Hi monks,

I am wondering if there is any way to write data to a C structure in memory, knowing the address of the structure, using only Perl functions, modules, etc.

I am creating a GUI application using Win32::GUI and using a Listbox control which is to be owner-drawn. In order to get this to work, I need to handle the WM_MEASUREITEM message, among others. Since Win32::GUI doesn't have an event for this message, I need to create a hook. The message receives a pointer to a structure, which needs to be filled out with the desired item height.

I know how to use pack() and unpack() to access the data in the structure but I am unsure how to store the information back in the same structure. I have tried packing the data back into the argument passed to the sub (using @_) but this didn't seem to work. I also tried it with Inline::C, but the module seems to have a problem with directories with spaces in the names. The reason I would like to be able to do this with only Perl functions or core modules is that I will most likely distribute this app as a sample with a module I am creating and would like to keep the dependencies to a minimum.

Does anyone have any suggestions about getting this to work?

Here is my hook code:

$winMain->Hook(
    WM_MEASUREITEM,
    sub {
        my($self, $wParam, $lParam, $type, $msgcode) = @_;
        return 1 unless $type == 0;
        return 1 unless $msgcode == WM_MEASUREITEM;
        my %measureitem;
        # Unpack data from structure
        @measureitem{qw(CtrlType CtrlID itemID itemWidth itemHeight it
+emData)} = 
            unpack 'IIIIIL', unpack 'P24', pack 'L', $lParam;
        # Set the height of the items
        $measureitem{'itemHeight'} = 30;
        # Pack data back into structure
        my $struct = pack 'IIIIIL',
            @measureitem{qw(CtrlType CtrlID itemID itemWidth itemHeigh
+t itemData)};
        $lParam = unpack 'L', pack 'P24', $struct;
        # or
        #$_[2] = unpack 'L', pack 'P24', $struct;
        return 1;
    },
);

[download]

Here is a sample of how it would be done in C:

case WM_MEASUREITEM: 
    lpmis = (LPMEASUREITEMSTRUCT) lParam; 
 
    // Set the height of the list box items. 
 
    lpmis->itemHeight = 30; 
    return true;

[download]

Thanks in advance for any help.

In reply to Manipulating C Structures using only Perl by kejohm

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