Question

I want to add a field to a structure in C. So for example I have the following structure.

struct A
{
 some_type x;
 some_type y;
}

I declare a new structure, like this.

struct B
{
 A a;
 some_type z;
}

Now say I have a function like this.

int some_function( A * a )

Is it possible to pass a variable of type B to it like this in the program.

B * b;
......
A * a = (A*)b;
some_function( a );

And also be able to use the fields inside some_function by using a->x for example?

Answer 1

Yes, it is valid. Word of the Standard, C99 6.7.2.1/13:

… A pointer to a
structure object, suitably converted, points to its initial member (or if that member is a
bit-field, then to the unit in which it resides), and vice versa. There may be unnamed
padding within a structure object, but not at its beginning.