NSString *helloText=@"您好啊！";
        NSError *error;
        if ([helloText writeToFile:@"/tmp/test.txt" atomically:true encoding:NSUTF8StringEncoding error:&error]){
            NSLog(@"writeToFile success");
        }else{
            NSLog(@"writeToFile failed:%@",[error localizedDescription]);
        }
        NSString *readText=[[NSString alloc] initWithContentsOfFile:@"/tmp/test.txt" encoding:NSUTF8StringEncoding error:NULL];
        NSLog(@"%@",readText);

        NSURL *url=[NSURL URLWithString:@"http://www.baidu.com"];
        NSURLRequest *request=[NSURLRequest requestWithURL:url];
        NSError *error;
        NSData *data=[NSURLConnection sendSynchronousRequest:request returningResponse:NULL error:&error];
        if (!data){
            NSLog(@"fetch failed with error %@",[error localizedDescription]);
        }else{
            if ([data writeToFile:@"/tmp/test1.txt"
                      options:NSDataWritingAtomic
                      error:&error]){
                NSLog(@"write success");
            }else{
               NSLog(@"write failed with error %@",[error localizedDescription]);
            }
        }

        NSData *readData=[NSData dataWithContentsOfFile:@"/tmp/test1.txt"];
        NSLog(@"file size is %ld",[readData length]);