Calling win32:api dll with pointer to array

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

Hi, I want to call a dll, which requires a pointer to an array of doubles, followed by the length, or number of elements, in the array. I've created my object like:

my $obj = Win32::API->new("dll","Function", [P, I], I);

The problem is, I don't know how to set up the array for the call. I've tried:

$arr=pack("d",1) . pack("d",1.5), pack("d",3);

and

@arr=(pack("d3",1,1.5,3))

whatever I try causes perl.exe to crash (ActivePerl 5.6.1). Any ideas on how the call should be made would be appreciated

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(tye)Re: Calling win32:api dll with pointer to array by tye (Sage) on Jan 14, 2002 at 19:36 UTC
Well, you were close with both of your attempts. Change one comma to a period in your first try and you have something that should work: `$arr= pack("d",1) . pack("d",1.5) . pack("d",3);` which can more compactly be written as your second try with @ changed to $: `$arr= pack("d3",1,1.5,3);` or, to be more flexible: `my @arr= ( 1, 1.5, 3 ); my $arr= pack "d*", @arr; my $res= $obj->Call( $arr, 0+@arr );` [download] An array of doubles in C is simply a bunch of doubles packed right next to each other in a contiguous span of memory. In Perl, the way to get a contiguous span of memory is in a string, so that is what pack produces. - tye (but my friends call me "Tye")	[reply] [d/l] [select]
Re: (tye)Re: Calling win32:api dll with pointer to array by dhorne (Initiate) on Jan 15, 2002 at 14:00 UTC
Thanks guys it works now	[reply]
Re: Calling win32:api dll with pointer to array by larryk (Friar) on Jan 14, 2002 at 18:51 UTC
From perldoc Win32::API: `The third parameter, the input parameter list, specifies how many arguments the function wants, and their types. It MUST be passed as a list reference. The following forms are valid: [a, b, c, d] \@LIST` [download] so make up a reference to an anonymous list as: `my $arr = [ pack("d",1), pack("d",1.5), pack("d",3) ]; # or as I would do it... my $arr = [ map { pack 'd', $_ } ( 1, 1.5, 3 ) ];` [download] The `[]` square brackets make an anonymous array so the scalar `$arr` (which you need to supply to the Call method of Win32::API) contains a reference to that array. You could as easily do: `my @arr = map { pack 'd', $_ } ( 1, 1.5, 3 );` [download] and then pass a reference to the list (`\@arr`) - whichever you prefer. Hope this helps larryk perl -le "s,,reverse killer,e,y,rifle,lycra,,print"	[reply] [d/l] [select]
(tye)Re2: Calling win32:api dll with pointer to array by tye (Sage) on Jan 14, 2002 at 19:41 UTC
No, the documentation that you quote is talking about the third argument to the new() method, which certain shouldn't contain packed `double` values. It is talking about the argument `[P, I]` in the code above. The array of doubles is an argument to the C API being accessed and so gets passed in via the Call() method. Note that I avoid using the style found in the Win32::API documentation and would rewrite the code as: `my $obj= Win32::API->new( "dll", "Function", [qw( P I )], 'I' );` so that I could use strict with it. - tye (but my friends call me "Tye")	[reply] [d/l] [select]