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user/include/linux/list.h this declaration:
#define list_entry(ptr, type, member) \ ((type *)((char *)(ptr) – (unsigned long)(&((type *)0)->member))) can somebody please explain what is this and how does it work, thanks in advance
P.S. please simplify your answer as much as possible, I know about threads, processes in Linux, now I'm exploring possibilities and I'm a little bit stuck with this one.
981
The list_entry() macro helps you to convert data_ptr to a pointer to the struct container value that holds the struct data value, pointed to by ptr : struct container *cont_ptr = list_entry(data_ptr, struct container, data_item);
The simplest way to iterate over a list is with the list_for_each() macro. The macro takes two parameters, both list_head structures. The first is a pointer used to point to the current entry; it is a temporary variable that you must provide. The second is a list_head in the list you want to traverse.
The 'list.h' kernel header file defines the -only- acceptable linked list implementation for the Linux kernel (as per kernel.org folks). In tern, 'list.h' includes 'types.h', which define the following: struct list_head { struct list_head *next, *prev; }
container_of(ptr, type, member) member – the name of the member within the struct. It returns the address of the container structure of the member.
Consider two structs like this:
struct data { int something; }; struct container { int something_before; struct data data_item; int something_after; }; Assume you have a pointer to a struct data value:
struct data *data_ptr; The list_entry() macro helps you to convert data_ptr to a pointer to the struct container value that holds the struct data value, pointed to by ptr:
struct container *cont_ptr = list_entry(data_ptr, struct container, data_item); The macro works by computing the offset of data_item inside the struct container, and subtracting that many bytes from the data_ptr pointer. This, when cast to struct container *, gives a valid pointer to the struct container that holds this particular struct data "inside".
The macro can also be simplified a bit by using the builtin offsetof() macro:
#define list_entry(ptr, type, member) \ ((type *)((char *)(ptr) – offsetof(type, member))) If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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