Does seek() not work on STDOUT

Short answer:No. Longer answer: No. How could it?

There is nothing special about STDOUT, it's just another file handle. seek() does work on STDOUT if STDOUT is seekable, e.g. a plain file. But (pseudo) TTYs usually aren't seekable, at least not for writing, so seek() doesn't work when STDOUT is not redirected.

#!/usr/bin/perl -w
use strict;

seek(STDOUT,14,0);
print "Now I'm there\n";
seek(STDOUT,0,0);
print "Now I'm here\n";
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Run without redirection (perl foo.pl), and you get:

Now I'm there
Now I'm here
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The seek()s simply fail, and because the code does not check for errors, seek() seems to be a dummy for STDOUT.

Run again, redirecting to a file (perl foo.pl > foo.txt; cat foo.txt), and the file contains:

Now I'm here
Now I'm there
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Now let's play safe and add some error checking:

#!/usr/bin/perl -w
use strict;

seek(STDOUT,14,0) or warn $!;
print "Now I'm there\n";
seek(STDOUT,0,0) or warn $!;
print "Now I'm here\n";
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Run again, without redirection:

Illegal seek at foo.pl line 4.
Now I'm there
Illegal seek at foo.pl line 6.
Now I'm here
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So, seek() is not a dummy, it simply fails, because the TTY does not allow seek() for write access.

Redirecting to a file, as above, works without any warnings:

Now I'm here
Now I'm there
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Alexander

--
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

There is nothing special about STDOUT, it's just another file handle.

Yes there is. By default, its pre-opened--and in most sensible uses--is connected to either the console or a pipe. Both non-seekable devices.

Yes, the OP could re-open STDOUT to a seekable device, (he could also overload '+' to do subtraction), but if he was doing that, he wouldn't have a problem.

use strict;
use warnings;

$|=1;
open(STDOUT, '+<') or die "open: $!";

print STDOUT "11111111111111111111111111111111\n" or die "print 1: $!"
+;

for (0..31){
  seek(STDOUT, $_, 0) or die "seek: $!";
  print STDOUT '0' or die "print 0: $!";
  sleep(1);
}
close(STDOUT);
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open(STDOUT, '+<', "&STDOUT") or die "open: $!";
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After reading the posting again and again, I think that you really search for a progress bar or something like that. You want to move the cursor and change a character on the (virtual) terminal. And seek just don't let you do that. Am I right?

If yes, you are searching for information about terminal control codes. One of the most primitive ones is the backspace, and it works on nearly every terminal. Stupid example:

#!/usr/bin/perl -w
use strict;
$|=1;
my @spinner=qw( / - \ | );
print "Wait ... ";
for my $i (0..19) {
        print "$spinner[$i % 4]\b";
        sleep 1;
}
print "- done\n";
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There are much more terminal control codes. Unless you work with exotic hardware or exotic operating systems, the generic VT100 or xterm codes will work. There are libraries that take care of the actual codes, like termcap and the newer http://en.wikipedia.org/wiki/Terminfo. Libraries like curses and the newer ncurses add functions for text mode user interfaces. You can get Perl interfaces for all of those libraries on CPAN.

Alexander

Thanks,

The program I'm writing is a simple terminal game called snake that was a popular arcade game in the '70s. It has 50x50 grids where the snake-like creature moves around and the player steers it not to hit the blockage or itself. It is intended to run on xterm.

I finished the program, but currently, it refreshes the display every second(and getting faster over the time) with print `clear` and redraws the entire 50x50 grids with the new pattern. However, since all the grids are refreshed every time, the flickering is annoying especially when the snake's move gets faster. Then I realized that only 2 out of 50x50 grids are required to be updated every time(The head and tailend of the snake need to be updated to move forward) and I need to move the cursor to those 2 locations to update it every time. I thought seek() could do that on STDOUT where I got it wrong.

So, basically, like you said, I need to move the cursor to the random points of 50x50 grid on xterm, update them with keeping all the other grids unchanged.

Is ncurses able to do it ?

Thanks for your help..

On *nix, you can probably use ansi escape sequences to move the cursor around (untested):

sub moveTo{  
    my( $x, $y ) = @_;
    return chr(27) . "[$y;$xH";
}

print moveTo( 25, 25), 'some text';

print moveTo( 10, 10 ), ' ';
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---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

RIP an inspiration; A true Folk's Guy