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

Prob some stupid mistake, but why does sizeof give 8 and not 5. One "char" (1byte) + One "long" (4bytes) = 5? 

    use Win32::API;
    use Win32::API::Struct;
    use strict;
    use warnings;

        #0.41            
    #print Win32::API->VERSION;

    Win32::API::Struct->typedef('test', qw( 
      char i;
      long l;
    ));
    
    my $Bl = Win32::API::Struct->new('test');
        #8. Why not 5?
    print "\nSize:", $Bl->sizeof();

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Re: Win32::API::Struct problem
by rhesa (Vicar) on Oct 27, 2006 at 00:19 UTC
    The reason is structure alignment (i.e. how the structure is stored in memory). I'd guess that it gets rounded up to the nearest DWORD boundary.

    Running your snippet with $Win32::API::DEBUG=1 reveals some detail about what's going on: 

    use Win32::API;
use Win32::API::Struct;
$Win32::API::DEBUG = 1;

Win32::API::Struct->typedef( test => qw(char i; long l;) );
my $b = Win32::API::Struct->new( 'test' );
print $b->sizeof;

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    Output: 
    (PM)Struct::recognize got 'i', 'char' -> 'c'
(PM)Struct::recognize got 'l', 'long' -> 'l'
(PM)Struct::new: got 'test'
(PM)Struct::sizeof: sizeof with member(i) now = 1
(PM)Struct::sizeof: sizeof with member(l) now = 5
(PM)Struct::sizeof first=1 align=4
(PM)Struct::sizeof returning 8
8

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Re: Win32::API::Struct problem
by Old_Gray_Bear (Bishop) on Oct 27, 2006 at 00:24 UTC
    I suspect that you are on an architecture that insists that a long() must begin on a word boundary (the address%4 = 0). When you inserted the char(), you moved the next available memory position off of a word boundary, and so you get the three slack-bytes inserted. If you positively must have the smallest in-memory foot-print, then order the components of your struct in order of decreasing alignment -- float/double before long before half-word before int/char/byte.

    ----
    I Go Back to Sleep, Now.

    OGB

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      Size of this struct is reported as 3!? 
      Win32::API::Struct->typedef('test', qw( 
      long array[10];
      char i;
      char z;
    ));

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      Size of this is reported as 40 (logical, since one "long" = 4bytes * 10 = 40). 
      Win32::API::Struct->typedef('test', qw( 
   long array[10];
));

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      What is this all about?
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        What is this all about?

        Looks like a bug - this one looks like it might be related to http://rt.cpan.org/Public/Bug/Display.html?id=6757 which refers you to http://www.perlmonks.org/index.pl?node_id=369601. As mentioned in the update to that perlmonks post, there seems to be some problem with the member alignment aspect of the computation - which becomes apparent when you turn on $Win32::API::DEBUG (as recommended in an earlier post). You can handle structs in Win32::API without having to resort to using Win32::API::Struct. I think that not using Win32::API::Struct is probably the best way to workaround any problems that Win32::API::Struct throws at you :-)

        Cheers,
        Rob
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