Can I pass a pointer to a matrix created in MATLAB to C via a MEX file and change the data in it?

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Tommy Thomasson
Tommy Thomasson 2018-3-25
评论： James Tursa 2018-4-23
In MATLAB I would like to create an 1xN array of zeros, pass it to a C routine as an input argument, have the C routine update the (type double) values in the array (matrix in MATLAB). I would like to do this instead of having the MEX routine return a matrix (as is done in the example arrayProduct.c)
Is this possible, and if so, does anyone have any examples they would be willing to share?

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Tommy Thomasson
Tommy Thomasson 2018-3-25
Make the second to last line in step 2 this and it will work:
workOnExistingMatrix(mxGetPr(prhs[0]));
Here's the corrected MEX file:
//This is the MEX file (written in C)
#include "mex.h"
void workOnExistingMatrix(double* aMatrixCreatedInMatlab) {
aMatrixCreatedInMatlab[0] = 0.1234;
}
//The gateway function
void mexFunction( int nlhs, mxArray *plhs[],
int nrhs, const mxArray *prhs[]) {
//This works =)
workOnExistingMatrix(mxGetPr(prhs[0]));
}
  1 个评论
Jan
Jan 2018-3-26
Be careful.
a = 1:10;
b = a;
yourMexFcn(b);
disp(a)
disp(b)
Both arrays are modified! A more evil example:
function main
a = zeros(1, 10);
subfunction(a);
disp(a)
end
function subfunction(b)
yourMexFcn(b)
disp(b);
end
There is no chance to see in the main function, why a(1) is modified. Do not underestimate the effects of shared data copies.

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更多回答（1 个）

Jan
Jan 2018-3-25
编辑：Jan 2018-3-25
This works directly, but it can have side-effects, because Matlab uses a copy-on-write strategy and shared data copies:
a = rand(100,100);
b = a;
Now the data pointer of a and b point to the same block of memory. A deep data copy is performed only, if b is modified:
b(1) = 0;
Now b has its own data.
Now to your question: If you provide a shared data copy to a C-Mex function and modify it, you touch both variables:
a = rand(100,100);
b = a;
YourInplaceOperation(b);
If you set e.g. b(1) to -1, a(1) is -1 also, because both variables share the data.
Therefore you have to check at first, if the input to the C-Mex function is a shared data copy. See James Tursa's FEX submissions:
Note that the method of shared data copies is not documented and the documentation of Matlab states, that it is not supported to modify the input data directly.
[EDITED] Keep in mind, that the input of your MEX can be a temporary variable:
a = rand(100,100);
YourInplaceOperation(a + 1);
Now modifying the input would be meaningless.
  2 个评论
Tommy Thomasson
Tommy Thomasson 2018-3-25
编辑：Tommy Thomasson 2018-3-25
Thanks Jan - that's helpful information, but it's still not clear how to pass a matrix pointer to a MEX function. Below is a simplified code example of what I want to do. For now, assume I'm not concerned about error checking the input arguments in the MEX gateway function.
Step 1: In MATLAB, I would do something like this:
%this is MATLAB code
myMatlabMatrix = zeros(1,5)
%workOnExistingMatrix is a MEX function defined in Step 2
workOnExistingMatrix(myMatlabMatrix)
Step 2: And here's the MEX file:
//This is the MEX file (written in C)
#include "mex.h"
void workOnExistingMatrix(double* aMatrixCreatedInMatlab) {
aMatrixCreatedInMatlab[0] = 0.1234;
}
//The gateway function
void mexFunction( int nlhs, mxArray *plhs[],
int nrhs, mxArray *prhs[]) {
//This doesn't work =(
//I assume I need a cast or a conversion of some sort
workOnExistingMatrix(prhs[0]);
}
Step 3: Back in MATLAB, I would then be able to do this:
>> %now we're back in MATLAB
>> myMatlabMatrix
myMatlabMatrix =
0.1234 0 0 0 0
>>% =)
I need help with the second-to-last line of the code in Step 2. Something else needs to happen here, but I haven't been able to figure out what.

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