Position in seek() confusion

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

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Re: Position in seek() confusion by BrowserUk (Patriarch) on Jul 01, 2015 at 02:57 UTC
That was a crude demonstration knocked up in haste. If you run it, you'll find that in addition what you've noticed, it also prints the lines double spaced. This is due to the use of -l on the shebang line. Newlines are added when the lines are printed to the file and then again when they are read back and printed to STDOUT. I took the liberty of discarding the spaces when posting. If you want a better version, try: #! perl -sw use strict; use threads; use threads::shared; my $sem :shared; open my $fh, '>:raw', 'junk.dat' or die $!; $_->join for map { async { for my $i ( 0 .. 999 ) { lock $sem; seek $fh, $i320 + ($_-1)80, 0; print $fh $_ x 79, "\n"; } }; } 1 .. 4; close $fh; open $fh, '<', 'junk.dat' or die $!; print while <$fh>; close $fh; __END__ C:\test>t-write.pl 1111111111111111111111111111111111111111111111111111111111111111111111 +11111111 2222222222222222222222222222222222222222222222222222222222222222222222 +22222222 3333333333333333333333333333333333333333333333333333333333333333333333 +33333333 4444444444444444444444444444444444444444444444444444444444444444444444 +44444444 1111111111111111111111111111111111111111111111111111111111111111111111 +11111111 2222222222222222222222222222222222222222222222222222222222222222222222 +22222222 3333333333333333333333333333333333333333333333333333333333333333333333 +33333333 4444444444444444444444444444444444444444444444444444444444444444444444 +44444444 1111111111111111111111111111111111111111111111111111111111111111111111 +11111111 2222222222222222222222222222222222222222222222222222222222222222222222 +22222222 3333333333333333333333333333333333333333333333333333333333333333333333 +33333333 4444444444444444444444444444444444444444444444444444444444444444444444 +44444444 1111111111111111111111111111111111111111111111111111111111111111111111 +11111111 2222222222222222222222222222222222222222222222222222222222222222222222 +22222222 3333333333333333333333333333333333333333333333333333333333333333333333 +33333333 4444444444444444444444444444444444444444444444444444444444444444444444 +44444444 1111111111111111111111111111111111111111111111111111111111111111111111 +11111111 2222222222222222222222222222222222222222222222222222222222222222222222 +22222222 3333333333333333333333333333333333333333333333333333333333333333333333 +33333333 4444444444444444444444444444444444444444444444444444444444444444444444 +44444444 1111111111111111111111111111111111111111111111111111111111111111111111 +11111111 2222222222222222222222222222222222222222222222222222222222222222222222 +22222222 3333333333333333333333333333333333333333333333333333333333333333333333 +33333333 4444444444444444444444444444444444444444444444444444444444444444444444 +44444444 1111111111111111111111111111111111111111111111111111111111111111111111 +11111111 2222222222222222222222222222222222222222222222222222222222222222222222 +22222222 3333333333333333333333333333333333333333333333333333333333333333333333 +33333333 4444444444444444444444444444444444444444444444444444444444444444444444 +44444444 1111111111111111111111111111111111111111111111111111111111111111111111 +11111111 2222222222222222222222222222222222222222222222222222222222222222222222 +22222222 3333333333333333333333333333333333333333333333333333333333333333333333 +33333333 4444444444444444444444444444444444444444444444444444444444444444444444 +44444444 1111111111111111111111111111111111111111111111111111111111111111111111 +11111111 2222222222222222222222222222222222222222222222222222222222222222222222 +22222222 3333333333333333333333333333333333333333333333333333333333333333333333 +33333333 4444444444444444444444444444444444444444444444444444444444444444444444 +44444444 1111111111111111111111111111111111111111111111111111111111111111111111 +11111111 2222222222222222222222222222222222222222222222222222222222222222222222 +22222222 3333333333333333333333333333333333333333333333333333333333333333333333 +33333333 4444444444444444444444444444444444444444444444444444444444444444444444 +44444444 ... [download] Note: I truncated some of the output when posting. With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' 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. I'm with torvalds on this Agile (and TDD) debunked I told'em LLVM was the way to go. But did they listen!	[reply] [d/l]
Re: Position in seek() confusion by Anonymous Monk on Jul 01, 2015 at 02:02 UTC
