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By Captain C | Tuesday, 15 September 2009 16:28 | 0 Comments
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:

BOOL SetWindowText( HWND hwnd, LPCTSTR lpString );

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

BOOL SetWindowTextW( HWND hwnd, LPCWSTR lpString );

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.

0001     * // Null-terminate the string
0002     strToPass = "Some text" : char( 0 )

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:

0001     declare function str_Unicode
0003     * // Ensure the string we are going to pass is in Unicode
0004     * // format
0006     strToPass = "Some text"
0007     strWide = str_Unicode( strToPass )

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:

0001     declare function ANSI_Unicode, UTF8_Unicode, isUTF8
0003     * // Ensure the string we are going to pass is in Unicode
0004     * // format. This is the long way round and emulates the
0005     * // str_Unicode() function
0007     strToPass = "Some text"
0008     if isUTF8() then
0009        strWide = UTF8_Unicode( strToPass )
0010     end else
0011        strWide = ANSI_Unicode( strToPass )
0012     end

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:

0001     * // Adding X and Y below will produce a numeric result 
0002     * // that will be held internally in the engine as a binary 
0003     * // integer
0005     x = 1
0006     y = 2
0007     z = x + y ; * // z is in a binary numeric format
0009     z := ""   ; * // z is now in a string format
0010               ; * // (the ASCII character "3" )

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:

0001     myVar  = "Some text"
0003      *// Always use LockVariable before GetPointer
0004     lockVariable myVar as CHAR
0006     * // Get the pointer
0007     pMyVar = getPointer( myVar )
0009     * // Use the pointer
0010     call someFunc( pMyVar )
0012     * // Cleanup
0013     unlockVariable myVar

(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:

0001     declare function SetWindowTextW, str_Unicode, get_Property
0003     hwnd    = get_Property( @window, "HANDLE" )
0004     newText = get_Property( @window : ".EDITLINE_1", "TEXT" )
0006     // Null terminate
0007     newText := char( 0 )
0009     // Ensure we have a UNICODE string
0010     newText = str_Unicode( newText )
0012     // Force string type - redundant but make sure
0013     // we're future proof
0014     lockVariable newText as CHAR
0016     // Get a pointer to the string
0017     pNewText = getPointer( newText )
0019     // Invoke the function
0020     x = SetWindowTextW( hwnd, pNewText )
0022     // Cleanup
0023     unlockVariable newText


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:

0001     * // We're using the following User32 DLL prototypes:
0002     * //
0003     * // INT STDCALL GetWindowTextLength( HANDLE )
0004     * // INT STDCALL GetWindowTextW( HANDLE, LPVOID, INT )
0006     declare function getWindowTextLength, getWindowTextW
0007     declare function str_Unicode, unicode_Str
0009     hwnd    = get_Property( @window, "HANDLE" )
0011     * // Find out how much text the window contains in CHARACTERS 
0012     * // (not BYTES) and create a buffer large enough to contain it
0014     textLen = getWindowTextLength( hwnd )
0015     textBuf = str( char(0), textLen + 1 ) ; * // add space for a 
0016                                           ; * // null terminator!
0018     * // Make sure that the buffer contains enough space for
0019     * // Unicode chars as we're calling the "W" function
0020     textBuf = str_Unicode( textBuf )
0022     * // Not needed at the moment but it won't hurt!
0023     lockVariable textBuf as CHAR
0025     * // Get the pointer
0026     pBuf = getPointer( textBuf )
0028     * // Get the window text
0029     x = getWindowTextW( hwnd, pBuf, textLen + 1 )
0031     * // Clean up - we need to ensure the Unicode string we've got 
0032     * // back is translated to UTF8/ANSI
0033     unlockVariable textBuf
0035     * // Convert the string
0036     textBuf = unicode_Str( textBuf )
0038     * // Get the text, removing the null terminator which we don't 
0039     * // need in Basic+ 
0040     winText = textBuf[1,char(0)]

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:

0001     * // We're using the following Kernel32 DLL prototypes:
0002     * //
0003     * // UINT STDCALL GetShortPathNameW( LPWSTR, LPWSTR, UINT )
0005     declare function getShortPathNameW
0007     * // Attempt to get the size of the buffer for the short path
0008     longPath  = "c:\temp\somelongfilename.txt"   
0009     shortPath = ""
0011     bufLen = getShortPathNameW( longPath, shortPath, 0 )
0013     * // and so on ...

Neither will this:

0001     * // We're using the following Kernel32 DLL prototypes:
0002     * //
0003     * // UINT STDCALL GetShortPathNameW( LPWSTR, LPWSTR, UINT )
0005     declare function getShortPathNameW
0007     * // Attempt to get the size of the buffer for the short path
0008     longPath  = "c:\temp\somelongfilename.txt"   
0009     shortPath = 0
0011     bufLen = getShortPathNameW( longPath, shortPath, 0 )
0013     * // and so on ...

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:

0001     * // We're using the following Kernel32 DLL prototypes:
0002     * //
0003     * // UINT STDCALL GetShortPathNameW( LPWSTR, LPVOID, UINT )
0005     declare function getShortPathNameW
0007     * // Attempt to get the size of the buffer for the short path
0008     longPath  = "c:\temp\somelongfilename.txt"   
0010     * // Pass a NULL (0) pointer ...
0011     bufLen = getShortPathNameW( longPath, 0, 0 )
0013     * // and so on ...

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:

0001     * // We're using the following Kernel32 DLL prototypes:
0002     * //
0003     * // UINT STDCALL GetShortPathNameW( LPWSTR, LPWSTR, UINT )
0004     * // UINT STDCALL GetShortPathNameW( LPWSTR, LPVOID, UINT ) 
0005     * //                                 As GetShortPathNameWByPtr
0007     declare function getShortPathNameW, getShortPathNameWByPtr
0009     * // Attempt to get the size of the buffer for the short path
0010     longPath  = "c:\temp\somelongfilename.txt"   
0012     * // Pass a NULL (0) pointer ... 
0013     bufLen = getShortPathNameWByPtr( longPath, 0, 0 )
0015     * // buflen includes space for null-terminator, so create the buffer
0016     shortPath = str( char(0), bufLen )
0018     * // And get the path
0019     x = getShortPathNameW( longPath, shortPath, bufLen )


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.

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