Quick check on signed promotion of bytes

[toe]

I have a byte called "x". I want byte "y" to be the complement of
"x" (i.e. all the bits flipped).

Initially I wrote:

char unsigned x, y;

...

x = 72;

y = ~x;

But then I thought that the following might happen on your average
system (CHAR_BIT == 8, sizeof(int) == 4):

1) x is promoted to signed int.
2) The complement is take of this signed int.
3) This signed int is then converted to an unsigned char

Am I right in thinking that this is what will happen? If so, would I
be wise to do the following instead:

y = ~(unsigned)x;

--
Tomás Ó hÉilidhe

[vippstar]

On Jan 31, 12:45 pm, [e-mail] wrote:

I have a byte called "x". I want byte "y" to be the complement of
"x" (i.e. all the bits flipped).

Initially I wrote:

char unsigned x, y;

...

x = 72;

y = ~x;

But then I thought that the following might happen on your average
system (CHAR_BIT == 8, sizeof(int) == 4):

1) x is promoted to signed int.

That won't happend, therefore the rest of your thoughts is false.
Your code is perfectly valid.

[toe]

On Jan 31, 10:48*am, [e-mail] wrote:

* * 1) x is promoted to signed int.

That won't happend, therefore the rest of your thoughts is false.
Your code is perfectly valid.

My understanding was as follows:

"If you're dealing with an integer type smaller than int, then it
must undergo promotion before you can perform any operations on it."

--
Tomás Ó hÉilidhe

[Jack Klein]

On Thu, 31 Jan 2008 02:48:28 -0800 (PST), vi******@gmail.com wrote in
comp.lang.c:

On Jan 31, 12:45 pm, [e-mail] wrote:

I have a byte called "x". I want byte "y" to be the complement of
"x" (i.e. all the bits flipped).

Initially I wrote:

char unsigned x, y;

...

x = 72;

y = ~x;

But then I thought that the following might happen on your average
system (CHAR_BIT == 8, sizeof(int) == 4):

1) x is promoted to signed int.

That won't happend, therefore the rest of your thoughts is false.
Your code is perfectly valid.

No, your correction is false. 'x' must be promoted to either signed
or unsigned int before the bitwise inversion operator is applies. In
a system with CHAR_BIT is 8, a signed int must be able to hold all
possible values of unsigned char, so 'x' is indeed promoted to signed
int.

--
Jack Klein
Home: http://JK-Technology.Com
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