Reply to thread

Message

[QUOTE="Mark Wooding, post: 3809900"]
Comments:

* Important style: Use a typedef rather than #define for making type
    synonyms.

* Indentation is nice.

* You never read the values of `bit' or `g'.

* `mask' has type `unsigned int' but you're passing it to `printf'
    with a `%d' format specifier which expects a signed `int'.  If
    `mask' is actually representable as an `int' then all will be well;
    if not, you have undefined behaviour.

* The constant FULL_MASK may have signed type.  If it has, then
    FULL_MASK << n may overflow causing undefined behaviour.  You
    probably mean to define FULL_MASK to be 0xffffffffu, but, in fact...

* It looks to me as if you're better off including <limits.h> and
    using UINT_MAX rather than guessing FULL_MASK, which may be either
    too large to fit in unsigned int, or too small to cover all of the
    bits.  Of course, I don't actually know what you're trying to
    achieve, so this might be wrong.

I'll assume that FULL_MASK either has unsigned type or is capable of
representing all of the values necessary.

5 & 31 = 5 /= 0, so the consequent expression is evaluated.

~(FULL_MASK << (5 & 31)) = ~(FULL_MASK << 5)

= ~0x1fffffffe0

= -0x1fffffffdf

This is then reduced modulo 2^n (maybe it helps to think of this as a
2-adic value, but probably not...), where n is the width of the smaller
of unsigned int and whatever type 0xffffffff happens to have on your
platform.  The result will end 0x...1f; there may or may not be higher-
order set bits, depending on the widths of the types involved.

So it looks as if all of the bits of mask are properly initialized,
though maybe not with what you expected them to be initialized to.  So
to me it sounds like (a) Purify is wrong, and (b) so is your code.

For the record, Valgrind didn't find anything to complain about.

-- [mdw]
[/QUOTE]

Verification