c - Struct pointer compatibility

Question

Suppose we have two structs:

typedef struct Struct1
{
    short a_short;
    int id;
} Struct1;

typedef struct Struct2
{
    short a_short;
    int id;
    short another_short;
} Struct2;

Is it safe to cast from Struct2 * to Struct1 * ? What does the ANSI spec says about this? I know that some compilers have the option to reorder structs fields to optimize memory usage, which might render the two structs incompatible. Is there any way to be sure this code will be valid, regardless of the compiler flag?

Thank you!

Answer 1

It is safe, as far as I know.

But it's far better, if possible, to do:

typedef struct {
    Struct1 struct1;
    short another_short;
} Struct2;

Then you've even told the compiler that Struct2 starts with an instance of Struct1, and since a pointer to a struct always points at its first member, you're safe to treat a Struct2 * as a Struct1 *.