Screen Output Buffering

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

This is a little question, but it bothers me that I can't figure out what's going on. Here is the code in question:

print "\nStage 1 of 3: Processing employee data... ";
retrieve_employee_data();
print "finished.\nStage 2 of 3: Processing punch data... ";
retrieve_punch_data();
print "finished.\nStage 3 of 3: Creating output... ";
output_db();
print "finished.\n\nProgram execution complete.\n";
[download]

When I run this, the program prints the first line: "Stage 1 of 3...", then prints "finished" and newlines, then it pauses flashing (presumably while it runs retrieve_punch_data()) and doesn't print "Stage 2 of 3..." until after it finishes. What's going on?

Comment on Screen Output Buffering Select or Download Code

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Re: Screen Output Buffering by mortis (Pilgrim) on Aug 19, 2004 at 20:59 UTC
I'm surprised this hasn't been answered already. STDOUT in perl is buffered by default. To make sure STDOUT is autoflushed, set $\| to a true value early in your program. STDOUT is not buffered by default. If another file handle has been selected, you may need to select STDOUT before setting $\|. See perldoc perlfun and perldoc perlvar fore more information on flushing and $\|. hth, Kyle	[reply]
Re^2: Screen Output Buffering by Limbic~Region (Chancellor) on Aug 19, 2004 at 21:39 UTC
mortis, To make sure STDOUT is autoflushed, set $\| to a true value early in your program I am sorry about nitpicking but setting $\| to a true value will turn on auto-flushing for the currently selected file handle. I know it is being pedantic, but just because STDOUT is the default selected file handle doesn't mean it has to be: `#!/usr/bin/perl use strict; use warnings; open (FILE, '>', 'foo.txt') or die "Unable to open foo.txt for writing + : $!"; select FILE; $\| = 1; # FILE is now unbuffered print "hello, world\n"; print STDOUT "goodbye cruel world\n"; # still buffered` [download] Cheers - L~R	[reply] [d/l]
Re^2: Screen Output Buffering by mvaline (Friar) on Aug 19, 2004 at 21:12 UTC
Thanks! That worked. I guess I didn't grep perlfun and perldoc accurately enough. I appreciate the help.	[reply]
Re^3: Screen Output Buffering by ikegami (Patriarch) on Aug 19, 2004 at 21:54 UTC
$\| is in perlvar.	[reply]
Re: Screen Output Buffering by ikegami (Patriarch) on Aug 19, 2004 at 21:00 UTC
you're seeing the effects of buffering. Try "$\|=1;" to disable the buffering, or "$temp=$\|; $\|=1; $\|=$temp;" to flush it. If STDOUT is sent to a terminal/console, printing a "\n" will flush the buffer too.	[reply]
Re: Screen Output Buffering by ccn (Vicar) on Aug 19, 2004 at 21:01 UTC
Did you turn on STDOUT autoflushing with `$\|=1` as described in perlvar?	[reply] [d/l]


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