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Object-oriented design patterns in the kernel, part 2Object-oriented design patterns in the kernel, part 2Posted Jun 7, 2011 22:29 UTC (Tue) by cmccabe (guest, #60281)In reply to: Object-oriented design patterns in the kernel, part 2 by nix Parent article: Object-oriented design patterns in the kernel, part 2 I don't think any macros are really needed, just something like this:
struct base {
int a;
int b;
char end[0] __attribute__((aligned(4)));
};
struct derived1 {
int a;
};
struct derived2 {
char b;
};
Then you have stuff like this:
void foo(struct base *b) {
if (type == DERIVED1) {
struct derived1 *d = (struct derived1*)b->end;
...
}
}
It isn't as nice as C++, but it's ok.Flexible array members were standardized in C99. Every compiler can do it, but I don't know if the syntax for forcing alignment is standardized yet.
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Object-oriented design patterns in the kernel, part 2 Posted Jun 8, 2011 4:52 UTC (Wed) by jzbiciak (✭ supporter ✭, #5246) [Link] I don't know if the syntax for forcing alignment is standardized yet. What if you just made it an array of "double" or "long double"? Or, an array of a union of all of the "large" types, so you're guaranteed to get the most restrictively aligned type on the platform?
typedef union
{
long long ll;
double d;
long double ld;
void *vp;
void (*fp)(void); /* function pointer */
} align_u;
Then replace char end[0] __attribute__((aligned(4))); with align_u end[0];. Wouldn't that work?
Object-oriented design patterns in the kernel, part 2 Posted Jun 8, 2011 13:16 UTC (Wed) by nix (subscriber, #2304) [Link]
That does indeed work and is exactly what I did. A hallowed ancient trick: I don't know *how* long ago I first saw it. Decades back.
Object-oriented design patterns in the kernel, part 2 Posted Jun 8, 2011 13:14 UTC (Wed) by nix (subscriber, #2304) [Link]
Ah yes. You have access to flexible array members and can assume that you're using GCC. Alas, when I perpetrated this trick I could assume neither of these, so needed a suitable union as a final member to ensure correct alignment.
The macros are just to make things less horrendously unreadable. :)
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