Low-level version of Win32::API::Struct

sgv_81 has asked for the wisdom of the Perl Monks concerning the following question:

I'm working with Win32::API against a Win32 dll. My machine is 64 b and I'm using Cygwin. I'm trying to create a struct defined in C as:

typedef struct cRefStr 
{ 
    size_t length;        
    char const* data;   
} cStrRef;
[download]

I've tried to define the following structure with Win32::API::Struct:

Win32::API::Struct->typedef(CSTRREF =>qw{int n; char* buffer});
[download]

The complete perl code is:

# Sub all 
# cTblAtKey -> findInTableByKey
# cTbl -> myTable
# cCell -> myCell
# ctbl_v2 -> myDll

my $aKey = "UserName";
print $aKey."\n";
my $lengthaKey = length($aKey);
print $lengthaKey."\n";
Win32::API::Struct->typedef(CSTRREF =>qw{int n; char* buffer});
my $structParameter = Win32::API::Struct->new('CSTRREF');
$structParameter->{n}=$lengthaKey;
$structParameter->{buffer}=$aKey;
# C function defined as:
# myCell* findInTableByKey(myTable that, struct cRefStr key, unsigned 
+offset);
# with: 
# struct myCell { 
#     unsigned char opaque[10]; 
#};
my $aKeyFunc = Win32::API->new("myDll.dll",'findInTableByKey', 'NSN','
+C','_cdecl');
if(not defined $aKeyFunc)
{
    die "Couldn't load the function findInTableByKey from the dll";
}
$cellPtr = $aKeyFunc->Call($outputTable,$structParameter,0);
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This breaks with a "Segmentation fault (core dumped)" error.

I've managed to use other functions of the same dll against $outputTable (this was the thread) and I know it is a valid parameter. So I've determined that the issue is with the Struct. I'm trying to substitute $structParameter by its low-level version as suggested in the Win32::API doc, this is using something like pack('NC',$size,$aKey).

I've tried to use a simplified example to understand how this would work. The code in the Win32::API doc, to test struct works as expected:

 #### define the structure
    Win32::API::Struct->typedef( POINT => qw{
        LONG x; 
        LONG y; 
    });
    
    #### import an API that uses this structure
    my $getCursor = new Win32::API('user32', 'GetCursorPos','S','N');
    
    #### create a 'POINT' object
    my $pt = Win32::API::Struct->new('POINT');
    
    #### call the function passing our structure object
    $getCursor->Call($pt);
    
    #### and now, access its members
    my ($x, $y) =($pt->{x}, $pt->{y});
    print "The cursor is at: $x, $y\n";
[download]

However the low-level version suggested in the same doc, throws an error: "Can't call method 'Pack' on an undefined value at ./TestStruct.pl line 27". The broken code:

    #### import an API that uses this structure
    my $getCursor = new Win32::API('user32', 'GetCursorPos','S','N');
    
    #### create a 'POINT' object
    $lpPoint=pack('LL',0,0);
    #### call the function passing our structure object
    $getCursor->Call($lpPoint);
    
    #### and now, access its members
    my ($x,$y) = unpack('LL',$pt);
    print "The cursor is at: ".$x.", ".$y."\n";
[download]

Any suggestion?

