2 Dec
2014
2 Dec
'14
11:03 a.m.
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...
Jon