2 Dec
2014
2 Dec
'14
10:45 a.m.
Jon Elson wrote:
[..]
Boiling it down, we had :
long long int A;
int B;
A = B << 32;
This means that some field of B gets shifted to the right by
32 bits, and
fit into the upper 32-bits of A.
Now, any decent compiler should either extend B to the
length of A, or
as the C rules specify, NOT extend B, and therefor ought to
warn you
that it is losing significant bits. No warning, no
extending the variable
before shifting the bits off the end of the word. So, A
always gets
a zero! UGH! Stupid! I could almost write a book of these
sorts
of gotchas.
Jon
No, thats not stupid. Nobody says that the compiler should think about your
code.
You clearly shift the int variable 32 times to the left and the compiler is
warning you..
BTW: there are some standards in stdint.h that clarify up a lot which tpe
has how many bits, eg int32_t int64_t uint8_t.
On my system int is 64 Bits and a long long is 128 bits, for shure your
code will work here, bit your code isn't portable! It doesn't work on a
32 Bit system and it doesn't work on 16 or 8 Bit machines.
C is much like Assembler, you should know what you really want todo. The
comiler let you do all kind of things with the assumption that you know what
you are doing. This is a programming language for system programming
purposes (written to write Unix) where you have to load unusual registers
and such things...
How about this:
A = (int64_t)B << 32;
Regards,
Holm
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