mIndex += *_retval;
if (NS_SUCCEEDED(rv)) mIndex += *_retval;
    NS_IMETHOD ReadSegments(nsWriteSegmentFun writer, void * closure, PRUint32 count, PRUint32 *_retval) {        PRUint32 readCount = PR_MIN(count, (mSize-mIndex));        *_retval = 0;        nsresult rv = writer (this, closure, mBuffer+mIndex, mIndex, readCount, _retval);        mIndex += *_retval;        return rv;    }