sizes mismatch error with reduction variable inside parfor inside mex function

Minimal example:

function [A] = mexwoe(n,a,b)

A = zeros(a,b);

parfor i = 1:n

      B = rand(a,b);
      A = A + B;

end

Run as normal function:

>> mexwoe(4,2,3)

ans =

      1.6644    2.6927    3.3542
      1.6938    2.4979    1.1478

Then, successfully compile into a C++ mex function called mexwoe_mex: MEX configured to use 'Microsoft Visual C++ 2013 Professional (C)' for C language compilation.

n, a, and b are defined as double scalars in the Coder app.

>> coder -build mexwoe.prj
Code generation successful: View report

But,

>> mexwoe_mex(4,2,3)
Sizes mismatch: [0][0] ~= [2][3].
More information

Error in mexwoe (line 10)
    A = A + B;

"More information" links to the help topic "Incompatibilities with MATLAB in Variable-Size Support for Code Generation", but I don't understand how anything listed there applies to this example?

Possibly useful info: Matlab 2014a. Windows 7 x64. 32GB RAM. Intel i7 CPU. I don't seem to have other issues with compiling and using mex functions containing parfor loops - it seems to be the reduction variable in the parfor. If I change to "for" instead of "parfor", the mexed version then works. Without the reduction statement, it works. It seems to think A is empty - why? Thanks for any help!

Answer 1

[John Elliott]

As a workaround for this problem, you could try moving the sum operation into a sub-function.

Instead of

A=A+B;

use

A=f(A,B);

where f is defined as

function A=f(A,B)

A=A+B;

[Rudi]

I can confirm that this works - thanks a lot. I'll wait to "accept" just in case anyone has an explanation for this apparent bug in either Matlab or Microsoft Visual C++ 2013.