Troubles with do...while loop and sleep

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

I just started working in Perl the other day and it's been going pretty well but today I hit a hitch I can't seem to overcome. I was creating a do...while loop (this also occurs in a simple while loop) with an embedded sleep statement and it was giving me some strange behavior. Finally, I narrowed down to this:

do {
    sleep(5);
    printf "Hello...\n";
} while(1);
[download]

it outputs "Hello..." at 5 second intervals. where as this:

do {
    sleep(5);
    print "Hello...";
} while(1);
[download]

displays nothing and just hangs until I hit C-c. Why will the first one display "Hello..." while the second without "\n" not? It must have to do with sleep because if I remove it both will be displayed ad infinitum.
Thanks much for your help in advance.
-K

Comment on Troubles with do...while loop and sleep Select or Download Code

Replies are listed 'Best First'.
Re: Troubles with do...while loop and sleep by ikegami (Patriarch) on May 26, 2008 at 05:14 UTC
You are suffering from buffering. Here are two fixes: `use IO::Handle qw( ); STDOUT->autoflush(1); do { sleep(5); print "Hello..."; } while(1);` [download] `use IO::Handle qw( ); do { STDOUT->flush(); sleep(5); print "Hello..."; } while(1);` [download]	[reply] [d/l] [select]
Re: Troubles with do...while loop and sleep by runrig (Abbot) on May 26, 2008 at 04:10 UTC
Add a "\n" to the second one and it should display just as often. Without a linefeed, you're just filling up the buffer (which should get flushed and display when it gets full). Or you can set `$\|` (i.e. `$OUTPUT_AUTOFLUSH`...see perlvar) to a true value.	[reply] [d/l] [select]
Re: Troubles with do...while loop and sleep (line- vs. block-buffered) by almut (Canon) on May 26, 2008 at 05:39 UTC
Essentially, there are three buffering modes to be aware of: unbuffered line-buffered block-buffered "line-buffered" is typically used with interactive things such as an interactive shell / terminal that your command line runs in. "block-buffered" is the default, used with almost everything else. And "unbuffered" is only used for special purposes, if you explicitly request that the performance optimisation not be used. Line-buffered output is flushed when encountering a newline in the stream, while block-buffered output is flushed when the block has filled up. Put differently, if you for example redirect the output (an operation which is considered non-interactive) of your snippet which does output the `"\n"` to a file `$ 688477.pl >suffbuf` [download] and then try `"tail -f suffbuf"` (from another terminal) in an attempt to watch the lines being output, it wouldn't work either... (while it would with `$\|=1;`). I.e., before you get any output, you'd have to wait quite a while until the buffer has been filled up (typically several kByte), at which moment the whole buffer would be output at once as one block...	[reply] [d/l] [select]
Re: Troubles with do...while loop and sleep by ABuiteman (Initiate) on May 27, 2008 at 14:17 UTC
It probably has something todo with buffering of your output. And you have to use print "Hello...\n"; Otherwise no newline will not be printed and no buffer flush will occur.	[reply]