Mex file gives segmentation fault only on second use - matlab

Question

The following code compiles successfully and returns the correct results when called the first time. Making the same call a second time, I get a segmentation fault error.
//% function TF = InWindow(Date,WindowStartDates,WindowEndDates,EndHint)
//% INWINDOW returns true for window that contains Date. All inputs must be
//% uint32 and WindowEndDates must be sorted.
//% EndHint is an optional input that specifies the row number to start
//% searching from.
#include "mex.h"
#include "matrix.h"
#include "math.h"
void CalculationRoutine(mxLogical *ismember, uint32_T *Date, uint32_T *WindowStartDates, uint32_T *WindowEndDates, unsigned int *StartIndex, int NumObs) {
mwIndex Counter;
// Find the first window that ends on or after the date.
for (Counter = (mwIndex) *StartIndex; Counter < NumObs; Counter++) {
if (*Date <= *(WindowEndDates+Counter)) {
break;
}
}
*StartIndex = (unsigned int) Counter;
// Now flag every observation within the window. Remember that WindowStartDates
// is not necessarily sorted (but WindowEndDates is).
for (Counter = (mwIndex) *StartIndex; Counter < NumObs; Counter++) {
if (*Date >= *(WindowStartDates+Counter)) {
*(ismember+Counter) = true;
}
}
}
void mexFunction( int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[] ) {
mxArray *ismember;
unsigned int *StartIndex;
//Input Checking.
if (nrhs == 3) {
// Default Hint to first entry.
mexPrintf("SI Starts OK.\n");
*StartIndex = 1;
mexPrintf("SI Ends OK.\n");
} else if (nrhs == 4) {
if (!mxIsUint32(prhs[3])) {
mexErrMsgTxt("EndHint must be uint32.");
}
StartIndex = mxGetData(prhs[3]);
} else {
mexErrMsgTxt("Must provide three or four input arguments.");
}
// Convert the hint to base-zero indexing.
*StartIndex = *StartIndex - 1;
// Check the inputs for the window range.
if (!mxIsUint32(prhs[0])) {
mexErrMsgTxt("DatesList must be uint32.");
}
if (!mxIsUint32(prhs[1])) {
mexErrMsgTxt("WindowStartsDates must be uint32.");
}
if (!mxIsUint32(prhs[2])) {
mexErrMsgTxt("WindowEndsDates must be uint32.");
}
if (mxGetM(prhs[1]) != mxGetM(prhs[2])) {
mexErrMsgTxt("WindowStartDates must be the same length as WindowEndDates.");
}
// Prepare the output array.
ismember = mxCreateLogicalArray(mxGetNumberOfDimensions(prhs[1]), mxGetDimensions(prhs[1]));
CalculationRoutine(mxGetLogicals(ismember),mxGetData(prhs[0]),
mxGetData(prhs[1]), mxGetData(prhs[2]), StartIndex, (int) mxGetM(prhs[1]));
plhs[0] = ismember;
}
I call it with:
>>InWindow(uint32(5),uint32((1:6)'),uint32((3:8)'))
The code reaches the line between the two mexPrintf calls before the segmentation fault (ie the first call prints, but not the second).
I am on Matlab 2007a (yes, I know), Win7 64bit and VS 2008.

Answer 1

You need to initialize the pointer StartIndex - you're "lucky" that it works the first time, since it is not pointing to a defined memory location. Why not do something more like:
unsigned int StartIndex;
// and either:
StartIndex = 1;
// or:
StartIndex = * (static_cast< unsigned int * >( mxGetData(prhs[3]) ) );