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;
} 5 1750
Grant wrote: 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.
--
Tim Prince
Grant wrote: Are the bitwise operators >>, <<, &, |, ~, ^ only defined for non-negative values?
Yes.
--
pete
Jack Klein wrote: On Sat, 11 Oct 2003 02:29:51 GMT, pete <pf*****@mindsp ring.com> wrote in comp.lang.c:
Grant wrote: Are the bitwise operators >>, <<, &, |, ~, ^ only defined for non-negative values?
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.
"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
pete wrote: Jack Klein wrote: On Sat, 11 Oct 2003 02:29:51 GMT, pete <pf*****@mindsp ring.com> wrote in comp.lang.c:
Grant wrote: > > Are the bitwise operators >>, <<, &, |, ~, ^ only defined for > non-negative values?
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.
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.
--
pete This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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