What is the value of (-1 >> 6)?

[Grant]

Are the bitwise operators >>, <<, &, |, ~, ^ only defined for
non-negative values?

The little program below produces the output
3 255
I expected to get the 3 but why does it produce the 255 when the
variable c is not typecast first?






#include <stdio.h>

int main(int argc, char **argv) {

char c = -1;
unsigned char uc1 = (unsigned char)c >> 6;
unsigned char uc2 = c >> 6;

printf("%hhu %hhu\n", uc1, uc2);
return 0;
}

 

[Tim Prince]

Are the bitwise operators >>, <<, &, |, ~, ^ only defined for
non-negative values?

The little program below produces the output
3 255
I expected to get the 3 but why does it produce the 255 when the
variable c is not typecast first?






#include <stdio.h>

int main(int argc, char **argv) {

char c = -1;
unsigned char uc1 = (unsigned char)c >> 6;
unsigned char uc2 = c >> 6;

printf("%hhu %hhu\n", uc1, uc2);
return 0;
}

When you don't specify whether you want signed or unsigned char, you leave
it up to the designers of your compiler. Apparently, you tested on a
system where it's signed 2's complement.

 

[pete]

Are the bitwise operators >>, <<, &, |, ~, ^ only defined for
non-negative values?

Yes.

 

[Jack Klein]

Yes.

NO!

They are only well-defined. Their effects when right shifting signed
types is implementation-defined, but that is a far cry from undefined.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++ ftp://snurse-l.org/pub/acllc-c++/faq

 

[pete]

NO!

They are only well-defined. Their effects when right shifting signed
types is implementation-defined, but that is a far cry from undefined.

"undefined aspects" is undefined, enough for me.

N869
6.5 Expressions
[#4]
... bitwise operators ...
These operators return values that depend on the internal
representations of integers, and have implementation-defined
and undefined aspects for signed types.

 

[pete]

right shift *is* implementation defined,
but OP asked about bitwise operators as a group,
so I took my answer from the part of the standard
which addresses the issue for bitwise operators
as a group.

"undefined aspects" is undefined, enough for me.

N869
6.5 Expressions
[#4]
... bitwise operators ...
These operators return values that depend on the internal
representations of integers, and have implementation-defined
and undefined aspects for signed types.