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";

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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?

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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]
      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

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[d/l]
      Thanks! That worked. I guess I didn't grep perlfun and perldoc accurately enough. I appreciate the help.
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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.
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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?

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[d/l]

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