When I print out other info, from a simple line return to actual data all the backed up dots print out. If I use printf(".\n") then it works fine.

Am doing this on an Ubuntu system if that makes a difference.

Thanks

Don't use `printf` when there are no format specifiers. This isn't C, and `print` is fine. — Borodin, Jul 23 '18 at 14:11

score 5 · Accepted Answer · edited Jul 23 '18 at 14:10

5

This is a buffering issue. Try setting autoflush: $| = 1;

{
    $| = 1;
    print ".";
}
sleep 1 for 1..5;

STDOUT is line-buffered, so print ".\n" works fine

More information: Suffering from buffering?

Another useful answer: Can you force-flush output in Perl?

edited Jul 23 '18 at 14:10

Borodin

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answered Jul 23 '18 at 04:53

k-mx

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4

Or `STDOUT->autoflush(1);`. – melpomene Jul 23 '18 at 04:58
2

Why `sleep 1 for 1..5` instead of `sleep 5`? – Borodin Jul 23 '18 at 14:05
perldoc -f sleep ...On some older systems, it may sleep up to a full second less than what you requested, depending on how it counts seconds. – k-mx Jul 23 '18 at 15:15
So you prefer to sleep for 0 to 5 seconds over sleeping for 4 to 5 seconds? – Mark Setchell Jul 23 '18 at 16:21
You can add validation and sleep more or skip extra seconds ;) P.S. Okay, next time I will use sleep(5) – k-mx Jul 23 '18 at 16:54

score 3 · Answer 2 · answered Jul 23 '18 at 15:21

Like other have said, it is a buffering issue. In recent enough versions of Perl you can do:

 use IO::Handle
 STDOUT->autoflush()

I think this is cleaner than using the $| variable, even if it will work in the same way.

The use IO::Handle is needed to attach the autoflush method to any future handle. And autoflush has its quirks in the sense that without passing a value it assumes 1 as written in the perlvar man page:

The methods each take an optional EXPR, which, if supplied, specifies the new value for the "IO::Handle" attribute in question. If not supplied, most methods do nothing to the current value--except for "autoflush()", which will assume a 1 for you, just to be different.

Printing a dot in Perl won't come on the screen till I print something else

2 Answers2