How to acces lib.pointer in C++ Mex Files

Hello everyone,

I want to access lib.pointer within a mexFile using Matlab's C++ API. The pointer is transported as matlab::data::ArrayType::HANDLE_OBJECT_REF, but I don't know how to access the values within this structure. I want to convert this to a void pointer later. The lib.pointer contains a handle to a device and I am using this to communicate with the device via a separate API.

Hopefully someone can help me further.

Thank you in advance.

Answer 1

[埃博拉酱]

lib.pointer has a Value property. Pass that to your MEX function.

[Richard Z.]

Hi and thanks for your reply.

I thought that passing this value is essentially a 64bit Number and I have to cast this later on to void Pointer? I thought there is a safer or even better way to protect the handle and therefor keep it in this lib.pointer. Passing the handle as a uint64 already works.

Hopefully you or someone can enlight me what is the best way.

Thanks in advance.

[埃博拉酱]

What do you mean by "protect the handle"? What does it mean to "keep it in this lib.pointer"? Passing the Value to a MEX function doesn't mean you're taking it away from lib.pointer as long as you don't delete this object in the MATLAB workspace - it's still in lib.pointer. And lib.pointer is just a container, a segment of memory, containing that value, and it doesn't have any "protect" functionality that I understand.

[Richard Z.]

Hmm ok but matlab is giving an error if I want to access the property:

The datatype and size of the value must be defined before the value can be retrieved.

The datatype of this libpointer is 'voidPtr'. Do you have a tip for me?

Thanks in advance!

[埃博拉酱]

You can study these documents as follows:

https://ww2.mathworks.cn/help/releases/R2024b/matlab/ref/libpointer.html

https://ww2.mathworks.cn/help/releases/R2024b/matlab/matlab_external/passing-arguments-to-shared-library-functions.html

https://ww2.mathworks.cn/help/releases/R2024b/matlab/ref/lib.pointer-class.html