Access violation reading location

I can't find anything wrong with my code. When i debug, this occurs.

Unhandled exception at 0x00fb74b1 in pacman.exe: 0xC0000005: Access violation reading location 0xfdfdfdfd.

Code:

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printf("Lowest F = %d, x:%d, y:%d", lowestf, openX[lowestf], openY[lowestf]);
       
ScanAdjacentNodes(openX[lowestf], openX[lowestf]);

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The printf works fine, some integers are printed.

If i parse values like this there is no problem.

Code:

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ScanAdjacentNodes(5, 5);

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Code:

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int openX[550];
int openY[550];

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Code:

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void ScanAdjacentNodes(int x, int y) {

        if(FREE(x+1, y)) {
                f[index]=hscore(x+1, y);
                openX[index]=x+1;
                openY[index]=y;
                index++;
        } else {
                closedX[j] = x+1;
                closedY[j] = y;
                j++;
        }
        if(FREE(x-1, y)) {
                f[index]=hscore(x-1, y);
                openX[index]=x-1;
                openY[index]=y;
                index++;
        } else {
                closedX[j] = x-1;
                closedY[j] = y;
                j++;
        }
        if(FREE(x, y+1)) {
                f[index]=hscore(x, y+1);
                openX[index]=x;
                openY[index]=y+1;
                index++;
        } else {
                closedX[j] = x;
                closedY[j] = y+1;
                j++;
        }
        if(FREE(x, y-1)) {
                f[index]=hscore(x, y-1);
                openX[index]=x;
                openY[index]=y-1;
                index++;
        } else {
                closedX[j] = x;
                closedY[j] = y-1;
                j++;
        }

}

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What's the signature for ScanAdjacentNodes()? And the definition of the variables? You haven't given enough information.

IIRC, 0xfdfdfdfd means that you're trying to use a freed variable.