Ok, Deparse shows `BEGIN { $^W = 1; } BEGIN { $/ = "\n"; $\ = "\n"; } use threads; use strict; die $! unless open my $fh, '>', 'junk.dat'; $_->join foreach (map {async sub { seek $fh, $_ * 80, 0; print $fh $_ x 78; } ;} 1..4); close $fh; die $! unless open $fh, '<', 'junk.dat'; print $_ while defined($_ = <$fh>); close $fh;` [download] So on windows "\n" becomes the bytes "\r\n" when you print, so 78 + 2 = 80, thats the lines so seek to 180 from zero print "1" 78 times followed by "\r\n" so seek to 280 from zero print "2" 78 times followed by "\r\n" so seek to 380 from zero print "3" 78 times followed by "\r\n" so seek to 480 from zero print "4" 78 times followed by "\r\n" Then when reading the file the first 80 chars are null ("\0") then its 1111...222...333...4444 Thats Basic debugging checklist for you	[reply] [d/l]
Re: Position in seek() confusion by Anonymous Monk on Jul 01, 2015 at 08:23 UTC
It's not at all clear to me how `B::Deparse` makes it obvious for people not familiar with `seek`... OTOH, `Devel::Peek` does. This is on Linux: `open my $fh, '>', 'junk.dat' or die $!; for ( 1 .. 4 ) { seek $fh, $_ * 80, 0; print $fh $_ x 78, "\r\n"; } open $fh, '<', 'junk.dat' or die $!; while ( <$fh> ) { use Devel::Peek; Dump $_; }` [download] output: SV = PV(0x189fd70) at 0x18c0a38 REFCNT = 1 FLAGS = (POK,pPOK) PV = 0x18a4610 "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0 +\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\ +0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0001111111111111111111111111111 +11111111111111111111111111111111111111111111111111\r\n"\0 CUR = 160 LEN = 200 SV = PV(0x189fd70) at 0x18c0a38 REFCNT = 1 FLAGS = (POK,pPOK) PV = 0x18a4610 "2222222222222222222222222222222222222222222222222222 +22222222222222222222222222\r\n"\0 CUR = 80 LEN = 200 SV = PV(0x189fd70) at 0x18c0a38 REFCNT = 1 FLAGS = (POK,pPOK) PV = 0x18a4610 "3333333333333333333333333333333333333333333333333333 +33333333333333333333333333\r\n"\0 CUR = 80 LEN = 200 SV = PV(0x189fd70) at 0x18c0a38 REFCNT = 1 FLAGS = (POK,pPOK) PV = 0x18a4610 "4444444444444444444444444444444444444444444444444444 +44444444444444444444444444\r\n"\0 CUR = 80 LEN = 200 [download] So, when you `seek` past the end of file, it's as if the filesystem fills that part of file (0 .. 79 bytes in our case) with zero bytes (actually it creates a 'hole' in the file, which, when read, returns zeros - apparently NTFS also does that?). These zero bytes are then actually printed, just not displayed on the terminal: `$ perl -E 'say "\0\0\0ABC" ABC $ perl -E 'say "\0\0\0ABC"' \| perl -nE 'printf "%vx\n", $_' 0.0.0.41.42.43.a` [download]	[reply] [d/l] [select]
Re: Position in seek() confusion by Anonymous Monk on Jul 01, 2015 at 01:34 UTC
Use B::Deparse and it will become obvious	[reply]
Re: Position in seek() confusion by locked_user sundialsvc4 (Abbot) on Jul 01, 2015 at 12:35 UTC
You folks seriously told the OP to “de-parse” a Perl subroutine into its underlying bytecode-tree, assuring him or her that thereby it would be “obvious?” Really?? ... `:-/` ... Gee, you people really are nerds! The one and only thing to remember about `seek()`, in any programming language at all, is that it works strictly on byte-positions within the file. It does not know about newline sequences (in any of their one- or two-byte flavors ...), nor does it know about Unicode. The file consists of nothing more and nothing less than a collection of zero-or-more bytes, and `seek()` sets the read/write cursor to an absolute or relative byte position within that collection.
Re^2: Position in seek() confusion by Anonymous Monk on Jul 01, 2015 at 18:27 UTC
You seriously replied to the OP indirectly? Not every context is a beginner's context. In this case, the OP was looking for deep knowledge and not just the Cliffs Notes. Can you not see this?	[reply]
Re^2: Position in seek() confusion by Anonymous Monk on Jul 01, 2015 at 18:40 UTC
sundialsvc4: You folks seriously told the OP to “de-parse” a Perl subroutine into its underlying bytecode-tree, assuring him or her that thereby it would be “obvious?” Really?? ... `:-/` ... Gee, you people really are nerds! The one and only thing to remember about `seek()`, in any programming language at all, is that it works strictly on byte-positions within the file. It does not know about newline sequences (in any of their one- or two-byte flavors ...), nor does it know about Unicode. The file consists of nothing more and nothing less than a collection of zero-or-more bytes, and `seek()` sets the read/write cursor to an absolute or relative byte position within that collection. Poor flushells, you really don't know anything about B::Deparse, print, $/, -l Why starts at "column 0"? Why 78/80? Because \r\n is added	[reply]