pack/unpack woes

james289o9 has asked for the wisdom of the Perl Monks concerning the following question:

Originally i tried to ask in the chatterbox, but i think that i may have mis-translated what i am trying to do. So without further ado:

I have an ogg/opus audio file that i am trying to read the header, which is 28 bytes or 0x1C in length. Here is the code i am using to read this header:

#!/usr/bin/perl
use strict;
use warnings;
                        
open my $ogg_file, '<', shift or die $!;
binmode($ogg_file);

read_ogg_header($ogg_file);

sub read_ogg_header {
    
    my ( $file ) = @_;
    
    read($file, my $buf, 28 );

    my ( $ogg_magic, $ogg_version, $flags, $granule_position, $serial_
+number, $sequence_number, $checksum, $total_segments, $segment_size )
+ = unpack('C4 C1 C1 C8 C4 C4 C4 C1 C1', $buf);

    print $ogg_magic;  ### Expected it to print 'OggS'
}
[download]

and here is the 28 byte header:

0x4F6767530002000000000000000083CA3DC0000000009F59730A0113
[download]

The more i think about this the more i wonder if unpack C is not what i want to do anyway. are the single bytes already in unpack C format? if so, how can i just store that value into the variable without any conversion so any further printing or processing the actual raw values eg print $ogg_magic will print "OggS"

Or maybe i am just completely confused by pack and unpack. i can use read() and i can easily print those values or translate them into expected formats, but i think unpack C is messing me up somehow? or ... OR... maybe i am just completely confused. If i need to elaborate any more please just let me know.

P.S. i guess what i am wondering in the end is, how can i make pack or unpack assign the same values as read($file, my $buf, n);? i can work with whatever data read stores (im assuming binary?) but i cannot get the same values/data to store in those variables with unpack or pack. hope i am clear, if not just give me a swift kick in the hind quarters and tell me what other info you need. and as always any help is certainly appreciated :)

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Re: pack/unpack woes by AnomalousMonk (Archbishop) on Jun 14, 2022 at 21:10 UTC
Win8 Strawberry 5.30.3.1 (64) Tue 06/14/2022 17:01:38 C:\@Work\Perl\monks >perl use strict; use warnings; use Data::Dump qw/dd pp/; my $ogg_head = pack 'H', '4F6767530002000000000000000083CA3DC0000000009F59730A0113'; dd $ogg_head; # for debug my ($ogg_magic, $opus_version, $flags, $granule_position, $serial_number, $sequence_number, $checksum, $total_segments, $segment_size) = # unpack('C4 C1 C1 C8 C4 C4 C4 C1 C1', $buf); unpack('a4 C C Q N N N C C', $ogg_head); print "'$ogg_magic' \n"; dd $ogg_magic, $opus_version, $flags, $granule_position, $serial_number, $sequence_number, $checksum, $total_segments, $segment_size; printf "%s %02x %02x %08x %04x %04x %04x %02x %02x \n", $ogg_magic, $opus_version, $flags, $granule_position, $serial_number, $sequence_number, $checksum, $total_segments, $segment_size; ^Z pack("H","4f6767530002000000000000000083ca3dc0000000009f59730a0113") 'OggS' ("OggS", 0, 2, 0, 2211069376, 0, 2673439498, 1, 19) OggS 00 02 00000000 83ca3dc0 0000 9f59730a 01 13 [download] Note: Check that your version of Perl supports the `Q` pack/unpack template specifier. Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re^2: pack/unpack woes by ikegami (Patriarch) on Jun 14, 2022 at 21:31 UTC
A little explanation of the changes, with some corrections: `C4` is the same as `C C C C`. It extracts 4 numbers. `$ perl -e'printf "0x%X\n", $_ for unpack "C4", "\x12\x34\x56\x78"' 0x12 0x34 0x56 0x78` [download] But that's not what you want. You want to extract a single 32-bit unsigned integer in little-endian byte order.^[1] After consulting Mini-Tutorial: Formats for Packing and Unpacking Numbers, we determine that we need `L<` or `V` for that. `$ perl -e'printf "0x%X\n", $_ for unpack "L<", "\x12\x34\x56\x78"' 0x78563412` [download] So, what you want is `unpack( 'a4 C C Q< L< L< L< C', $ogg_head )` [download] Note that this is only 27 bytes, not 28. The number of remaining bytes is indicated by that last field (the 27th byte), and may be a number other than one. As hinted by the parent, `Q` is only available on builds of Perl with 64-bit integers (i.e. where `perl -V:uvsize` aka `perl -Mv5.10 -MConfig -e'say $Config{uvsize}'` prints 8 or more). The parent assumed big-endian order and thus used `N`. But the spec says little-endian byte order is used, which is repeated here.	[reply] [d/l] [select]
Re^3: pack/unpack woes by james289o9 (Acolyte) on Jun 14, 2022 at 22:12 UTC
I had never read the mini tutorial that you made. After looking at that and then re-reading this comment, everything at least makes sense to me. Not only that, but it actually will help me clean up some code in other modules that ive written. Thanks.	[reply]
Re^4: pack/unpack woes by AnomalousMonk (Archbishop) on Jun 15, 2022 at 06:12 UTC
Re^2: pack/unpack woes by james289o9 (Acolyte) on Jun 14, 2022 at 21:21 UTC
Thank you AnomalousMonk, this most definitely solves my original question, and gives me some extra things to study.	[reply]
Re: pack/unpack woes by ikegami (Patriarch) on Jun 14, 2022 at 20:52 UTC
Use `a4` instead of `C4` to get `OggS`.	[reply] [d/l] [select]
Re^2: pack/unpack woes by james289o9 (Acolyte) on Jun 14, 2022 at 21:17 UTC
That did the trick. Thank you good sir.	[reply]


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