Re: Does casting lvalue lead to Undefined Behaviour ?

p_cricket_guy@xxxxxxxxxxx said:

Please see the code below

-- start listing is_it_ub.c --

#include <stdio.h>
#include <stdlib.h>

int main (void)
{
unsigned char buff[20];
unsigned int i;

i = 0xaabbccddUL;
*((int *)buff) = i; /* Is this UB ? */

Yes. You're evaluating buff (which is a pointer to char), and then
converting that value to a pointer to int, but there is no guarantee that
it will be properly aligned. You then dereference the possibly-invalid
pointer thus obtained. On systems where this works, it's harmless. On
systems where it doesn't, we're talking potential bus errors, which are
definite showstoppers.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at the above domain, - www.
.

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