Can I load a dll into shared memory?

Question

I have access a 32 bit dll from a 64 bit application. For this I am using Shared Memory IPC and I have done something like this

TCHAR szName[]=TEXT(Path of DLL on local machine); 
TCHAR szMsg[]=TEXT("abc");

HANDLE file = CreateFile(szName,                
                       GENERIC_READ,          
                       0,                      
                       NULL,                   
                       CREATE_NEW,             
                       FILE_ATTRIBUTE_NORMAL,  
                       NULL);

Is it the correct approach to share a dll over an IPC? Can I access the functions defined inside DLL at the reader interface?

Answer 1

It is, by definition, not possible to call a 32-bit function from 64-bit code. The processor behaves differently in 32-bit and 64-bit mode. The OS can do different things for 32-bit and 64-bit applications by setting the code-segment selector to 32- and 64-bit mode respectively, but that is a big switch for the entire application.

• 32-bit calls have a different calling convention to 64-bit code. (64-bit mode uses registers for the first 5-6 arguments for all calls, 32-bit mode only 3 arguments, and not for all functions). General register usage (which registers are used for what) is different, so registers that need to be preserved in 32-bit are not needing that in 64-bit bit, and so on.

• 32-bit operations clear the upper part of 64-bit registers, whether they want to or not. So just setting eax to 5 will alter the upper 64-bits of eax, which the 32-bit code has no knowledge even existed.

• Pointers in 64-bit mode are allocated in the full 64-bit range [well, 47-bits, but that's still 15 bits more than 32], so you won't be able to pass any pointers to the called code, even if not a single of the other problems existed.

• push and pop instructions with register operand are now 64-bit (regardless of whether you want that or not), so the 32-bit code will save registers thinking they take up 4 bytes of stack, and they will take up 8, meaning any offset from stack pointer calculation will be wrong inside the called function - including the common sequence: push ebp and mov ebp, esp, the value of esp is now wrong.

• Some instructions are no longer available or are only available in their alternative form. In particular, byte values 0x40-0x4f is "prefix" for 64-bit instructions, rather than the instructions they used to be.

• Any calls from your "imported code" will be seen as 64-bit, so OS-calls, C library calls, etc, won't work correctly.

Of course, you can READ the file, transcribe it into 64-bit mode, but that would require a huge amount of work in coming up with a translator that understands the code and can translate it it (since you'd need to know what is actual code, and what is for example jump-tables for switch statements or text-strings embedded in the code-segment, which shouldn't be translated, at least not in the same way).

Most likely it's 10 times easier to recompile the code as 64-bit. Or recompile your 64-bit app as 32-bit (which is almost certainly the EASIEST option).