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.”)