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passing varibales to mex

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PT
PT 2019-1-8
关闭： MATLAB Answer Bot 2021-8-20
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

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回答（1 个）

Jan
Jan 2019-1-8
编辑：James Tursa 2019-1-8
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.

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