Hi all, I was playing with some pack/unpack code dealing with long integers, these just happened to be generated from IP addresses, but that is off topic. I had the working code to figure out whether the code was running on a big or little endian machine as such: 
sub is_big_endian {
    my ($a,$b,$c,$d) = unpack("C4",pack("L",0x01020304));
    ($a==0x01) && ($b==0x02) && return 1; #big endian
    ($a==0x04) && ($b==0x03) && return 0; #little endian
}

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but I got to thinking that, "Why do I need to pack the long integer 0x01020304 in the first place?" I thought I'd be able to do something like this: 
sub is_big_endian {
    $num=0x01020304;
    my ($a,$b,$c,$d) = unpack("C4",$num);
    ($a==0x01) && ($b==0x02) && return 1; #big endian
    ($a==0x04) && ($b==0x03) && return 0; #little endian
}

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Which didn't work, running via the debugger, I tried LOTS of variations attempting to make sense of what was actually going on: 
  DB<1> c 242
main::is_big_endian(test-dns.pl:242):
242:            my ($a,$b,$c,$d) = unpack("C4",0x01020304);
  DB<2> n
main::is_big_endian(test-dns.pl:243):
243:            ($a==0x01) && ($b==0x02) && return 1; #big endian
  DB<2> p $a
49
  DB<3> p $b
54
  DB<4> p $c
57
  DB<5> p $d
48
  DB<6> p unpack("C4",0x01020304)
49545748
  DB<7> p unpack("L",0x01020304) 
825637168

  DB<12> $a=0x01020304

  DB<13> p $a
16909060
  DB<14> p unpack("CCCC",$a)
49545748
  DB<15> p unpack("C4",$a)  
49545748

  DB<17> p unpack("C4",pack("L",($a)))
1234
[..stuff removed..]
  DB<51> p $a
16909060
  DB<52> p unpack("b32",pack("L",$a))
10000000010000001100000000100000
  DB<55> p unpack("B32",pack("L",$a))
00000001000000100000001100000100
  DB<3> p unpack ("C*",x01020304)
1204849485048514852
  DB<4> p unpack ("C*",0x01020304)
4954574857485448
  DB<5> p unpack ("C*",0x00000001020304)
4954574857485448

  DB<10> print join(" ", map { sprintf "%#02x", $_ } unpack("C*",pack(
+"L",0x12345678))), "\n";
0x12 0x34 0x56 0x78

  DB<11> print join(" ", map { sprintf "%#02x", $_ } unpack("C*",0x123
+45678)), "\n";          
0x33 0x30 0x35 0x34 0x31 0x39 0x38 0x39 0x36

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Then I read that internally, all numbers are represented as double precision floats, so, I tried those as well: 

  DB<15> print join(" ", map { sprintf "%#02x", $_ } unpack("C*",pack(
+"D",$a))), "\n";
0x41 0xb2 0x34 0x56 0x78 00 00 00

  DB<16> print join(" ", map { sprintf "%#02x", $_ } unpack("C*",pack(
+"D*",$a))), "\n";
0x41 0xb2 0x34 0x56 0x78 00 00 00

  DB<17> print join(" ", map { sprintf "%#02x", $_ } unpack("C*",pack(
+"d*",$a))), "\n";
0x41 0xb2 0x34 0x56 0x78 00 00 00

  DB<18> print join(" ", map { sprintf "%#02x", $_ } unpack("C*",pack(
+"d",$a))), "\n"; 
0x41 0xb2 0x34 0x56 0x78 00 00 00

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And those didn't match what I'd gotten before. So my question boils down to: "What is *actually* going on internally to give me the answer I get when doing an unpack("C",$a) where $a=0x01020304?"

In reply to pack, unpack and internal represenation of numbers by 5mi11er

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