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:
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:
INT STDCALL SetWindowTextW( HANDLE, LPVOID )
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 0002 0003 * // Ensure the string we are going to pass is in Unicode 0004 * // format 0005 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 0002 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 0006 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 0004 0005 x = 1 0006 y = 2 0007 z = x + y ; * // z is in a binary numeric format 0008 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" 0002 0003 *// Always use LockVariable before GetPointer 0004 lockVariable myVar as CHAR 0005 0006 * // Get the pointer 0007 pMyVar = getPointer( myVar ) 0008 0009 * // Use the pointer 0010 call someFunc( pMyVar ) 0011 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 0002 0003 hwnd = get_Property( @window, "HANDLE" ) 0004 newText = get_Property( @window : ".EDITLINE_1", "TEXT" ) 0005 0006 // Null terminate 0007 newText := char( 0 ) 0008 0009 // Ensure we have a UNICODE string 0010 newText = str_Unicode( newText ) 0011 0012 // Force string type - redundant but make sure 0013 // we're future proof 0014 lockVariable newText as CHAR 0015 0016 // Get a pointer to the string 0017 pNewText = getPointer( newText ) 0018 0019 // Invoke the function 0020 x = SetWindowTextW( hwnd, pNewText ) 0021 0022 // Cleanup 0023 unlockVariable newText
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:
0001 * // We're using the following User32 DLL prototypes: 0002 * // 0003 * // INT STDCALL GetWindowTextLength( HANDLE ) 0004 * // INT STDCALL GetWindowTextW( HANDLE, LPVOID, INT ) 0005 0006 declare function getWindowTextLength, getWindowTextW 0007 declare function str_Unicode, unicode_Str 0008 0009 hwnd = get_Property( @window, "HANDLE" ) 0010 0011 * // Find out how much text the window contains in CHARACTERS 0012 * // (not BYTES) and create a buffer large enough to contain it 0013 0014 textLen = getWindowTextLength( hwnd ) 0015 textBuf = str( char(0), textLen + 1 ) ; * // add space for a 0016 ; * // null terminator! 0017 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 ) 0021 0022 * // Not needed at the moment but it won't hurt! 0023 lockVariable textBuf as CHAR 0024 0025 * // Get the pointer 0026 pBuf = getPointer( textBuf ) 0027 0028 * // Get the window text 0029 x = getWindowTextW( hwnd, pBuf, textLen + 1 ) 0030 0031 * // Clean up - we need to ensure the Unicode string we've got 0032 * // back is translated to UTF8/ANSI 0033 unlockVariable textBuf 0034 0035 * // Convert the string 0036 textBuf = unicode_Str( textBuf ) 0037 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 ) 0004 0005 declare function getShortPathNameW 0006 0007 * // Attempt to get the size of the buffer for the short path 0008 longPath = "c:\temp\somelongfilename.txt" 0009 shortPath = "" 0010 0011 bufLen = getShortPathNameW( longPath, shortPath, 0 ) 0012 0013 * // and so on …
Neither will this:
0001 * // We're using the following Kernel32 DLL prototypes: 0002 * // 0003 * // UINT STDCALL GetShortPathNameW( LPWSTR, LPWSTR, UINT ) 0004 0005 declare function getShortPathNameW 0006 0007 * // Attempt to get the size of the buffer for the short path 0008 longPath = "c:\temp\somelongfilename.txt" 0009 shortPath = 0 0010 0011 bufLen = getShortPathNameW( longPath, shortPath, 0 ) 0012 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 ) 0004 0005 declare function getShortPathNameW 0006 0007 * // Attempt to get the size of the buffer for the short path 0008 longPath = "c:\temp\somelongfilename.txt" 0009 0010 * // Pass a NULL (0) pointer … 0011 bufLen = getShortPathNameW( longPath, 0, 0 ) 0012 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 0006 0007 declare function getShortPathNameW, getShortPathNameWByPtr 0008 0009 * // Attempt to get the size of the buffer for the short path 0010 longPath = "c:\temp\somelongfilename.txt" 0011 0012 * // Pass a NULL (0) pointer … 0013 bufLen = getShortPathNameWByPtr( longPath, 0, 0 ) 0014 0015 * // buflen includes space for null-terminator, so create the buffer 0016 shortPath = str( char(0), bufLen ) 0017 0018 * // And get the path 0019 x = getShortPathNameW( longPath, shortPath, bufLen )
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.