Why is mxCreateNumericMatrix maximum size smaller than system maximum array size?

Hi,

I am trying to create a matrix in a MEX function. The following works:

   uint64_t N;
   N = 2147483647; // N = 2*2^30 -1
   plhs[0] = mxCreateNumericMatrix(N,1,MXUINT8_CLASS,mxREAL);

However, I am unable to create an array that is this size:

   uint64_t N;
   N = 2147483648; // N = 2*2^30
   plhs[0] = mxCreateNumericMatrix(N,1,MXUINT8_CLASS,mxREAL);

The preceding code throws the error:

    maximum variable size allowed by the function exceeded

Which is confusing since my system (64-bit Linux running 64-bit Matlab 2010b) tells me the maximum array size is, in fact, very large.

    [~,M] = computer
    M = 
        281474976710655 % 2^48 -1 for those of you keeping track

Furthermore, from the command line, I am able to create very large arrays, and have been quite happily for some time, with calls like the following:

    a = zeros(16*2^30,1,'uint8');
    disp(uint64(numel(a)))
    17179869184

My question is, why am I not able to create arrays in my mex function that I am clearly able to create from the command line, or from other *.m functions?

Thank you.

Answer 1

[Daniel]

The answer lies in the compiler options. By default, Matlab limits the size to 2^31-1. To increase the size, the following option must be included in your mex compile command.

   mex -largeArrayDims myFunction.c