TIFFFillStripPartial( TIFF *tif, int strip, tmsize_t read_ahead, int restart )
{
	static const char module[] = "TIFFFillStripPartial";
	register TIFFDirectory *td = &tif->tif_dir;
        tmsize_t unused_data;
        uint64 read_offset;
        tmsize_t cc, to_read;
        /* tmsize_t bytecountm; */
        
        if (!_TIFFFillStriles( tif ) || !tif->tif_dir.td_stripbytecount)
            return 0;
        
        /*
         * Expand raw data buffer, if needed, to hold data
         * strip coming from file (perhaps should set upper
         * bound on the size of a buffer we'll use?).
         */

        /* bytecountm=(tmsize_t) td->td_stripbytecount[strip]; */
        if (read_ahead*2 > tif->tif_rawdatasize) {
                assert( restart );
                
                tif->tif_curstrip = NOSTRIP;
                if ((tif->tif_flags & TIFF_MYBUFFER) == 0) {
                        TIFFErrorExt(tif->tif_clientdata, module,
                                     "Data buffer too small to hold part of strip %lu",
                                     (unsigned long) strip);
                        return (0);
                }
                if (!TIFFReadBufferSetup(tif, 0, read_ahead*2))
                        return (0);
        }

        if( restart )
        {
                tif->tif_rawdataloaded = 0;
                tif->tif_rawdataoff = 0;
        }

        /*
        ** If we are reading more data, move any unused data to the
        ** start of the buffer.
        */
        if( tif->tif_rawdataloaded > 0 )
                unused_data = tif->tif_rawdataloaded - (tif->tif_rawcp - tif->tif_rawdata);
        else
                unused_data = 0;
        
        if( unused_data > 0 )
        {
		assert((tif->tif_flags&TIFF_BUFFERMMAP)==0);
                memmove( tif->tif_rawdata, tif->tif_rawcp, unused_data );
        }

        /*
        ** Seek to the point in the file where more data should be read.
        */
        read_offset = td->td_stripoffset[strip]
                + tif->tif_rawdataoff + tif->tif_rawdataloaded;

        if (!SeekOK(tif, read_offset)) {
                TIFFErrorExt(tif->tif_clientdata, module,
                             "Seek error at scanline %lu, strip %lu",
                             (unsigned long) tif->tif_row, (unsigned long) strip);
                return 0;
        }

        /*
        ** How much do we want to read?
        */
        to_read = tif->tif_rawdatasize - unused_data;
        if( (uint64) to_read > td->td_stripbytecount[strip] 
            - tif->tif_rawdataoff - tif->tif_rawdataloaded )
        {
                to_read = (tmsize_t) td->td_stripbytecount[strip]
                        - tif->tif_rawdataoff - tif->tif_rawdataloaded;
        }

	assert((tif->tif_flags&TIFF_BUFFERMMAP)==0);
        cc = TIFFReadFile(tif, tif->tif_rawdata + unused_data, to_read);

        if (cc != to_read) {
#if defined(__WIN32__) && (defined(_MSC_VER) || defined(__MINGW32__))
                TIFFErrorExt(tif->tif_clientdata, module,
                             "Read error at scanline %lu; got %I64u bytes, expected %I64u",
                             (unsigned long) tif->tif_row,
                             (unsigned __int64) cc,
                             (unsigned __int64) to_read);
#else
                TIFFErrorExt(tif->tif_clientdata, module,
                             "Read error at scanline %lu; got %llu bytes, expected %llu",
                             (unsigned long) tif->tif_row,
                             (unsigned long long) cc,
                             (unsigned long long) to_read);
#endif
                return 0;
        }
        
        tif->tif_rawdataoff = tif->tif_rawdataoff + tif->tif_rawdataloaded - unused_data ;
        tif->tif_rawdataloaded = unused_data + to_read;

        tif->tif_rawcp = tif->tif_rawdata;
                        
        if (!isFillOrder(tif, td->td_fillorder) &&
            (tif->tif_flags & TIFF_NOBITREV) == 0) {
		assert((tif->tif_flags&TIFF_BUFFERMMAP)==0);
                TIFFReverseBits(tif->tif_rawdata + unused_data, to_read );
	}

        /*
        ** When starting a strip from the beginning we need to
        ** restart the decoder.
        */
        if( restart )
                return TIFFStartStrip(tif, strip);
        else
                return 1;
}

The product divides a value by zero.