passing varibales to mex

Hi,

I have a question regarding MEX memory allocation. I am novice to writing mex files, so I apologize in advance for a very fundamental question.

I have a code which looks something like this -->

double *x
x = mxGetPr(prhs[0]);
blahblah(x) ; // changes value of x
plhs[0] = mxCreateNumericArray(1,dim,mxDOUBLE_CLASS, mxREAL);  // dim is a const size_t
mxSetPr(plhs[0],x);

(I have Matlab17b so I am not using mxSetDoubles)

Does this piece of code make a copy of the variable x or simply assigns the pointer of x into plhs? In my opinion the use of mxCreateNumericArray will make a copy of x. Is my understanding correct?

Also, what is the best way to pass a variable to my mex function, change it, and return it back to Matlab without making a copy in c++.

Thank you

-PT

[OCDER]

Are you trying to do in-place replacement of values of a large array via MEX routines? Take a look at these.

https://www.mathworks.com/help/matlab/matlab_external/avoid-copies-of-large-arrays.html

https://www.mathworks.com/matlabcentral/answers/383188-mex-inplace-change-problems-r2015b-and-later

Answer 1

[Jan]

mxCreateNumericArray creates a new variable. Overwriting the pointer to the reserved memory by mxSetPr will leak the memory and sharing the memory of the input and output will confuse Matlab's memory manager remarkably: It will crash.

To duplicate a variable use mxDuplicateArray. If you are a newcomer, avoid smart inplace techniques at first. They are tricky, not documented and prone to hard errors in the old R2017b API.