Why is strtoul returning 0 from a "1" string?

I'm trying to compact a raster file in a way that is easy to read without GDAL library (my web server cannot install GDAL). Following this question, I'm doing the following to convert a raster's bytes (only 0 and 1 values) to bits:

int main(int argc,char *argv[]) {
    if (argc < 3) {
        return 1;
    }
    GDALDataset *poDataset;
    GDALAllRegister();
    poDataset = (GDALDataset*)GDALOpen(argv[1],GA_ReadOnly);
    if (poDataset == NULL) {
        return 2;
    }
    int tx=poDataset->GetRasterXSize(), ty=poDataset->GetRasterYSize();
    GDALRasterBand *poBand;
    int nBlockXSize,nBlockYSize;
    poBand = poDataset->GetRasterBand(1);
    printf("Type: %s\n",GDALGetDataTypeName(poBand->GetRasterDataType()));
    // Type: Byte
    poBand->GetBlockSize(&nBlockXSize,&nBlockYSize);
    int i, nX = tx/nBlockXSize, nY = ty/nBlockYSize;
    char *data = (char*)CPLMalloc(nBlockXSize*nBlockYSize + 1);
    uint32_t out[nBlockXSize*nBlockYSize/32];
    char temp;
    CPLErr erro;
    FILE* pFile;
    pFile = fopen(argv[2],"wb");
    for (y=0; y<nY; y++) {
        for (x=0; x<nX; x++) {
            erro = poBand->ReadBlock(x,y,data);
            if (erro > 0) {
                return 3;
            }
            for (i=0; i<nBlockXSize*nBlockYSize; i+=32) {
                temp = data[i+32];
                data[i+32] = 0;
                out[i/32] = strtoul(&data[i],0,2);
                if (data[i] != 0) {
                    printf("%u/%u ",data[i],out[i/32]);
                }
                data[i+32] = temp;
            }
            ch = getchar(); // for debugging
        }
        fwrite(out,4,nBlockXSize*nBlockYSize/32,pFile);
    }
    fclose(pFile);
    CPLFree(data);
    return 0;
}

After the first set of bytes is read (for (i=0; i<nBlockXSize*nBlockYSize; i+=32)), I can see that printf("%u/%u ",data[i],out[i/32]); is printing some "1/0", meaning that, where my raster has a 1 value, this is being passed to strtoul, which is returning 0. Obviously I'm messing with something (pointers, probably), but can't find where. What am I doing wrong?

strtoul is for converting printable character data to an integer. The string should contain character codes for digits, e.g. '0', '1' etc.

Apparently in your case the source data is actually the integer value 1 and so strtoul finds there are no characters of the expected form and returns 0 .