On 2018/2/6 23:03, Peter Breuer wrote:
> int main() {
> signed int x = 0x80000005u;
> unsigned int y = 0x00000002u;
> signed int z = x >> y;
> printf("0x%0x\n", z);
> return 0;
> }
> % gcc -std=c99 test.c
> test.c: In function 'main':
> test.c:6:3: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
> printf("0x%0x\n", z);
> ^
> test.c:6:3: warning: incompatible implicit declaration of built-in function 'printf'
>
> (I'll live with that warning - just for the purpose of a clean example!)
>
Well you could have declared the `printf()` function:
extern int printf(const char *restrict format, ...);
> % ./a.out
> 0xe0000001
> ^ SIGNED right shift
>
> OK, so x>>y was done signed. That means both x,y were converted to
> signed int (trivially).
No. See below.
>
> However, that seems against C99 rules, which seem to say one should
> convert to unsigned int in mixed sign equal rank situations.
>
> First: NO INTEGER PROMOTION is done, by this rule
>
> any operand whose type ranks lower than int is automatically converted
> to the type int, provided int is capable of representing all values of
> the operand's original type. If int is not sufficient, the operand is
> converted to unsigned int.
>
> Both x and y are int, so do not rank "lower than int". Rule does not
> apply. All as is. So INTEGER CONVERSION applies from there:
It doesn't.
I don't have C99 at hand, so I will quote C11 (WG14 N1570) for you:
```
6.5.7 Bitwise shift operators
...
Semantics
3 The integer promotions are performed on each of the operands. ...
```
Note that the standard references _the integer promotions_, in contrast
to what you see elsewhere:
```
6.5.5 Multiplicative operators
...
Semantics
3 The usual arithmetic conversions are performed on the operands.
```
```
6.5.9 Equality operators
...
Semantics
4 If both of the operands have arithmetic type, the usual arithmetic
conversions are performed.
```
Clearly, the type of the operand on the right-hand side is not altered -
it remains `unsigned int`.
--
Best regards,
LH_Mouse
