Calling C Functions in existing DLL's from Java

Hi Guys ,
The tutorial in this site talks about creating ur own DLL's and then calling them from Java . I need to call C functions in existing DLL's from Java . How do I go about doing this ? . Any help on this would be much appreciated.
regards
murali

What you are interested in can be done with what's called "shared stubs", from the JNI book (http://java.sun.com/products/jdk/faq/jnifaq.html), although you don't need the book to do it (I didn't).
The example code will call functions with any number and kind of parameters, but doing that requires some assembly language. They supply working examples for Win32 (Intel architecture) and Solaris (Sparc).
If you can limit yourself to functions to a single function signature (number and types of parameters), or at least a small set that you know you'll call at compile time, you can modify the example so that the assembly language part isn't needed, just straight C code.
Then you'll have one C library that you compile and a set of Java classes and you can load arbitrary functions out of arbitrary dynamic libraries. In my case you don't even have to know what the libraries and functions are ahead of time, the user can set that up in a config file.
You mentioned doing this with Delphi. One thing to watch out for is C versus Pascal (Win32) function calling convention. A good rule of thumb; if it crashes hard, you probably picked the wrong one, try the other. :-)

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"rsam" wrote in message
news:[email protected]..
> Thx Wiebe,
>
> did you load both dll at the same time? For example in 1 vi. Somehow
> the first loaded function keeps to overrule the second. Notice that
> the interface is
exactly the same.
>
> I loaded 2 dll created in Labview with results described above.
>
> Regards Ruud

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rjpierce wrote:
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Hello everyone,
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Solved!
Go to Solution.

There is no need for the dllexport stuff. There is also the option for a static library without any DLL. But the 'extern "C"' is essential, because it forces the C++ compiler, which was probably used to compile the library , to use C calling convention.
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Solved!
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rolfk wrote:
Well somebody using your library should not have to dig into your VIs and do all this on his own. Instead your library should wrap that and hide the troubles of this entirely.
That is EXACTLY what I want to do.
The error cluster was added when the dynamic path option was added. It is not useful to hide that error output so it is always there. Together with the dynamic path there was improved error handling added to the CLN. One of them is that the level of error checking during the function call (exception handling) can be specified. I would assume that some of those options can generate an error code instead of popping up a dialog as they did before and for that the error code output can be useful even in the static call case.
As to what you want to do, I would long ago
I would long ago have done a lot of things differently on this project -- but no one asked me then :-) (Good thing it wasn't me, or maybe I'd have taken offense ;-) )
have handled that with a wrapper DLL that has basically the same functions as your other DLLs and some initiliasation function that returns a pointer to a function dispatch structure based on the actual DLL you want to call.
OK, I am getting the feeling that this will be a brilliant and elegant solution when I fully understand it. I understand what a wrapper DLL is. But do you mean that each version of my product would include a different wrapper DLL (with the same name so that CLN would always work), or the same one and it would somehow be told which actual DLL I want to call? Who would decide which DLL, the VI or the wrapper DLL?
By a function dispatch structure do you mean a table of function pointers gotten by GetProcAddress, one such entry for each function?
Quite like what an object oriented function dispatch table is. Then when initilising your interface you call that initialise function and specify the interface/device type you want to use and after that all the other functions take one additional function dispatch pointer parameter as first parameter in addition to the parameters the actual function has. This function dispatch pointer would be just a pointer to a structure containing the table of function pointers for that interface and for the sake of LabVIEW would be simply a pointer sized integer.
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I think I understand what the wrapper functions need to do. But I am missing some pieces for the initialize function. It would have to do LoadLibrary and lots of GetProcAddresses -- that much I get. But I still don't understand who decides which DLL. Maybe you mean that the wrapper DLL's initialize function will get the DLL name and path from the registry, load it, try a different path if the load fails, etc. Is that what you mean? That could work very nicely. But from where would it get the registry key? From the VI? This is the part that would clearly be different for each product. I suppose that you had something in mind when you described your solution.
This is some C programming
Great! That's what I do for a living. At least I know how to do that! Hey, I should have thought of this idea!
and might require some planning and desigining of the different interfaces to facilitate this kind of dispatch technique but it will for sure pay of in the long run, and make your library even usable in earlier LabVIEW versions, as well as VB etc. without tricky dynamic loading in the high level programming environment.
Ah, see how much I don't know in LabVIEW? I didn't know this was a new feature...
Thank you for the great idea.
Batya Perlman

Can i use malloc function while using DLL created from C program

Hi,
I have written a simple program in C , which uses malloc function for allocating memory for the array.When iam calling that DLL using call library function node in labview, it is not giving any output, my doubt is can we use malloc function in C program from which we will create DLL and is called in labview? I am attaching my C code and DLL and also labview file please tell me where i am doing wrong?
Thanks & Regards,
Vishnu
Attachments:
first.zip ‏1010 KB

Hi Vishnu,
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How to call a function of a dll

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