On 2016-09-26 10:32:14 +0100, Andrew Haley wrote: > On 25/09/16 22:46, David Brown wrote: > > I think the bug is here: > > > temp = *t2p; // Read as T2 > > t1p2 = (T1*)t2p; // Visible T2 to T1 pointer conversion > > *t1p2 = temp; // Write as T1 > > 6.3.2.3 Pointers > > 7 A pointer to an object type may be converted to a pointer to a > different object type. If the resulting pointer is not correctly > aligned for the referenced type, the behavior is undefined. > Otherwise, when converted back again, the result shall compare equal > to the original pointer. > > Note that you have permission only to convert the pointer back to the > original type and compare it. You don't have permission to > dereference it as a different type. IMO your program is undefined. I disagree. The above paragraph says nothing about dereferencing the pointer; in particular, it does not disallow one to dereference it as a different type. The restrictions about dereferencing are given by 6.5#7 (itself based on 6.5#6, which defines the "effective type"). Moreover 6.5#7 restrictions are about write-read combinations, not about read-write like in the above 3 lines of code. -- Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <https://www.vinc17.net/> 100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/> Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
