2 Dec
2014
2 Dec
'14
7:25 p.m.
On 12/02/2014 01:03 PM, Roe Peterson wrote:
Should have been gcc on a 64-bit Ubuntu 12.04 build, I
THINK. (Not absolutely sure what
machine they were running this on at work.) I have NO IDEA
what compiler flags were used,
however. It was buried deep in a several page Make file,
with those 5 line long compile
lines filled with switches and includes. It definitely did
NOT produce any warning at
all.
Jon
On Dec 2,
2014, at 11:08 AM, Jon Elson <elson at pico-systems.com> wrote:
still discovering new ways that
C code that looks perfectly correct can screw up horribly. Last one was
yesterday.
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.
Hmm. What C compiler did this, on what distro? I'm pretty sure
that this throws a compiler warning in gcc with -Wall...