On 29 May 2006, Neil Brown suggested tentatively: > On Sunday May 28, [EMAIL PROTECTED] wrote: >> - mdadm-2.4-strict-aliasing.patch >> fix for another srict-aliasing problem, you can typecast a reference to a >> void pointer to anything, you cannot typecast a reference to a >> struct. > > Why can't I typecast a reference to a struct??? It seems very > unfair...
ISO C forbids it. Pretty much the only time you're allowed to cast a pointer to A to a pointer to B is if one or the other of them is a pointer to void or char. The compiler can (and does) assume that all other pointers-to-T are in different `aliasing sets', i.e. that a modification through a pointer-to-T1 cannot affect variables of any other type (so they can be cached in registers, moved around without reference to each other, and so on). It's *very* advantageous, and with the interprocedural optimizations in GCC 4.1+ is getting more so all the time (the compiler can now move code between *functions* if it needs to, or propagate constants consistently used as function arguments down int into the function themselves; pointer aliasing torpedoes both of these optimizations, and many more.) (One particularly evil thing is the glibc printf handler extension. As a result of that, we must assume that any call to printf() or scanf() can potentially modify any global variable and any local to which a pointer may have escaped. This is decidedly suboptimal given that nobody sane ever uses those extensions!) I think I'll dare to upgrade mdadm soon. I tried upgrading from 2.3.1 to 2.4 (with uClibc from SVN dated 2006-03-28) and it promptly segfaulted at array assembly time. If it happens with 2.5 I'll get a backtrace (hard though that is to get when you don't have any accessible storage to put the core dump on... I upgraded uClibc at the same time so it may well be an uClibc bug.) -- `Voting for any American political party is fundamentally incomprehensible.' --- Vadik - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
