filewrite不允许读

plus.io.requestFileSystem(plus.io.PRIVATE_WWW, function(fs) {  

                fs.root.getFile("webConfig.txt", {create:false}, function(fentry) {  

                    fentry.createWriter(function(writer) {  
                        writer.onwrite = function(e) {  
                            document.getElementById("ces").innerHTML = "success";  
                        };  
                        writer.onerror = function(e) {  
                            document.getElementById("ces").innerHTML =e.target.error.message;  
                        };  
                        writer.write("cc");  
                    }, function(e) {  
                        document.getElementById("ces").innerHTML = "error";  
                    });  
                });  
            });  

最后报了“不允许读”的错误，是什么原因？FileReader可以正常的读。