In C, does a pointer to a structure always point to its first member?

Question

Suppose I have a number of C structs for which I would like a particular set of functions to operate upon.

I'm wondering if the following is a legitimate approach:

typedef struct Base {      int exampleMember;      // ... } Base;  typedef struct Foo {      Base base;      // ... } Foo;  typedef struct Bar {      Base base;      // ... } Bar;  void MethodOperatesOnBase(void *);  void MethodOperatesOnBase(void * obj) {      Base * base = obj;      base->exampleMember++; } 

In the example you'll notice that both structs Foo and Bar begin with a Base member.

And, that in MethodOperatesOnBase, I cast the void * parameter to Base *.

I'd like to pass pointers to Bar and pointers to Foo to this method and rely on the first member of the struct to be a Base struct.

Is this acceptable, or are there some (possibly compiler-specific) issues I need to be aware of? (Such as some sort of packing/padding scheme that would change the location of the first member of a struct?)

Answer 1

Yes, the C standard specifically guarantees that this will work.

(C1x §6.7.2.1.13: "A pointer to a structure object, suitably converted, points to its initial member ... and vice versa. There may be unnamed padding within as structure object, but not at its beginning.")

Answer 2

The whole gtk+ is implemented like that. I cannot think of a better example. Take a look at http://git.gnome.org/browse/gtk+/tree/gtk/