I have the following code. Why its undefined to derefrence union pointers in the following way ?
extern union MyUn *P1;
extern union MyUn *P2;
extern void myfunc(void)
{
*P1 = *P2;
}
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7
If you haven't also defined the union in this source file, the compiler doesn't know how much to copy.
What is the size of the union?
Bo Persson
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If the union isn't known at that point, won't `*P1` and `*P2` be incomplete types ? Thus the dereference itself is not OK. – cnicutar Sep 01 '12 at 09:41
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Yes, the main problem with incomplete `struct` and `union` types is that they are of unknown size (and contain unknown members). – Bo Persson Sep 01 '12 at 09:51
3
That has nothing to do with unions in particular, and it's not "undefined", either: It's simply a compiler error if you try to dereference a pointer to an incomplete type (for obvious reasons).
Kerrek SB
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By simple compiler error you mean a constraint violation, don't you :) – Jens Gustedt Sep 01 '12 at 13:53