Why does the FORTRAN MEX API function mxCopyPtrToInteger4 return incorrect results on 64-bit MATLAB?

I am writing a FORTRAN MEX file in which I am using the MEX API function MXCOPYPTRTOINTEGER4 as shown in the line of code below:

        call mxCopyPtrToInteger4( mxGetDimensions( PRHS(2) ), 

       $                                DIMS, NDIMW )

where DIMS is INTEGER*4 and NDIMW is MWSIZE. This works fine on a 32-bit MATLAB.

However, when I recompile and execute this MEX file on 64-bit MATLAB, this code snippet gives me erroneous results. It does not provide the correct dimensions of a 2-D cell array or a 3-D matrix.

Answer 1

[MathWorks Support Team]

This issue arises because of the difference in how the FORTRAN pre-compiler interprets mwPointer on 32-bit and 64-bit systems. The FORTRAN preprocessor converts mwPointer to INTEGER*4 when building binary MEX-files on 32-bit platforms and to INTEGER*8 on 64-bit platforms.

The function mxGetDimensions returns an mwPointer. So when using mxCopyPtrToInteger4 to copy the data, you assume that the array size is INTEGER*4 which is not the case on 64-bit platforms.

As a workaround you can follow one of these options:

1) Use the FORTRAN MEX API function MXCOPYPTRTOPTRARRAY as shown below:

        call mxCopyPtrToPtrArray(mxGetDimensions( PRHS(2) ), 

       $                       DIMSW, NDIMW)

where DIMSW is MWPOINTER and NDIMW is MWSIZE

2) Compile the MEX file using the "-largeArrayDims" flag in the MEX command.