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do not understand mxSetData

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gujax
gujax on 8 Mar 2012
Hi, I have declared a pointer to a float
float *myvariable;
I initialize this to
myvariable = mxCreateNumericMatrix(256,1,SINGLE (sorry for not the exact syntax),0);
Then I call a function func(myvariable);
This function does math on myvariable and returns the pointer.
Now if I have to output this variable, I get Matlab crash with this:
mxSetData(plhs[0],(float*)myvariable);
I have tried mxFree before the line but still a problem.
Questions: 1. If the second parameter is void according to definition how come my program compiles fine without error.
2. Why does Matlab crash (implying memory leak)?
3. The other option works and this is
plhs[0] = mxCreateNumericArray(256 matrix of SINGLE 0) and finally myvariable=mxGetData(plhs[0]); func(myvariable);
The reason I do not like this way is
a. I would prefer to run the function first and then assign the return to plhs[0];
b. It is a very ugly way of doing it like this, though it works.
c. I would at some point like to declare "myvariable" to be static so that every time it is requested by Matlab function some.m, I do not initialize it to 0. And I have many such variables to work with. I am updating data every fraction of a second for several minutes.
And I have to work with floats
Thanks

Accepted Answer

Jan
Jan on 8 Mar 2012
mxCreateNumericMatrix creates a Matlab variable and returns a mxArray * pointer to it. This variable has a header, e.g. the type of the values, and contains a pointer to the actual data, see mxGetPr for a DOUBLE array or mxGetData for all other types.
mxSetData injects a pointer to the data manually to the Matlab variable. This overwrites the pointer to the existing values. If you assign an mxArray pointer to the data, a crash is expected.
You have to distinguish between the Matlab arrays and their data:
float *data;
plhs[0] = mxCreateNumericMatrix(...);
data = (float *) mxGetData(plhs[0]);
func(data);
I prefer to use Matlab arrays in the main function (mexFunction) only and call all subfunctions using the C-pointers to the underlying data. Then the mexFunction is the gateway between Matlab and C and it is easy to use the subfunctions in a pure C program also.
  13 Comments
James Tursa
James Tursa on 10 Mar 2012
@gujax: I am never offended in the least by someone pointing out my mistakes. That is how I learn, and I like to learn. Thanks for your gracious regard for my feelings, but it is really not necessary in my case. Post anything you want. (It would not surprise me that I have some errors in my oldest posts)
Regarding what will or will not work in your case, I would suggest you post your code as you currently have it and then we can comment on it. There are subtleties of creating mxArray variables from pieces that are *not* in the documentation ... I simply had to learn them from other posters (e.g. Jan, Bruno, Walter, et al) or from trial and error on my own part. Using the mxCreateETC functions are hard enough for a beginning mex programmer to deal with without venturing into the mxSetData etc stuff, hence the wise advice from Jan to avoid it if you can ... at least for now. But post your code and we can go from there.

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