Unpack a buffer which is packed by perl with templ

Question

I am new to perl. I need to write a golang code which read UDP package sent from a perl UDP socket client. Basically, the perl client packs data using the template "N/a* N/a*" like the following:

$them = pack($sockaddr,&AF_INET, $data_port, $broadaddr);
$actual_data = pack("N/a* N/a*", $string1, $string2);
send(S,$actual_data,0,$them) || die $!;

My question is:

1. What does "N/a* N/a*" actually means? A simple explanation would be helpful. How two strings are actually packed?
2. How i can write a similar packing function in golang given two unicode strings; and if I got a packed buffer sent from perl UDP client, how i can write a corresponding function "in golang" to unpack it. Is there any library which can handle this so that I can use golang on par with perl for socket programming.

Answer 1

From the pack documentation:

• N is an unsigned long in network format
• a is a string with arbitrary binary data
• * is a repeat count that uses up the rest of the characters in the string
• the / notes that you're packing a length of the item and then the packed item.

You can write a small program to check it out:

use v5.10;
my $first  = 'dog';
my $second = 'parrot';

my $packed = pack "N/a* N/a*", $first, $second;

say $packed;

Run it and you won't see the number, but you can hexdump it to see everything:

$ perl5.22.0 pack.pl
dogparrot

$ perl5.22.0 pack.pl | hexdump -C
00000000  00 00 00 03 64 6f 67 00  00 00 06 70 61 72 72 6f  |....dog....parro|
00000010  74 0a                                             |t.|
00000012

There's a 00_00_00_03 in front of dog and a 00_00_00_06 in front of parrot. Those are the lengths of the packed strings. This probably means the other side scans the string by reading the number, then grabbing that number of succeeding octets.