How to create a TIFF file with tiles

Question

I would like to create a large TIFF RGBA image using tiles. I wrote the following program which loops over the positions of the tiles, but the resulting image is always transparent. Why ?

#include <math.h>
#include <stdlib.h>
#include "tiffio.h"
int main(int argc,char** argv)
{
uint32 width=400, height=400;
uint32 tileWidth=16, tileHeight=16;
TIFF* tif = TIFFOpen("foo.tif", "w");
int i,j,x;

int spp=4;
int bps = 8;
TIFFSetField(tif, TIFFTAG_IMAGEWIDTH, width);
TIFFSetField(tif, TIFFTAG_IMAGELENGTH, height);
TIFFSetField(tif, TIFFTAG_SAMPLESPERPIXEL,spp);//R G B A
TIFFSetField(tif, TIFFTAG_BITSPERSAMPLE, bps);
TIFFSetField(tif, TIFFTAG_TILEWIDTH, tileWidth);
TIFFSetField(tif, TIFFTAG_TILELENGTH, tileHeight);
TIFFSetField(tif, TIFFTAG_PLANARCONFIG, PLANARCONFIG_CONTIG);
TIFFSetField(tif, TIFFTAG_ORIENTATION, ORIENTATION_TOPLEFT);
int tilesize = tileWidth * tileHeight * bps/8 *  spp;
uint8 *buf = malloc(tilesize);

for ( j = 0; j < width; j += tileWidth) {
    for ( i = 0; i < height; i += tileHeight)
        {
           for ( x = 0; x < tilesize; x+=spp)
            {
                uint8 a = 100;
                buf[x+0] =a;
                buf[x+1] =a;
                buf[x+2] =a;
                buf[x+3] =a;
            }
        if(TIFFWriteTile(tif, buf, i, j, 0, 0)<0) exit(-1);
        }
    }
free(buf);
TIFFClose(tif);
return 0;
}