c - USB设备id表理解

Question

我试图了解 linux 内核初始化结构的不同方式。在此查询中，我编写了一个示例 usb 驱动程序，但我不理解某些要点，指出了 ??

之前的注释

static struct usb_device_id pen_table[] =  //?? why pen_table is an array
{
    { USB_DEVICE(0xaaaa , 0x8816) },       //??what type of array initialization is this
    {} /* Terminating entry */             //??how this terminates
};

我尝试以这种方式初始化设备 ID 表，但在接近初始化时出现错误

static struct  usb_device_id pen_table = {
    .match_flags = USB_DEVICE_ID_MATCH_DEVICE,
    .idVendor=0xaaaa,
    .idProduct = 0x8816,
};

Answer 1

您应该手头有 Linux 内核源代码才能真正理解这一点。

• 为什么 pen_table 是一个数组？

在 MODULE_DEVICE_TABLE(参见 Hard time in understanding MODULE_DEVICE_TABLE(usb, id_table) usage)和定义 usb_driver 结构的实例中，它是必需的，参见 http://opensourceforu.efytimes.com/2011/11/usb-drivers-in-linux-2/ .

• 这是什么类型的数组初始化？

USB_DEVICE 是 include/linux/usb.h 中定义的宏:

#define USB_DEVICE(vend, prod) \
    .match_flags = USB_DEVICE_ID_MATCH_DEVICE, \
    .idVendor = (vend), \
    .idProduct = (prod)

• 这是如何终止的？

C 标准说:

The initialization shall occur in initializer list order, each initializer provided for a particular subobject overriding any previously listed initializer for the same subobject; all subobjects that are not initialized explicitly shall be initialized implicitly the same as objects that have static storage duration.

和:

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. If an object that has static storage duration is not initialized explicitly, then:

• if it has pointer type, it is initialized to a null pointer;
• if it has arithmetic type, it is initialized to (positive or unsigned) zero;
• if it is an aggregate, every member is initialized (recursively) according to these rules;
• if it is a union, the first named member is initialized (recursively) according to these rules.

因此，id_table 被定义为一个指针，而不是 usb_driver 中的一个数组:

const struct usb_device_id *id_table;

使用 id_table 的函数不是单独传递数组大小，而是递增指向 id_table 的指针，直到其中一个元素为 NULL。请参阅代表此技术的这个简短示例:

#include <stdio.h>
#include <stdlib.h>

struct small
{
    int a;
    int b;
};

struct big
{
    struct small *s;
};

struct small table[] =
    {
        {1, 1},
        {2, 2},
        {3, 3},
        {}
    };

int main(void)
{
    struct big b = {
        .s = table
    };

    const struct small *s;
        /* traverse through table using pointer arithmetic */
    for (s = b.s; s->a; s++)
        {
            printf("%d\n", s->a);
            printf("%d\n", s->b);
        }

    exit(0);
}

• 我尝试用这种方式初始化设备 ID 表，但我得到了 接近初始化的错误

我不知道，你确定你不是要重新定义 pen_table 吗？什么是错误消息？