I've written a very simple DLL to call from another language. The parameter LastOBC is passed as a pointer to a 10-element array, but this DLL treats it as a single value, not an array. My parameter LastOBC:PTR DWord is apparently not correct. Can anyone tell me what the proper syntax is for passing an array pointer? Here's the relevant code segment:
**********
AddUp Proc USES ebp LastOBC:PTR DWord
mov ebp,LastOBC
mov eax,[ebp]
mov ebx,10
add eax,ebx
Ret
AddUp EndP
**********
Thanks very much for any help.
You should be more specific - which HLL are you using? The pointer declaration (pointer to array of unsigned long) is correct.
Using EBP that way is a bad idea, because it is used for the stack frame. EBX should be saved.
Thanks for your reply.
The high level language is APL. As you say the pointer declaration is correct, then it looks like the problem is in the call from APL.
Thanks again.
If you want to access different elements in the array, you will need to have an offset with ebp.
Assuming your parameter is a pointer to an array of 4-byte elements, you would change [ebp] to [ebp+4], [ebp+8], etc.
Try something like this, you can use EAX, ECX and EDX without having to preserve them. If you need to use EBX, ESI or EDI you should preserve and restore them in the procedure.
AddUp Proc LastOBC:DWORD
mov ecx,LastOBC
mov eax,[ecx]
mov edx,10
add eax,edx
Ret
AddUp EndP
Thanks for all the replies.
The answer turned out to be this: when calling from APL, I need to pass two parameters, the pointer and the length of the array. As I am passing fixed length arrays, my program doesn't need to know the length, but apparently the DLL needs that information in order to treat it as an array.
Thanks to Hutch for the indexing tips -- when calling from this language, the USES directive will preserve the values of the pointer registers, but I prefer not to use them in mixed-language programming. I've found that push and pop can cause problems on return to the APL interpreter; it's a bit tricky.
Thanks again.