Safe use of unions

The quoted text below is from comp.std.c which originated
from a discussion on comp.lang.c. I've edited out the parts
that do not apply to my question.

Robert Gamble wrote:
Dann Corbit wrote:
#include <stdio.h>

int main(void)
{
typedef union foo_u {
struct a {
unsigned char carr[sizeof(unsigned int)];
} aa;
struct b {
unsigned int ui;
} bb;
} foo;

foo bar;
bar.bb.ui = 1;
printf("%u\n", (unsigned)bar.aa.carr[0]);
return 0;
}

#include <stdio.h>

int main(void)
{
typedef union foo_u {
unsigned char carr[sizeof(unsigned int)];
unsigned int ui;
} foo;

foo bar;
bar.ui = 1;
printf("%u\n", (unsigned)bar.carr[0]);
return 0;
}

Is the first sample safe but the second not safe?

Neither are safe.

Why is either example unsafe? I understand the output of
the printf calls is unspecified. But I do not see anything
that would be cause for concern other than that.

.

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