Well, the pointer is at the beginning of the file and that's where the data is written. So the functions behave as they should.
If you wanted to attach some data at the end of the file, you could use the permission 'a+'. In case you want to extend the file at the beginning, I think you need to create an entire new file:
readID=fopen('XYZ.c','r');
writeID=fopen('NEW.c','w');
fprintf(writeID,'%s\n','XYZ');
fwrite(writeID,fread(readID));
fclose(readID);
fclose(writeID);
In principle, you can of course first read the file and then overwrite it (so both files are the same):
fileID=fopen('test.txt','r+');
data=fread(fileID);
frewind(fileID);
fprintf(fileID,'%s\n','XYZ');
fwrite(fileID,data);
fclose(fileID);
edit test.txt
