CODEWITHSUNDEEP
cgccmacros
Gaut
Gaut

Reputation: 1397

efficiently get struct member with macro

I am implementing double linked list. I produce the following macro, based on linux kernel list and the macro container_of:

typedef struct  s_list
{
    struct s_list   *prev;
    struct s_list   *next;
}               t_list;

# define LIST_ENTRY(ptr, type, lst_member) \
                (type*)((char*)ptr - offsetof(type, lst_member))

# define LIST_MEMBER(ptr, type, lst_member, member) \
                *(typeof(&(((type*)0)->member))) \
                    (LIST_ENTRY(ptr, type, lst_member) + offsetof(type, member))

I wonder how efficient are those macro. I would use LIST_MEMBER() by defining a new macro depending on my struct, for instance, for instance:

struct  test
{
     void    *ptr;
     t_list  lst;
     double  x;
};

# define TEST_LIST_C2(ptr) LIST_MEMBER(ptr, struct test, list, c2)

I have two questions:

Upvotes: 0

Views: 121

Answers (1)

martinkunev
martinkunev

Reputation: 1405

As explained in C99 7.17.3 offsetof() is a macro, so it's evaluated at compile time. typeof() is a GNU extension, but it's also a macro. Macros are replaced with their values at compile time (the preprocessing phase).

So yes, gcc will compute everything it can at compile time and reduce the expression the way you expect.

Upvotes: 1

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