Re: Type-punning / casting problem

 James Kanze <>
Sat, 15 Sep 2007 13:48:14 -0700
On Sep 15, 7:46 pm, terminator <> wrote:

On Sep 15, 5:26 pm, "Victor Bazarov" <> wrote:

Phil Endecott wrote:

The four bytes pointed to by the const char* that this function
returns eventually get sent to a socket, and I can observe them at
the other end. They are wrong, i.e. if I pass 2.0 then the value at
the other end might be 8.923461290e-44. There is some
non-determinism in the values I see, which makes me think that I'm
looking at uninitialised memory rather than a systematic corruption. =


first thought was to add somethng like this:

std::cout << "cptr[0] = " << static_cast<int>(cptr[0]) << ....etc f=


[1] to [3] ... << "\n";

However, as soon as I add this (just before the return statement), it
works: the correct value is seen in the other process. My feeling is
that the cptr[n] expressions in the debugging tell the compiler that
these bytes are needed; when the debuging is not there, it thinks that
they are not used, and optimises them away. Can you think of any
other explanation?

Compilers are written by humans. Errare humanum est. Hence all
compilers have bugs, known and unknown. If you want to know whether
the bug you're encountering is known, contact the compiler writers.
If you just want a work-around, disable optimization for the small
module in which this function (these functions) is (are), and see if
it makes any difference. I've seen a significant improvement from
seemingly random behaviour with the HP and Sun compilers before, and
even <gasp!> with Microsoft's VC++ compiler, if optimizations are
disabled locally.

It looks like an auto optimization error.Maybe - as a result of
pointer algebra on small argument type - the enregisterd parameter is
copied to stack and operated upon ,and filled with rubish at
return(you are returning a pointer to the automatic variable).

It's not a bug. The compiler is allowed to assume that a
variable is only modified/read through expressions of its type,
or through expressions of char or unsigned char type. The fact
that something is modified through an uint32_t* is irrelevant
when the compiler is optimizing accesses to a float, for

can you write this:

union exg
   float f;
   char cl[sizeof(float)];
   exg& IndianSwap();

exg e={value};

and leave the optimization to the compiler(I think modern
compilers can do fine on this special case)?

If "IndianSwap" accesses cl, then it's undefined behavior.

James Kanze (GABI Software)
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