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

Oh Dispensers of Wisdom, please come to my aid

Using the neat Win32::TieRegistry module I'm reading stuff from the Windows registry. The data gets written to a file, which is later read and processed. I'm having trouble with bit-twiddling after reading the data file, because of my limited understanding of pack, unpack and perl's internal data representation. I'm just missing something here -- possibly a piece of grey matter.

We're using ActiveState perl: "This is perl, version 5.005_03 built for MSWin32-x86-object" (but will probably be moving to a newer version soon.)

For REG_DWORD items read by Win32::TieRegistry's GetValue, the value is read thus: 

    my ( $value, $type ) = $key->GetValue( "$_" );
    # ---- format the value as text
    if ( $type == 4 )    # reg_dword
    {
        $value = hex( $value )
    }

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So for a reg_dword value 0x00500000 in the registry, the string 5242880 is written to file.

Later I want to test and set bits in the value against a mask, so presumably it needs to be packed back into four bytes. Unfortunately none of my attempts to do this work, and I can't figure out how to "see" how perl is storing the data. Please can someone who has seen the light tell me how?

maybe something like this: 


    $dword  = pack <somehow> ( $value );   # eg $value = 5242880 dec

    $mask   = pack <somehow> ( 4194304 );  # 0x00400000
    $flags  = pack <somehow> ( 1048576 );  # 0x00100000

    $testdw = $dword & $mask;
    $newdw  = $dword | $flags;
    $newval = hex ( $newdw );   # gets written to file

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Many thanks

Replies are listed 'Best First'.
Re: how to pack bits, flags and masks
by Zaxo (Archbishop) on Nov 15, 2004 at 18:42 UTC

    You're making this too difficult. The dword is a 32-bit native integer - 

    perl -e'print 0x00500000,$/
5242880
$

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    All you need to test or set bits are the bitwise operators &, |, and ^, and their assignment forms, &=, |=, and ^=.

    For example, to test if bit 7 is set in $value, 

    if ($value & 1<<6) {
    #do something
}

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    The bit manipulations you show should work fine on your data as it is.

    Even if you read your number as a decimal string, it should automatically be converted to an unsigned numeric type by the bitwise operators.

    After Compline,
    Zaxo

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      thanks, it does seem simpler without 'pack' and I can even count the set bits with a for loop to test each in turn.

      I'd been hoping to use this fragment from Programming Perl (ch.29.2.189. unpack):
      "The following efficiently counts the number of set bits in a bitstring:
      $setbits = unpack "%32b*", $selectmask;

      but for me 

          $value = 5242880;
    $value = $value | 0x0011;
    $setbits = unpack "%32b*", $value;
    printf "$setbits bits are set in %#.8x\n", $value;

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      gives
      25 bits are set in 0x00500011

      so my understanding is still inadequate. If you can bear to explain, it would be much appreciated (though I can use the for loop to count bits instead)

      thanks

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        $setbits = unpack "%32b*", $selectmask;
        should be:
        $setbits = unpack "%32b*", $packedmask;
        so:
        $setbits = unpack('%32b*', pack('N', $selectmask));
        does the trick
        print(unpack("%32b*", pack('N', 0x00500011)), $/);  # 4

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Re: how to pack bits, flags and masks
by jimbojones (Friar) on Nov 15, 2004 at 19:04 UTC
    Hi

    I'm confused. Why can't you just leave everything in Perl's internal format?

    
#-- set up the vars to be like what Win32::TieRegistry returns (a stri
+ng and a number);
my ($dword, $type) = ( '0x00500000', 4);
if ( $type == 4 )
{
 $dword = hex $dword;
}

printf "Dword:\tdec:%d\thex:%x\n", $dword, $dword;
my $mask = 0x00400000;
printf "mask:\tdec:%d\thex:%x\n", $mask, $mask;
my $flags = 0x00100000;
printf "mask:\tdec:%d\thex:%x\n", $flags, $flags;

my $testdw = $dword & $mask;
my $newdw  = $dword | $flags;

printf "TestDW:\tdec:%d\thex:%x\n", $testdw, $testdw;
printf "NewDW:\tdec:%d\thex:%x\n", $newdw, $newdw;

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    Results

    Dword:  dec:5242880     hex:500000
mask:   dec:4194304     hex:400000
mask:   dec:1048576     hex:100000
TestDW: dec:4194304     hex:400000
NewDW:  dec:5242880     hex:500000

    [download]
    I think you still need the first $dword = hex $dword; as the return from Win32::TieRegistry is a string.

    - j
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