How can I perform pointer arithmetic on an array of structures returned from a shared library in MATLAB 7.8 (R2009a)?

I use LOADLIBRARY, CALLLIB, and LIBPOINTER to obtain a pointer to an array of structures from an external library. I wish to iterate through each element in the array using pointer arithmetic. The following code worked in R2007a:

msgSizePtr   = libpointer('int32Ptr',  0);   %size of structure in bytes

msgPtr = calllib('TestDLL', 'Get_report_Messages', msgCountPtr, msgSizePtr, rc );

ptr = msgPtr

for j = 1:msgCountPtr.value

     ptr.value

     ptr = ptr + msgSizePtr.value;

end

When running this same code in MATLAB R2008a or later, I get the following error or a SEGV:

 ??? Can not offset a pointer to be greater then the size of the data pointed to

 Error in ==> testdllprog at 38

    ptr = ptr + msgSizePtr.value;

Answer 1

[MathWorks Support Team]

There has been a change in how MATLAB handles pointer arithmetic. Starting in R2009a, you no longer have to perform byte arithmetic to increment a pointer; instead, you can use standard pointer arithmetic. In other words, the following code will succeed on R2009a:

msgSizePtr   = libpointer('int32Ptr',  0);   %size of structure in bytes

msgPtr = calllib('TestDLL', 'Get_report_Messages', msgCountPtr, msgSizePtr, rc );

ptr = msgPtr

for j = 1:msgCountPtr.value

     ptr.value

     ptr = ptr + 1;

end

In R2008a and R2008b, neither of these approaches will work. There are no workarounds for these versions.