Bit shifting past length of type

[British0zzy]

Is it a defined operation when I do something like this:
char a = 15;
a = a<<9;

[Jack Klein]

On Wed, 1 Oct 2008 23:43:12 -0700 (PDT), Peter Nilsson
<ai***@acay.com .auwrote in comp.lang.c:

Richard Heathfield <[e-mail].i nvalidwrote:

British0zzy said:

Is it a defined operation when I do something like
this:
char a = 15;
a = a<<9;

Only on systems where char has at least 10 bits.

The relevant Standard cite is 3.3.7 Bitwise shift
operators:

"If the value of the right operand is negative or is
greater than or equal to the width in bits of the
promoted left operand, the behavior is undefined."

^^^^^^^^

That is almost incidental. The promoted type must have
a width of at least 16.

The problem is that plain char may be signed or
otherwise promote to a narrow int.

What exactly do you mean by "promote to a narrower int"? How can int
be narrower than int?

Did you mean "promote to a narrower integer type than int"? If so,
chapter and verse, please.

--
Jack Klein
Home: http://JK-Technology.Com
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[Eric Sosman]

Peter Nilsson wrote:

Eric Sosman <[e-mail] e:

>>>British0zz y said:
char a = 15;
a = a<<9;

...The value of CHAR_BIT affects the outcome because
it affects the value of CHAR_MAX used in the conversion,
but it does not affect the operation of or validity of
the shift.

... changing the second line to `int i = a << 9;'
yields a fragment whose behavior is the same on all
implementation s.

If CHAR_MAX <= INT_MAX and a (INT_MAX >9), then the
behaviour of a << 9 is undefined.

Yes. But since INT_MAX is at least 32767, then
INT_MAX>>9 is at least 63, a number greater than the
value of `a'. The second half of your premise never
holds, so the conclusion does not apply.

--
Eric Sosman
es*****@ieee-dot-org.invalid

[Peter Nilsson]

Jack Klein <jackkl...@spam cop.netwrote:

Peter Nilsson <[e-mail] .auwrote in comp.lang.c:

Richard Heathfield <[e-mail].i nvalidwrote:

British0zzy said:
Is it a defined operation when I do something like
this:
char a = 15;
a = a<<9;
>
Only on systems where char has at least 10 bits.
>
The relevant Standard cite is 3.3.7 Bitwise shift
operators:
>
"If the value of the right operand is negative or is
greater than or equal to the width in bits of the
promoted left operand, the behavior is undefined."

* ^^^^^^^^
That is almost incidental. The promoted type must have
a width of at least 16.

The problem is that plain char may be signed or
otherwise promote to a narrow int.

What exactly do you mean by "promote to a narrower int"?

Since I didn't write that, I can't answer you. :-)

What I mean by 'narrow int' is that if plain char promotes
to int, then int must be at least 9 bits wider than plain
char for a left shift of 9 bits to be guaranteed to work
work for arbitrary char values. [Obviously, there is still
the problem of assigning the shifted value back to char.]

--
Peter