Welcome to part 3 of the "Strings and Things" series on OpenInsight DLL prototyping. So far we've covered the theory of how Windows defines functions that use ANSI and Unicode versions (in Part 1), and we've also looked at the standard way of calling those functions from Basic+ (in Part 2).

This time we're going to look at another way of passing string data to a Windows API function, but we'll show you how to handle the entire process yourself, and afterwards we'll look at why this is sometimes necessary.

(Note: This article assumes you are familiar with the basics of DLL prototyping in OpenInsight. If not please consult the OpenInsight on-line help for more details)


Passing Basic+ variables as string parameters - The Long Way Round

Previously we looked at a method whereby OpenInsight took care of all the low-level details for you when passing string parameters, but you do have the option to handle this process yourself via the following steps:

1. Prototype the DLL function so that it expects a raw pointer to be passed rather than a Basic+ variable.


2. Ensure that the variable is null-terminated - i.e. that it ends with a Char(0).


3. Encode the Basic+ variable as a Unicode or ANSI string depending on the function you are passing the data to, taking into account your application's UTF8 mode.


4. Get a pointer to the variable that you want to pass to the DLL function.


5. Invoke the function


6. Clean up any loose ends.

It appears like quite a lot of work but it's not as bad as it looks when you put it into practice.


Creating the LPVOID prototype

The first task you have to do is prototype the DLL function, and for now we'll use the same Unicode SetWindowText function that we used before so you can see the difference. Here's how it's documented:



Which we know is actually means this for the Unicode version:



So far so good but as you're handling the string passing yourself the prototype must be created like this instead:



Notice that for the string parameter we are now using the LPVOID prototype rather than the LPWSTR prototype as we did in Part 2. This means that when we call SetWindowTextW OpenInsight is expecting us to pass a pointer to the string data rather than the string data itself.

Now we have the prototyped function we can look at using it in a program, but as mentioned above we have to do some preparation to the Basic+ variables before they can be used.


Null termination

Strings used in C use a Char(0) as an end of string marker (i.e. it is "null-terminated"), so you must ensure that any strings you pass also follow this convention - simply appending a Char(0) to the end of the string will suffice here.



String Encoding

Before the string is passed to the DLL function you must make sure that it is encoded correctly. OpenInsight provides an easy way to do this via the str_ANSI and str_Unicode functions, which automatically take into account your application's UTF8 setting:



The str_ANSI and str_Unicode functions are very useful and save you a bit of coding, otherwise you would have to do something like this to get a Unicode string:



A note on variable typing

Variables in Basic+ are typeless - i.e. they can change their type at runtime based on the context in which they are used (They are actually very similar to the Variant type in Microsoft COM/OLE programming). While having typeless variables is very convenient from a standard Basic+ programming viewpoint, it's not so helpful when you're dealing with a typed language like C/C++, so you must ensure that any Basic+ variables you pass to a DLL function expecting strings are being held internally by OpenInsight as strings also.

There are several ways this can be done but the most common is to use the concatenation operator and append a null variable like so:



For our purposes in the SetWindowText example the act of appending a Char(0) to the variable we are passing performs any required coercion to a string (as would the Unicode encoding functions too actually).


Getting the pointer

All that remains to do now is obtain a pointer to the OI string so it can be passed to the DLL function, and for this there is the aptly-named GetPointer function:



(You'll notice the use of the LockVariable statement here as well - We're going to fully cover LockVariable later in separate post.)


Putting it altogether

And you're now in a position to finally call your DLL function. As an example look at how you would use all this with SetWindowText:



Clean-up

Any clean-up tasks you may need to perform are somewhat dependant on the actual function called, but the primary cleanup task is to call the UnlockVariable statement if you used the LockVariable statement as you can see in the example above. Another common occurrence is to convert a returned string to the correct OI string type as you can see from the GetWindowText example below:



So, tell me again why I need to do this?

Well, as well as being a good intellectual exercise and helping understand how OpenInsight works behind the scenes there's one important area where the ability to pass a pointer is really critical - and that's when you need to pass a NULL pointer (i.e. the numeric value '0') to a function.

For example many Windows API functions exhibit special behaviour when passed a NULL pointer such as returning the length of a buffer needed to contain a value (e.g. the GetShortPathName function), so it is quite important to be able to do this. However, you cannot to this with the LPWSTR and LPASTR prototypes we looked at in part 2, because they will always pass a pointer to something even if it's a null OpenInsight variable!

e.g. This will NOT work:



Neither will this:



The first example will end up passing a pointer to a Char(0) (i.e. an empty string), while the second example will pass a pointer to an ASCII '0' character.

The proper way to tackle this is:



The best of both worlds

Of course having to do all of this work just to pass a NULL pointer seems slightly unreasonable, so thanks to the magic of DLL function aliasing you can "have your cake and eat it". It's simply a matter of how you prototype the function - you prototype one version to use the LPWSTR/LPASTR prototype, and another to use LPVOID, just ensuring that you give them different names.

E.g. continuing with the GetShortPathName example here's how you would prototype the functions:



Conclusion

That concludes this small series of posts on string handling in DLL Prototyping. In the next post in the series we'll take a closer look at the LockVariable statement and why it is still relevant in 32-bit OpenInsight.

Labels: DLL, DLL Prototyping, OpenInsight, Windows API