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C Mex crashes if array is too large

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JohnDapper
JohnDapper on 3 Apr 2016
Edited: James Tursa on 5 Apr 2016
Hi all,
I am passing an array to a mex file as follows:
float *var;
var = (float*)mxGetPr(prhs[0]);
The mex file compiles/work perfect when the input is "small" (a few GB for my application), but when the input is larger, it crashes and gives me a segmentation fault. The input is held in memory (loaded into matlab workspace), and furthermore, the mex file shouldn't make a copy of the array (I'm just pointing to the array which is already in memory). Not only that, but I only need to access few elements of the input array at a time.
Any ideas on what could be causing this behavior?
One obvious candidate: I'm not compiling with -largeArrayDims (didn't update my code for 64-bit API). Might this be an issue?
  3 Comments
James Tursa
James Tursa on 5 Apr 2016
Likely. -largeArrayDims controls whether mwSize is an int or a size_t. If you were using an int for the index and it wrapped around to a negative value then of course you would be pointing to invalid memory hence a crash. Keep in mind, however, that since mwSize is now likely a size_t in your code you need to be careful how you are doing your if-testing since size_t is unsigned. Are you actually "casting" an int to a size_t? I assume you meant typing the variable as a mwSize, not casting.

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Answers (1)

Dave Behera
Dave Behera on 5 Apr 2016
I think you should definitely try recompiling with -largeArrayDims. Also, paste a snippet of your code here.

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