Note that although it is possible to concatenate your data string onto '\n' to generate the format string this can have unintended side-effects if the strings being concatenated contain special characters (i.e. ones which need to be escaped). So the more robust and efficient approach is to provide the data string as an input to fprintf:
fprintf(fid,'%s\n',str)
Note that while you can specify the carriage return and linefeed characters by specifying them both explicitly in the format string (as Ameer Hamza's answer shows), the recommended platform independent approach is to open the file in text mode, which on Windows automatically does this conversion for you:
fid = fopen(..,'wt'); % open in text mode...
fprintf(fid,'%s\n',str) % \r is added automatically!
fclose(fid);
Using text mode means you can write the same code for any OS and not worry about what newlines it uses.
