I must be going mad. First up here's what double.h looks like:

C:\_32\c>type double.h
double __declspec(dllexport) my_double(int);
C:\_32\c>
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And here is double.c:

C:\_32\c>type double.c
#include "double.h"

double __declspec(dllexport) my_double(int num) {
       return (double) num;
}
C:\_32\c>
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Let's turn double.c into a dll, using Visual Studio 7.0 as the compiler, and making sure to invoke the __stdcall calling convention:

C:\_32\c>cl /Gz /LD double.c
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 13.00.9466 for 
+80x86
Copyright (C) Microsoft Corporation 1984-2001. All rights reserved.

double.c
Microsoft (R) Incremental Linker Version 7.00.9466
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:double.dll
/dll
/implib:double.lib
double.obj
   Creating library double.lib and object double.exp

C:\_32\c>
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Let's check that double.dll does, in fact, export the my_double function:

C:\_32\c>dumpbin /exports double.dll
Microsoft (R) COFF/PE Dumper Version 7.00.9466
Copyright (C) Microsoft Corporation.  All rights reserved.


Dump of file double.dll

File Type: DLL

  Section contains the following exports for double.dll

    00000000 characteristics
    4837FFED time date stamp Sat May 24 19:45:49 2008
        0.00 version
           1 ordinal base
           1 number of functions
           1 number of names

    ordinal hint RVA      name

          1    0 00001000 _my_double@4

  Summary

        2000 .data
        2000 .rdata
        1000 .reloc
        7000 .text

C:\_32\c>
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That looks about right. Why the leading underscore ? Does that matter ?

By my reckoning, we should be able to access the my_double function using Win32::API. (I have version 0.53, and perl-5.10.0.) Here's the script I'm using:

C:\_32\c>type double.pl
use Win32::API;
use warnings;

$function = Win32::API->new('double', 'my_double', 'N', 'D');
$ret = $function->Call(123);
print $ret, "\n";
C:\_32\c>
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Let's run that script:

C:\_32\c>perl double.pl
Can't call method "Call" on an undefined value at double.pl line 5.

C:\_32\c>
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What gives ? (If I check the contents of $^E I find "The specified procedure could not be found")

Let's run a C program to check that the dll is ok. Here's the test program:

C:\_32\c>type double_test.c
#include <stdio.h>
#include "double.h"

int main(void) {
    double d = my_double(123);
    printf("%f\n", d);
    return 0;
}
C:\_32\c>
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Now let's build the app:

C:\_32\c>cl double_test.c double.lib
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 13.00.9466 for 
+80x86
Copyright (C) Microsoft Corporation 1984-2001. All rights reserved.

double_test.c
Microsoft (R) Incremental Linker Version 7.00.9466
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:double_test.exe
double_test.obj
double.lib
double_test.obj : error LNK2019: unresolved external symbol _my_double
+ reference
d in function _main
double_test.exe : fatal error LNK1120: 1 unresolved externals

C:\_32\c>
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Wtf ?

Let's try a slightly different approach. I'll compile double.c into double.obj, and then link to double.obj instead of double.lib:

C:\_32\c>cl -c double.c
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 13.00.9466 for 
+80x86
Copyright (C) Microsoft Corporation 1984-2001. All rights reserved.

double.c

C:\_32\c>
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Now build double_test.exe by linking to double.obj:

C:\_32\c>cl double_test.c double.obj
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 13.00.9466 for 
+80x86
Copyright (C) Microsoft Corporation 1984-2001. All rights reserved.

double_test.c
Microsoft (R) Incremental Linker Version 7.00.9466
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:double_test.exe
double_test.obj
double.obj
   Creating library double_test.lib and object double_test.exp

C:\_32\c>
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And finally run double_test.exe:

C:\_32\c>double_test.exe
123.000000

C:\_32\c>
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That looks fine ... so, two questions:
1) Why can't double.pl access double.dll ?
2) Why can't I build double_test.exe by linking to double.lib ?

Cheers,
Rob