Newsgroup: comp.lang.c

Alex wrote:
> If I have two struct. See below:
> struct s1
> {
>         int type;
>         int (*destroy)(struct s1* p);
> }
> 
> struct s2
> {
>         struct s1 base;
>         .../* here are some other memebers */
> }.
> 
> My doubt is that Will the standard C will guarantee the cast from
> 'struct s2' to 'struct s1' is safe. The problem comes because I don't
> know
> whether the align in struct will affect such cast.

You can only cast scalar types, not composite types like structures, so 
I assume that you mean a cast from (struct s2 *) to (struct s1 *).   The 
C99 standard (6.7.2.1) specifies that a pointer to a structure object, 
suitably converted, points to its initial member.  Therefore, your 
proposed cast is legal and guaranteed to work.

-- 
Diomidis Spinellis
Code Quality: The Open Source Perspective (Addison-Wesley 2006)
http://www.spinellis.gr/codequality

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