A Question About DLL's

[Clay Clear]

How can I use the same technique that Microsoft's DLL's use
when one of their functions can use different types of variables
for the same argument? In other words, if their function has
an argument declared as AS ASCIIZ, you may also use BYVAL Number&.
How can a DLL function KNOW which type of argument it is, without
GPFing? I'd like to use the technique in some of my custom DLL's.

Thanks in advance!


------------------
Clay C. Clear

Clay Clear's Software
mailto:[email protected][email protected]</A>

[Steve Hutchesson]

Clay,

I am reasonably sure that within the definition of an industry standard
DLL that it cannot be done. PowerBASIC produces DLLs that conform
to that standard so it currently does not have that capacity in a
direct manner.

There are a number of ways to do what you are after but none of them
are easy to code manually, if you dont mind the extra overhead, you
can pass different size data in a union but there is no native way
of automatically determining at the other end which data type is being
passed.

You could manually code an extra variable that passed this info to
the function in the DLL so that it could tell what is being passed to
it but this is different to the process used in Microsoft languages where
this is built in at the compiler level.

To do what you are after in an efficient manner, I would be inclined to
write a major function in the DLL and make a set of calling functions
that can accept different parameter sizes and types. Each calling
function will have to handle the different data correctly and then
call the major function.

Regards,

[email protected]

------------------

[Eric Pearson]

Clay --

When you pass an ASCIIZ string to a function you are actually passing a 4-byte pointer (a DWORD) -- a numeric value -- that tells the function where the string is located. When you use BYVAL x the PowerBASIC compilers override their usual type-checking and allow you to pass a literal number. You can also use BYVAL STRPTR(sString$) to pass a "normal" string instead of an ASCIIZ string. It's the BYVAL that allows you to use something other than the declared parameter type. (You can also declare the parameter AS ANY, to tell the compiler "a pointer to some type of variable will be passed for this parameter" but that practice is usually discouraged.)

So all you really have to do in your function is to always treat the incoming parameter as a number. For example, your function could have a parameter that is BYVAL lPointer AS LONG (or BYVAL lPointer&), but in the declaration for the function -- the DECLARE line that is used by programs that want to call the function -- declare it as zValue AS ASCIIZ.

Then if the calling program uses BYVAL 0 the value of lPointer (received by the function) will be zero. If they use an ASCIIZ string then lPointer will be nonzero, and the function can use it to find the string.

A good knowlege of pointers, using DIM...AS...PTR, the @ operator, and related topics will be helpful, but it's not all that difficult.

-- Eric


------------------
Perfect Sync Development Tools
Perfect Sync Web Site
Contact Us: mailto:[email protected][email protected]</A>



[This message has been edited by Eric Pearson (edited October 03, 2001).]

[Wayne Diamond]

This is probably a pretty rude way of doing it, but how about passing a single parameter - a string (but not asciiz) like this:
"S4:fredI01"
where S means string, it then reads until the ":" to find the length of the string - 4 bytes, so it reads the next 4 bytes in which it knows is a string - "fred"
It then moves onto the next byte which it knows will be a "type identifier" (such as S for string, I for Integer, L for Long etc). It sees that the second type in this case is an integer, and knows that Integers are 16-bit, so it reads the next 2 bytes in and assigns that to a local integer variable. etc etc...

Best of luck,
Wayne


[This message has been edited by Wayne Diamond (edited October 03, 2001).]

[Tom Hanlin]

How can I use the same technique that Microsoft's DLL's use
when one of their functions can use different types of variables
for the same argument? In other words, if their function has
an argument declared as AS ASCIIZ, you may also use BYVAL Number&.

Many curious things are hidden in the Windows APIs. The technique
to which you refer is somewhat unsavory, but readily duplicated.
A number of APIs allow you to pass either an ASCIIZ string or a
word value in a LONG or DWORD. They can tell the difference because
the high word (of the LONG or DWORD) is always going to be zero if
you're passing a word, which will never be true if the value is an
ASCIIZ pointer.

If you were to write such a routine, it would look something like:

SUB marine (BYVAL nParam AS DWORD)
 
    LOCAL wValue AS WORD
    LOCAL pszValue AS ASCIIZ PTR
 
    IF HIWRD(nParam) THEN
        pszValue = nParam
        MSGBOX "String: " + @pszValue
    ELSE
        wValue = nParam
        MSGBOX "Number: " + FORMAT$(wValue)
    END IF
 
END SUB

------------------
Tom Hanlin
PowerBASIC Staff

[Charles Dietz]

Tom,

Wouldn't you also be restricted to passing only a positive word?

------------------

[Tom Hanlin]

A WORD is always non-negative. You could just as easily pass an INTEGER,
though, as long as you encode and decode it properly, e.g.,

marine BYVAL MAKDWD(integervalue%, 0)

...and make wValue an INTEGER instead of a WORD in marine.


------------------
Tom Hanlin
PowerBASIC Staff

[Clay Clear]

Thanks for your replies, folks! I've got the info I need, now.

Thanks, again.

Regards,


------------------
Clay C. Clear

Clay Clear's Software
mailto:[email protected][email protected]</A>

[John Petty]

If you suspect the value is a pointer and dont want to GPF then you can use the following function
DECLARE FUNCTION IsBadWritePtr LIB "KERNEL32.DLL" ALIAS "IsBadWritePtr" (BYVAL lp AS LONG, BYVAL ucb AS LONG) AS LONG
lp is memory address, ucb is size of memory in bytes to check (should be at least 1).
Returns 0 if valid pointer


------------------