How to call an external .dll library in MATLAB ?

Hello!

I am quite new at using MATLAB and I am trying to load an external library to MATLAB. One of the function of this library is supposed to return me a value but instead it returned nothing (a libpointer) when I used loadlibray and calllib. So I am wondering if I can call the .dll and .h files in C++ and then call this C++ file in MATLAB (using MEX)? Having no knowledge in C++, any help would be very appreciated :)

Mana

[Guillaume]

So instead of just learning matlab, you want to learn matlab and C++. That makes no sense to me. And why C++ and not C anyway?

In any case, if your dll function returns a libpointer, it's either because you've made a mistake in your loadlibrary call, or it's meant to. If it's meant to and that pointer holds the value you want, you can simply extract it in matlab.

To help you more, we need to know the prototype of the function you're calling (from the h file you passed to |loadlibrary or from the documentation).

[Mana V]

I am using C++ because the library has to be called in that language (in the documentation, specific documents in format .h and .lib are provided by the editor).

How can I extract the value I want in Matlab? Here is the content of my .h file:

// 704IO.h : export header file for the 704IO DLL
//

#pragma once

extern "C" short WINAPI EXPORT PortRead(short Rack, short Port, short Offset);
extern "C" void WINAPI EXPORT PortWrite(short Rack, short Port, short Offset, short Value);

Is it what you wanted to know?

Regards,

Mana

[Walter Roberson]

In a duplicate question, Mana asked:

Hello,

I am currently trying to get the value from a device: it should returns 0 when unactived and a higher number when activated.

The device came along with a library that I used in my MATLAB program using loadlibrary and calllib with the right parameters. This library contains two functions. The first one allows me to send a value to the device and to activate it; I tested it and it works. The second one allows me to know wether or not a part of the device is activated; this is were the value issue happens. Indeed, the only thing I got is a libpointer that contains a voidPtr. I have a .h file in C++ and .dll file and I was told to work with a MEX-file to solve my issue, but I do not know how to proceed. Can you help me to do so or do you have any idea of how to solve my issue?

Cheers,

Mana

[Guillaume]

@Mana,

Your library use the C (not C++) calling convention, as defined by the extern "C"). It can be called by any code that understand that convention, be it written C, C++, C# or many other languages. It can be called straight from matlab, no need for a mex file.

I don't know which of the two functions you're calling but neither of them is defined as returning a pointer of any kind, so you must have made a mistake in your loadlibray call.

Show us the code where you're loading and calling the library.

[Guillaume]

@Walter, I can't see the duplicate. Did you close it?

[Mana V]

Thank you Guillaume. Here is my library loading code:

hfile =(fullfile(matlabroot, 'lib','win32', '704IO.h'));
loadlibrary('704IO',hfile,'alias','libPort');

In the document I read about this library, the 704IO.dll file needs to be placed in the folder C:\Windows\System32. As I doubted I also put 704IO.h file there, but I also put these files in C:\Program Files\MATLAB\R2015a\lib\win32.

Here is my calllib for the function I am failling to call:

value = calllib('libPort','PortRead',1,780,-1);

Where PortRead is the function and the numbers are the required parameters.

Cheers,

Mana

[Guillaume]

and

whos value

says it is a libpointer? That shouldn't be the case. What does

libfunctions('libPort', '-full')

return after you've done the call to loadlibrary?

And if value is a libpointer, what does

get(value)

return?

[Mana V]

libfunctions('libPort','-full')

returns me

>>Functions in library libPort:

>>lib.pointer PortRead(int16, int16, int16)
>>lib.pointer PortWrite(int16, int16, int16, int16)

And with whos value, I have:

voidPtr

    Name      Size            Bytes  Class          Attributes

    value         1x1                    lib.pointer

So it is a libpointer indeed. After calling get(value), I have the following error message:

Error using lib.pointer/get
The datatype and size of the value must be defined before the value can be retrieved.

Error in readTest (line 23)
get(value)

[Mana V]

Thanks to https://www.mathworks.com/matlabcentral/newsreader/view_thread/165683 It works with the following code:

a = 1;
b = 780;
c = -1;
value = calllib('x704IO','PortRead',a,b,c);
setdatatype(value,'uint8Ptr',1);
value.Value;

get(value);

However, it works randomly: if I run the function several times, "value"'s value will vary and sometimes MATLAB will crash...

[Ian Peikon]

Hi Mana, I hope you still check this thread. Anyway, we had exactly the same problem with you, with exactly the same library (from MedAssociates). The fix is very simply. All you need to do is modify the header file (I suggest making a separate header file so that your other programs that depend on 704IO.dll, etc. are still working). Here is the offending code:

extern "C" short WINAPI EXPORT PortRead(short Rack, short Port, short Offset);
extern "C" void WINAPI EXPORT PortWrite(short Rack, short Port, short Offset, short Value);

This just needs to be changed, by removing the EXPORT, to:

extern "C" short WINAPI PortRead(short Rack, short Port, short Offset);
extern "C" void WINAPI PortWrite(short Rack, short Port, short Offset, short Value);

Just be sure to update the calls to the library to use your new header file (we just named it 704IOb.h).

Hope that helps!