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Passing int to mex file

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Fran
Fran on 5 Aug 2014
Commented: Jed on 28 Jan 2020
Hello,
I wrote a MEX routine that calls a C function which uses "int" rather than double.
The way I fetch the array from MatLab is through a command like:
nc = (int *) mxGetPr(prhs[1]);
However, this works only if, when calling the function from MatLab, I do:
function(...,int32(nc),...);
Otherwise, it crashes, as it calculates an index incorrectly, which I then use to access some elements of an array.
I tried passing the array as a double, and then casting it to a new array within the mex file. Like this:
nc_double = (double *) mxGetPr(prhs[1]);
nc *int;
for (int i=0; i<size_nc; n++)
{
nc[i] = (int) nc_double[i];
}
It compiled, but have me an error: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0) using the debugger.
I wonder if there is a more robust way of doing this? Someone on the Matlab side might forget to convert the culprits to int32 and crash the program.
Cheers,
Francesco

  1 Comment

Adam
Adam on 5 Aug 2014
In terms of someone on the Matlab side forgetting to convert the data you could just write a .m wrapper function for the mex which does the conversion and anyone using the code uses that rather than calling the mex directly. That is what myself and colleagues tend to do.

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Accepted Answer

Friedrich
Friedrich on 5 Aug 2014
Edited: Friedrich on 5 Aug 2014
Hi,
have you tried
int a = (int)*mxGetPr(prhs[0]);
This will work for double values only. In order to make this work for all kinf of data you would need to get the classID of the input argument using mxGetClassID and cast accordingly.
Or if you are lazy call back to MATLAB and let it do the cast:
mxArray *lhs[1];
int a;
mexCallMATLAB(1,lhs,1,&(prhs[0]),"int32");
a = *(int*)mxGetData(lhs[0]);
mexPrintf("%d\n",a);

  3 Comments

Fran
Fran on 5 Aug 2014
Hi thanks for the quick answer. I have a question regarding:
int a = (int)*mxGetPr(prhs[0]);
I need an int array, so in your command "a" would just be a scalar, no?
Friedrich
Friedrich on 5 Aug 2014
Right, that would be a scalar only. This wont work for an array. However the mexCallMATLAB approach will work for arrays. Simply use
mxArray *lhs[1];
int* a;
mexCallMATLAB(1,lhs,1,&(prhs[0]),"int32");
a = (int*)mxGetData(lhs[0]);
Jed
Jed on 28 Jan 2020
If you just had a scalar and you didn't know that it was a double, you could also use this:
int a = (int)mxGetScalar(prhs[0]);

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