In a Mex file, a for loop goes beyond the relational expression?

I keep getting a crash in a mex file. I have no clue why there is a problem other than using mwSize instead of something else.

here is code cut down to only the problem

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])

{

mwSize t, T;

T = mxGetN(prhs[0]);

for( t = T - 2; t >= 0; t--){

mexPrintf("Smooth %d\n",t);

mexEvalString("drawnow;");

}/*end of t=T-2:0;*/

return;

}

this gets compiled with the -largeArrayDims flag.

then in matlab:

a= zeros(50,200); and run the above on a

I expect the printed t to go from 198 to 0 and then stop. Instead, it runs into negative numbers and doesn't stop unless killed.

Anyone know why?

Answer 1

[James Tursa]

Depending on MATLAB version and platform and compile flags, mwSize can either be an int (a signed integer) or a size_t (an unsigned integer ... key point here). For your case you have mwSize as size_t, an unsigned integer. So t and T are unsigned. Thus the result of the test t>=0 will ALWAYS be true. Once you get past the t=0 case, the expression t-- actually does a wrap-around in modulo arithmetic to get a very large unsigned value (the max possible then it keeps going). Then you print out the results but use the wrong format code for this, %d which is for signed values. So the mexPrintf essentially interpretes this very large unsigned bit pattern (the most significant bit is set) as a signed integer and prints it out.

To demonstrate that mwSize is in fact unsigned you can try this:

if( (t = -1) > 0 ) {
    mexPrintf("mwSize is unsigned\n");
}

[Jan]

I have accepted this answer.