How to unpack (64-bit) unsigned long in 64-bit Perl?

Question

I'm trying to unpack an unsigned long value that is passed from a C program to a Perl script via SysV::IPC.

It is known that the value is correct (I made a test which sends the same value into two queues, one read by Perl and the second by the C application), and all predecessing values are read correctly (used q instead of i! to work with 64-bit integers).

It is also known that PHP had something similar in bugs (search for "unsigned long on 64 bit machines") (seems to be similar: Pack / unpack a 64-bit int on 64-bit architecture in PHP)

Arguments tested so far:

..Q ( = some value that is larger than expected)
..L ( = 0)
..L! ( = large value)
..l ( = 0)
..l! ( = large value)
..lN! ( = 0)
..N, ..N! ( = 0)

use bigint; use bignum; -- no effect.

Details:

sizeof(unsigned long) = 8;
Data::Dumper->new([$thatstring])->Useqq(1)->Dump(); a lot of null bytes along some meaningful..
byteorder='12345678';

Solution: - x4Q with padding four bytes.

Show output of `Data::Dumper->new($string)->Useqq(1)->Dump` on your packed string and what number you are expecting. And maybe output of perl -V:byteorder — ysth, Mar 30 '11 at 15:44
@ysth, VAR1 = "\210\23\0\0\0\0\0\0\210\23\0\0\0\0\0\0\177\0\0\1\0\0\0\0\177\0\0\1\0\0\0\0W\273\@\37\0\0\0\0}^\330u\0\0\0\0\264\13\340u\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\16T\223M\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"; - wow, too much \0's, checking. Maybe `my $string` declaration doesn't fits return type somewhere. — kagali-san, Mar 30 '11 at 16:11
Have your tried just unpacking a scalar constant coded in the script? Once you get that working, try the pipe... — dawg, Mar 30 '11 at 16:30
@drewk, nope, I just can run all three programs simultaneously, (Perl version currently does no work unless I can get away this unpack trouble) and it can be restarted quickly.. not a time for unit test :) — kagali-san, Mar 30 '11 at 17:58
What is "predecessing values"? Do you mean "preceding values"? — Peter Mortensen, Jun 18 '17 at 20:17

score 3 · Answer 1 · answered Mar 30 '11 at 15:46

Unpacking using Q in the template works out of the box if you have 64-bit Perl:

The TEMPLATE is a sequence of characters that give the order
and type of values, as follows:

 ...

 q   A signed quad (64-bit) value.
 Q   An unsigned quad value.
       (Quads are available only if your system supports 64-bit
        integer values _and_ if Perl has been compiled to support those.
        Causes a fatal error otherwise.)

For a more robust solution, unpack the value into an 8-byte string and use the Math::Int64 module to convert it to an integer:

use Math::Int64 qw( :native_if_available int64 );

...

$string_value = unpack("A8", $longint_from_the_C_program);

# one of these two functions will work, depending on your system's endian-ness
$int_value = Math::Int64::native_to_int64($string_value);
$int_value = Math::Int64::net_to_int64($string_value);

Neither works correctly; source value = 563026057, same unpacked in secondary C app, native_to_int64 = 2418178501610831872, net_to_int64 = 2300088097 — kagali-san, Mar 30 '11 at 16:04

score 1 · Accepted Answer · answered Mar 30 '11 at 18:03

1

The solution was simple: added x4Q to skip four bytes before actual value; need to more visually think of padding/alignment..

answered Mar 30 '11 at 18:03

kagali-san

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How to unpack (64-bit) unsigned long in 64-bit Perl?

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