Comment on Low-level version of Win32::API::Struct Select or Download Code

Replies are listed 'Best First'.
Re: Low-level version of Win32::API::Struct by BrowserUk (Patriarch) on Aug 01, 2012 at 12:06 UTC
My machine is 64 b ... `pack('NC',$size,$aKey).` Firstly, template char 'N' is a 32-bit unsigned integer in network (big-endian) order. If you are running Windows on Intel (90%+ are), then you would need 'V' which is 32-bit unsigned in VAX (little-endian) order. But ... as you are on 64-bit OS, if you are running a 64-bit perl: (you don't say?) Then `size_t` will equate to a 64-bit unsigned int which would be template 'Q'. The simplest template would be 'J' which should work for 32-bit or 64-bit. (Excepting 32-bit perl's compiled for 64-bit ints.) With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. The start of some sanity?	[reply] [d/l] [select]
Re^2: Low-level version of Win32::API::Struct by sgv_81 (Novice) on Aug 01, 2012 at 16:21 UTC
Thank you, I understand that I'm in the wierd option of 32 bit perl for 64 int, Intel and Windows XP (but perl running through Cygwin). I think your post will be very helpful when I make the simplified example from my previous post work and I move into the original problem. As a quick refresh, the simplified example from my previous post: `#### import an API that uses this structure my $getCursor = new Win32::API('user32', 'GetCursorPos','S','N'); #### create a 'POINT' object $lpPoint=pack('LL',0,0); #### call the function passing our structure object $getCursor->Call($lpPoint); #### and now, access its members my ($x,$y) = unpack('LL',$pt); print "The cursor is at: ".$x.", ".$y."\n";` [download] No matter which parameter I use within pack ('J','L','N','Q' or 'V') in `$lpPoint=pack('LL',0,0);` I still get the error "Can't call method Pack on a undefined value". The error pops up in `$getCursor->Call($lpPoint);`. From CPAN doc on Win32::API seems that I can substitute the Win32::API::Struct for the packed variable and then pass it to the Win32::API object. Any idea what I'm missing?	[reply] [d/l] [select]
Re^3: Low-level version of Win32::API::Struct by bulk88 (Priest) on Aug 01, 2012 at 17:32 UTC
That is not an example from the docs. `my $getCursor = new Win32::API('user32', 'GetCursorPos','S','N');` [download] should be `my $getCursor = new Win32::API('user32', 'GetCursorPos','P','N');` [download] since $lpPoint is a scalar string formally. 'S'/'T' are only for ::Struct objects. The result of pack() is a scalar string. Not a blessed object. If you had a newer version of ::API, you would get a proper croak message `croak("Win32::API::Call: parameter %d must be +a%s", i+1, " Win32::API::Struct object!\n");` [download] much better than 0.68 blindly doing ->Pack() on whatever scalar happens to fall in as argument which is what is happening in your case.	[reply] [d/l] [select]
Re: Low-level version of Win32::API::Struct by Anonymous Monk on Aug 01, 2012 at 11:06 UTC
See Win32::API::Struct pointer / Re: Win32::API::Struct pointer	[reply]
Re: Low-level version of Win32::API::Struct by bulk88 (Priest) on Aug 01, 2012 at 17:04 UTC
I'm trying to substitute $structParameter by its low-level version as suggested in the Win32::API doc, this is using something like pack('NC',$size,$aKey). Win32::API's letter codes and pack()'s letter codes are different, do not use letters from one on the other, always reread the docs for pack and Win32::API. You want 'LP' (L=native (LE) 32 bit, P=pointer to scalar string) for pack. That pack 'L' is a 'I' for Win32::API BTW. `# myCell* findInTableByKey(myTable that, struct cRefStr key, unsigned +offset); # with: # struct myCell { # unsigned char opaque[10]; #}; my $aKeyFunc = Win32::API->new("myDll.dll",'findInTableByKey', 'NSN',' +C','_cdecl');` [download] Your prototype is wrong. 'C' as a return type either will be a undef, or a mangled something (number or single letter). You probably want 'N' instead of C to save that myCell * as a Perl scalar number. If you C header prototype is absolutly correct. 'S' and ::Struct won't work. "struct cRefStr key" is a pass by copy parameter, not a pointer or pass by reference. Thats going to require serious hacking on your part to get working since Win32::API can't do pass by copy structs/strings/anything except native machine numbers. I know its possible. Since that "struct cRefStr" has a size_t and a char , and is pass by copy now, on 32 bit machine code ONLY (x64 machine code has a different calling convention and something else must be done), replace the 'S' letter with a 'N' and then a 'P', you will not have a struct anymore in Perl but the C function will think it is getting the pass by copy struct. So 'NSN' becomes 'NNPN'. The recipe for pass by copy struct with ::API on 32 bit machine code is, do a pack() to create the binary packed struct as a scalar string. Then add nulls to the end of the scalar string to align it to a multiple of 4 (I think)(this can also be done with the pack() letter codes during the previous pack). Now do "length($packed)/4", 4 being the byte size of an ::API N on 32 bits, this is the number of Ns you will turn the packed scalar string into an array of scalar numbers, "@structasnums = unpack('L6', $packedstruct);", then for the prototype to ::API, you put "length($packed)/4" 'N's in a row where that struct is in the C header prototype so it looks like "$obj = new Win32::API('somedll.dll', 'aFunction', 'PPNNNNNNN', 'N');" . Then you do "$obj->Call($something, $foo1, @structasnums, $lastparam);". To break down the ::API prototype, "'PPNNNNNNN'", P=$something, P=$foo1, NNNNNN=@structasnums, N=$lastparam. From the last thread you made, WIN32::API and void, your running 32 bit cygwin perl with 64 bit ints on x64 Windows.	[reply] [d/l]
Re^2: Low-level version of Win32::API::Struct by sgv_81 (Novice) on Aug 02, 2012 at 07:12 UTC
I have made the following changes: `aKeyFunc = Win32::API->new("myDll.dll",'findInTableByKey', 'NNPN',' +C','_cdecl');` [download] returns a pointer (as expected). When I unpack it using 'P10' I recover the content of the cell in my table (i.e. the function works!!!).THANKS!	[reply] [d/l]
Re^3: Low-level version of Win32::API::Struct by bulk88 (Priest) on Aug 02, 2012 at 16:01 UTC
What are you unpacking with a 'P10'? That 'C' won't return a pointer, I think that 'C' as a return type is an undef in 0.68. You might also be leaking memory.	[reply]