C结构体-designated initializer

经常在CU看各位大牛的文章，就目前自己的情况来说，他们的水平对我来说只能是可望而不可及。在此发文，权当巩固每天所学或者不经意间了解的东西，希望自己好好学习，进步快点。若哪里说的不对，请各位指出，以免误导别人，自己也好长点知识。

最近看linux内核和light sensor driver的源码，其中出现最多的应该就是结构体了。可是它的结构体初试化感觉和以前所看的不太一样（本科专业是电子，C语言只看过谭的那本），并没有花时间细抠“为什么在每个成员前面加一个‘.’？”之类的小问题。今天起床无事，突然想起来，便随便查了一下，现在整理下来供自己和正在入门的兄弟姐妹参考。

如light sensor driver里面有如下结构体：

struct i2c_driver {
    unsigned int class;

    /* Notifies the driver that a new bus has appeared or is about to be
     * removed. You should avoid using this if you can, it will probably
     * be removed in a near future.
     */
    int (*attach_adapter)(struct i2c_adapter *);
    int (*detach_adapter)(struct i2c_adapter *);

    /* Standard driver model interfaces */
    int (*probe)(struct i2c_client *, const struct i2c_device_id *);
    int (*remove)(struct i2c_client *);

    /* driver model interfaces that don't relate to enumeration  */
    void (*shutdown)(struct i2c_client *);
    int (*suspend)(struct i2c_client *, pm_message_t mesg);
    int (*resume)(struct i2c_client *);

    /* Alert callback, for example for the SMBus alert protocol.
     * The format and meaning of the data value depends on the protocol.
     * For the SMBus alert protocol, there is a single bit of data passed
     * as the alert response's low bit ("event flag").
     */
    void (*alert)(struct i2c_client *, unsigned int data);

    /* a ioctl like command that can be used to perform specific functions
     * with the device.
     */
    int (*command)(struct i2c_client *client, unsigned int cmd, void *arg);

    struct device_driver driver;
    const struct i2c_device_id *id_table;

    /* Device detection callback for automatic device creation */
    int (*detect)(struct i2c_client *, struct i2c_board_info *);
    const unsigned short *address_list;
    struct list_head clients;
};

struct i2c_driver al3000_driver = {
    .driver = {
        .name    = AL3000_DRV_NAME,
        .owner    = THIS_MODULE,
    },
    .suspend = al3000_suspend,
    .resume    = al3000_resume,
    .probe    = al3000_probe,
    .command = al3000_command,
    .remove    = __devexit_p(al3000_remove),
    .id_table = al3000_id,
};

这种初始化写法就是所谓的C语言标记化结构初始化语法。

采用 .name = value这种形式，虽然没什么，但带来许多好处。举个小例子：

struct student { //结构体名
    char name[20];
    char s;
    int age;
    int score;
};

struct student stu1 ={ //传统初始化
    "x",'m',1,2
};
struct student stu1 ={ //上述标记式初始化
    .score=2,
    .age=1,
    .name="x",
};

通俗总结一下优点：

1.顺序。（不理会结构体成员顺序，允许对结构成员进行重新排列。）

2.个数。（可以有选择性的初始化，不需要针对所有成员都进行初始化。）

3.性能。(将频繁被访问的成员放在相同的硬件缓存行上，将大大提高性能。）

4.扩展性好。（如增加字段时，避免了传统的那种大量修改。）