Get pointer to struct by pointer to its member [duplicate] - c

Question

While looking at Linux kernel's implementation of doubly linked circular lists, I've found following macro:
#define container_of(ptr, type, member) ({ \
const typeof( ((type *)0)->member ) *__mptr = (ptr); \
(type *)( (char *)__mptr - offsetof(type,member) );})
The way this works is that it returns pointer to structure given only address of one of its members:
struct blabla
{
int value;
struct list_head *list;
}
Thus you can get pointer to blabla (and get to "value") given only pointer to list.
To my question, how would I make this as portable as possible (best case conforming to C89/C99?). Due to usage of typeof(), this is gcc only.
This is what I've got so far:
#define container_of(ptr, type, member) ( \
(type *) (char *)(ptr)-offsetof(type,member)\
)
Is this snippet conforming to ISO standards (and thus should be able to be compiled on any conforming compiler)?

Answer 1

As Ouah commented, the ({ ... }) statement expression is a GNU extension; you won't be able to use that. Your core expression is close to what's required, but doesn't have enough parentheses:
#define container_of(ptr, type, member) \
((type *) ((char *)(ptr) - offsetof(type, member)))
That looks clean to me. It's only spread across two lines for SO.

Answer 2

The macro is written the way it is to perfom a type check on ptr. It's possible to use a compound literal instead of the statement expression and fall back to a simple check for pointers instead of using __typeof__ if the compiler is not gcc-compatible:
#ifdef __GNUC__
#define member_type(type, member) __typeof__ (((type *)0)->member)
#else
#define member_type(type, member) const void
#endif
#define container_of(ptr, type, member) ((type *)( \
(char *)(member_type(type, member) *){ ptr } - offsetof(type, member)))

Answer 3

ISO C90 compatible version with type check. (However, caveat: two evaluations of ptr!)
#define container_of(ptr, type, member) \
((type *) ((char *) (ptr) - offsetof(type, member) + \
(&((type *) 0)->member == (ptr)) * 0))
struct container {
int dummy;
int memb;
};
#include <stddef.h>
#include <stdio.h>
int main()
{
struct container c;
int *p = &c.memb;
double *q = (double *) p;
struct container *pc = container_of(p, struct container, memb);
struct container *qc = container_of(q, struct container, memb);
return 0;
}
Test:
$ gcc -Wall containerof.c
containerof.c: In function ‘main’:
containerof.c:20:26: warning: comparison of distinct pointer types lacks a cast
containerof.c:20:21: warning: unused variable ‘qc’
containerof.c:19:21: warning: unused variable ‘pc’
We get the distinct pointer types warning for 26, but not 25. That is our diagnostic about pointers being misused.
I first tried placing the type check into the left hand side of a comma operator, gcc complains about that having no effect, which is a nuisance. But by making it an operand, we ensure that it is used.
The &((type *) 0)->member trick isn't well defined by ISO C, but it's widely used for defining offsetof. If your compiler uses this null pointer trick for offsetof, it will almost certainly behave itself in your own macro.

Answer 4

Yes, you can make "container_of" macros to be strictly ISO C conforming. To do this you need two things:
take rid of GNU exensions;
find a way to check types compatibility.
Basically, types checking is not run time operation, but compile time rather. And I not see any reasons, why original "container_of" implementation creates new variable just to assign it and perform type checking. This can be done without creation of new variable in some expression which is only computed (and types checked) in compile time. Fortunately, we have no much options in C and only choice is to use "sizeof(expression)" to check the type. See an example:
#define container_of(ptr, type, member) \
( (void)sizeof(0 ? (ptr) : &((type *)0)->member), \
(type *)((char*)(ptr) - offsetof(type, member)) )
In first line types compatibility is checked (for ternary operator compiler must insure, that types might be converted to common type, or that both types are compatible). Second line is the same, as in original "container_of" macros.
You can play with test program on GodBolt (https://godbolt.org/z/MncvzWfYn) and make sure, that this ISO conforming variant works even in Microsoft's Visual Studio compiler.
PS: After some time, I found that the following variant can be better:
#define CONTAINER_OF(ptr, type, member) \
( (void)sizeof(0 ? (ptr) : &((type*)0)->member), \
(typeof(_Generic((typeof(ptr))0, const typeof(*(typeof(ptr))0)*: (const type*)0, default: (type*)0))) \
((uintptr_t)(const void*)(ptr) - offsetof(type, member)) )
The difference, is that it preserves const qualifier from ptr and assigns it to the result, for example:
if ptr argument is const struct * pointer, the result then will have a type of const type *, despite if the type is const or not;
if ptr argument is non-const pointer (struct*), the result then will have type type*, which may be const or non-const, depending on the type of type argument.
As a result, preserving const qualifier reduces the possibility of the errors, when const pointer to some structure translated into non-const pointer via container_of macro.
Unfortunately, this version requires C23 or non-standard typeof() operator for earlier versions of C standard.
Another reason for ISO-compiant container_of macro, as opposed to implementation from Linux kernel, is that latter uses "statement expression" GCC-extension which works badly in a case, when argument ptr is a temporary variable. The latter might happen, when container_of macro is applied to the result of a function invocation (container_of(func().x, struct y, m), here is assumed, that func() returns a structure in which x is an array of structures), or to the compound statement (container_of((&(struct S){...}), struct B, m)). In both of these cases, a call to container_of macro borrowed from linux will result in a dangling pointer! This happens because a temporary object passed as ptr argument will be destroyed after the first semicolon (at the first line of Linux implementation of container_of macro), and because a variable created by a compound statement expression will be destroyed at the end of the nearest block, which is "statement expression" itself. ISO-compliant implementation of container_of macro have no such issues.