array pointers in matlab

Hi all,

I'm trying to implement a dll that I have created in Labview into Matlab. When the output value is a simple integer or floating point number, this poses no problems. However, when the dll created in Labview exports an array as output, an array pointer is used:

void __stdcall Test(int16_t outputArray[], int32_t len); //Testfunction in header file

My question is how do I read out the data present at this point in memory, in Matlab? As said before, when the output is a single number, I can simply

loadlibrary SharedLib.dll SharedLib.h alias lib

output = calllib('lib', 'Test')

unloadlibrary lib

in Matlab, but this obviously doesn't work now. Any guidance towards a solution would be greatly appreciated.

Best regards,

Sam

[Malcolm Lidierth]

v = zeros(10,1,'uint16');

v = calllib('lib','Test',v,int32(10));

should do the trick. MATLAB creates the libpointer automatically as I recall. On output, v is a int16 array, not a libpointer - hence no v.value property.

Answer 1

[Chirag Gupta]

Hi Sam,

I coded up a sample library with a signature of:

    void TestFunc(int outArray[],int len);

Then to use this, you need to make use of libpointers.

Sample code for using my function was:

loadlibrary('TestShared.dll','TestFile.h','alias','lib')
libfunctionsview lib
var = zeros(10,1);
% Creating a pointer from var of int32 type. In your case it would % be int16
v = libpointer('int32Ptr',var)
v.Value
v = calllib('lib','TestFunc',v,int32(10))
v.Value
unloadlibrary lib

You can find more examples with other types in the documentation

[Kaustubha Govind]

The documentation is here: http://www.mathworks.com/help/techdoc/matlab_external/f42650.html

Answer 2

[Sam Van der Jeught]

Hi Chirag,

Thanks for the fast reply. I implemented your code and adapted it to suit my specifics:

 loadlibrary('SharedLib.dll','SharedLib.h','alias','lib')
libfunctionsview lib
var = zeros(10,1);
% Creating a pointer from var of int32 type. In your case it would % be int16
1
v = libpointer('int16Ptr',var)
2
v.Value
3
v = calllib('lib','Test',v,int32(10))
4
v.Value
unloadlibrary lib

but I end up with the following error when I run the above code in Matlab:

ans =

     1

v =

libpointer

ans =

     2

ans =

      0
      0
      0
      0
      0
      0
      0
      0
      0
      0

ans =

     3

v =

      0
      0
      0
      0
      0
      0
      0
      0
      0
      0

ans =

     4

??? Attempt to reference field of non-structure array.

Error in ==> test at 12 v.Value

Maybe you have an idea of what still goes wrong.

Thanks in advance,

Sam

[Chirag Gupta]

Can you check what exactly the function signature is returning in MATLAB.

libfunctionsview lib should show that.

Also, try whos v to check v's type after the function call.

Answer 3

[Jim Hokanson]

Just in case anyone comes across this, I think you want the following:

For: void __stdcall Test(int16_t outputArray[], int32_t len); //Testfunction in header file

Assuming the library is referred to as 'lib'

len = 10; temp = zeros(1,len,'int16'); ptr = libpointer('int16Ptr',temp);

calllib('lib','Test',ptr,int32(len))

data = get(ptr,'Value')

The key point is that the output is void, so you don't assign an output to calllib, i.e. you don't do:

output = calllib

Since you are working with a pointer you need to go back to the pointer and extract its value.

NOTE: I wasn't sure what the value was, but if you do get(ptr) Matlab will display information about the object.