[gdal-dev] RasterBand::IWriteBlock

Jerry Tol jertol52 at gmail.com
Mon Dec 14 09:52:02 PST 2015

I'm implementing a gdal raster driver for an internal file format and am
getting results I do not understand. The output is a binary raster of
32-bit floating point data, but the resulting file contains single bytes of
value 0x0D at seemingly random intervals throughout the data.

Within my implementation of IWriteBlock, I see that the buffer pointed to
by pImage *does not* contain these 0x0D's, however after calling VSIFWrite the
output file does indeed have these individual bytes scattered throughout.
In all cases, I am creating a new output file (not overwriting an existing
file). Sometimes the 0D's occur in the middle of a 4-byte float, others are
between two 4-byte float sequences. I cannot figure how/when/why these
random 0D's are occurring in my output.

Thank you for any help provided. Here is my implementation of IWriteBlock

CPLErr RLYRRasterBand::IWriteBlock(int nBlockXOff, int nBlockYOff, void *pImage)
	RLYRDataset *poGDS = (RLYRDataset *)poDS;
	if (poGDS->fp == 0) {
		CPLError(CE_Failure, CPLE_FileIO, "IWriteBlock: fp is NULL");
		return CE_Failure;
	nBlockYOff = poDS->GetRasterYSize() - nBlockYOff - 1;

	size_t cdtSize = GDALGetDataTypeSize(GetRasterDataType()) / 8;
	size_t nSeek = RLYRMeta::offset_data +
		cdtSize * (nBlockXOff * nBlockYSize + nBlockYOff * nBlockXSize);
	int zeeksez = VSIFSeek(poGDS->fp, nSeek, SEEK_SET);
	if (zeeksez != 0) {
		CPLError(CE_Warning, CPLE_FileIO, "VSIFSeek returns %d", zeeksez);
		return CE_Failure;

	size_t nBlockLen = nBlockXSize * nBlockYSize;
	size_t nWrit = VSIFWrite(pImage, cdtSize, nBlockLen, poGDS->fp);
	if (nWrit != nBlockLen) {
		CPLError(CE_Warning, CPLE_FileIO, "VSIFWrite returns %d", nWrit);
		return CE_Failure;

	// test code - 'foo' does not exhibit bad data but the output file does
	//if (nBlockYOff == 1130) {
	//	FILE *tfp = VSIFOpen("h:\\rasters\\o\\foo", "wb");
	//	size_t nWrit2 = fwrite(pImage, cdtSize, nBlockLen, tfp);
	//	VSIFClose(tfp);

	return CE_None;
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