libusb源码学习：list_entry

雨点打透心脏的1/2处 2023-06-29 13:59 19阅读 0赞

USB开发涉及到libusb，看了下源码，现在已经到了1.0.23版；全部用的C，编译器和平台的普适性相对比较强；

其中的list\_head作为关键链表，也是各个链接结构的成员（member）变量。而利用这个成员变量完成各个结构链接，涉及到几个很有意思的宏，其中包括list\_entry，list\_for\_each\_entry，这种用法和我们最普通的用法有点区别：在C ++中，通常我用标准std::list, std::vector实现；在C中，我们往往直接使用结构自自身的指针来完成链接，而不是使用这种成员链接指针(list\_head)。

写了个list\_entry应用原理的解析，用3种完全等价的方式找到：dev->session\_data == 2

#include <iostream>
    
    struct list_head {
        struct list_head* prev, * next;
    };
    
    /* Get an entry from the list
     *  ptr - the address of this list_head element in "type"
     *  type - the data type that contains "member"
     *  member - the list_head element in "type"
     */
    #define list_entry(ptr, type, member) \
    	((type *)((uintptr_t)(ptr) - (uintptr_t)offsetof(type, member)))
    
    
    #define list_first_entry(ptr, type, member) \
    	list_entry((ptr)->next, type, member)
    
     /* Get each entry from a list
      *  pos - A structure pointer has a "member" element
      *  head - list head
      *  member - the list_head element in "pos"
      *  type - the type of the first parameter
      */
    #define list_for_each_entry(pos, head, member, type)			\
    	for (pos = list_entry((head)->next, type, member);		\
    		 &pos->member != (head);				\
    		 pos = list_entry(pos->member.next, type, member))
    
    #define list_for_each_entry_safe(pos, n, head, member, type)		\
    	for (pos = list_entry((head)->next, type, member),		\
    		 n = list_entry(pos->member.next, type, member);	\
    		 &pos->member != (head);				\
    		 pos = n, n = list_entry(n->member.next, type, member))
    
    #define list_empty(entry) ((entry)->next == (entry))
    
    struct libusb_device {
    	int refcnt;	
    	uint8_t bus_number;
    	uint8_t port_number;
    	struct libusb_device* parent_dev;
    	uint8_t device_address;
    	uint8_t num_configurations;
    	
    	struct list_head list;
    	unsigned long session_data;	
    };
    
    libusb_device* list_entry_func(list_head* ptr) {
    	uintptr_t upt = offsetof(libusb_device, list);
    	upt = (uintptr_t)(ptr) - upt;
    	return (libusb_device*)(upt);
    }
    
    int main() {
    	libusb_device ld1;
    	libusb_device ld2;
    	libusb_device ld3;
        ld1.list.next = &ld2.list;
        ld2.list.prev = &ld1.list;
        ld2.list.next = &ld3.list;
        ld3.list.prev = &ld2.list;
    	ld1.session_data = 1;
    	ld2.session_data = 2;
    	ld3.session_data = 3;
    
    	struct list_head usb_devs;
    	usb_devs.next = &ld1.list;
    	usb_devs.prev = &ld3.list;
    
    	struct libusb_device* dev;
    	struct libusb_device* ret = NULL;
    	
    //#define list_entry(ptr, type, member) \
    	//((type *)((uintptr_t)(ptr) - (uintptr_t)offsetof(type, member)))
    //#define list_first_entry(ptr, type, member)               \
    	//list_entry((ptr)->next, type, member)
    //#define list_for_each_entry(pos, head, member, type)		\
    	//for (pos = list_entry((head)->next, type, member);		\
    	//	 &pos->member != (head);				                \
    	//	 pos = list_entry(pos->member.next, type, member))
    
    	list_for_each_entry(dev, &usb_devs, list, struct libusb_device)
    		if (dev->session_data == 2) {
    			std::cout << "found the member in the list: 0 !" << std::endl;
    			break;
    		}
    	
    	for (dev = list_entry_func(usb_devs.next);
    		&dev->list != &usb_devs;
    		dev = list_entry_func(dev->list.next)) {
    		if (dev->session_data == 2) {
    			std::cout << "found the member in the list: 1 !" << std::endl;
    			break;
    		}
    	};
    	
    	for (dev = list_entry(usb_devs.next, libusb_device, list);
    		&dev->list != &usb_devs;
    		dev = list_entry(dev->list.next, libusb_device, list))
    		if (dev->session_data == 2) {
    			std::cout << "found the member in the list: 2 !" << std::endl;
    			break;
    		}
    	return 0;
    }