[DUG] Delphi Calling Convetion is Weird!!

[Stacey Verner]

After an odd bug and a bit of testing I have found that the parameters
in Delphi (tested 6 and 2006) are evaluated very strangely.

 

The 3rd parameter is evaluated first followed by 4, 5, 6 etc and then
the 2nd and then the 1st.

 

This appears to be because the default calling convention is register
which (from the documentation) puts up to 3 parameters in registers and
the rest on the stack. It seems to process the stack ones first from
parameters 3 onwards (Why 3? Shouldn't it be 4 if we are putting 3 into
registers. Must be only putting 2 in the registers), and then the first
2 in reverse order into the registers. The documentation also says that
is supposed to pass the from left to right, but it obviously doesn't.

 

You can download an example to see for yourself!

 

http://www.verner.co.nz/download/FunctionCall.zip

 

If you are interested how we found this, we have a simple function that
strips the first delimited item from a string returning the item and
removing it from the input string. You can have a string something like:

 

LString := 'Stacey,G,Verner';

 

and call:

 

LFirstName := Extract(LString, ',');

LInitial := Extract(LString, ',');

LLastName := Extract(LString, ',');

 

In this case if we had a function like:

 

function FullName(PFirstName, PInitial, PLastName: String): String;

 

and you call it with:

 

LName := FullName(Extract(LString, ','), Extract(LString, ','),
Extract(LString, ','));

 

then we'll get the wrong result.

 

Stacey

 

Stacey Verner             Ph:   +64-9-4154790
Software Developer        Fax:  +64-9-4154791
                          DDI:  +64-9-4154797
                          Email: stacey at cjntech.co.nz
<BLOCKED::mailto:stacey at cjntech.co.nz> 

CJN Technologies Ltd.

PO Box 302-278, North Harbour, Auckland 1330, New Zealand
12 Piermark Drive, North Harbour, Auckland, New Zealand
Visit our website at http://www.cjntech.co.nz/
<BLOCKED::http://www.cjntech.co.nz/> 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://ns3.123.co.nz/pipermail/delphi/attachments/20070216/b5422285/attachment.html