//一.读取

    //1.以只读方式打开

    NSFileHandle *fh = [NSFileHandle fileHandleForReadingAtPath:txtPath];

    //2.读取所有内容

    NSData *allData = http://www.mamicode.com/[fh readDataToEndOfFile];

    //转化str

    NSString *str = [[NSString alloc]initWithData:allData encoding:NSUTF8StringEncoding];

    NSLog(@"--%@",str );

    //3.读取指定长度

    //(注意:中文占3个字节)

    //(注意:会接着上一次继续读取)

    NSData *subData = http://www.mamicode.com/[fh readDataOfLength:3];

    NSString *str1 = [[NSString alloc]initWithData:subData encoding:NSUTF8StringEncoding];

    NSLog(@"--%@--%ld",str1,str1.length);

    //4.偏移(定位到某个字节,然后再读)

    [fh seekToFileOffset:4];

    NSData *offData = http://www.mamicode.com/[fh readDataToEndOfFile];

    NSString *str2 = [[NSString alloc]initWithData:offData encoding:NSUTF8StringEncoding];

    NSLog(@"--%@--%ld",str2,str2.length);

    //5.关闭文件

    [fh closeFile];

//二.写入

    //只写

    NSFileHandle *fh = [NSFileHandle fileHandleForWritingAtPath:txtPath];

    //读,写

//    NSFileHandle *fh1 = [NSFileHandle fileHandleForUpdatingAtPath:txtPath];

    //1.设置偏移到最后

    [fh seekToEndOfFile];

    //.注意:如seek到某个字节:是覆盖后面内容,不是插入

    //[fh seekToFileOffset:3];

    //2.写入

    NSData *data = http://www.mamicode.com/[@"writeToText" dataUsingEncoding:NSUTF8StringEncoding];

    [fh writeData:data];

文件句柄NSFileHandle