Re: Unsigned types are DANGEROUS??
On 17/03/2011 17:39, Leigh Johnston wrote:
On 17/03/2011 17:16, MikeP wrote:
"Leigh Johnston"<leigh@i42.co.uk> wrote in message
news:Z_-dnTYAK6Zrph_QnZ2dnUVZ8hOdnZ2d@giganews.com...
On 17/03/2011 16:43, MikeP wrote:
"Leigh Johnston"<leigh@i42.co.uk> wrote in message
I am curious; I have been using VC++ for years and have not been
aware
of a way to detected signed integer overflow; how exactly do you
do
this without using assembler?
Windows' Structured Exception Handling (SEH) will probably let you
hook
it.
I guess that requires CPU support;
SEH does? C++ exceptions are built on top of SEH (or at least they
used
to be, don't know about now).
SEH is only useful if an exception is actually thrown; I see no
support
in VC++ for causing such an exception to be thrown on integer
overflow
unless you actually write some inline assembler (INTO instruction for
example) after the integer operation.
A "structured exception" you mean? You'll have to map it to C++
exceptions. No assembly required though. (I haven't done this yet,
BTW,
just read about it... am considering it now though).
Yes I mean a "structured exception"; AFAICT there *is* assembly
required as VC++, targeting Intel, will not emit code to trigger an
overflow interrupt which in-turn causes a structured exception to be
thrown. I have tested this on Win32 and SEH only kicks in if I
manually add an inline "INTO" assembler instruction after an
overflowing expression.
You actually got the INTO to work though? You've caught the
EXCEPTION_INT_OVERFLOW exception?
Yes:
int exception_filter(unsigned int code)
{
if (code == EXCEPTION_INT_OVERFLOW)
return EXCEPTION_EXECUTE_HANDLER;
else
return EXCEPTION_CONTINUE_SEARCH;
}
int main()
{
__try
{
int n = INT_MAX;
++n;
_asm INTO
}
__except(exception_filter(GetExceptionCode()))
{
std::cout << "hmm";
}
}
But doing the above is obviously non-portable and totally non-standard
in addition to being utter garbage IMO; the C++ language makes no
gaurantees about what CPU flags are set and when so you would have to
double check the assembler output of the compiler to ensure the overflow
flag is set or unset before the INTO instruction and even then you
wouldn't catch such mistakes as:
int n = INT_MAX;
int n2 = ((n + 1) * 2);
_asm INTO
as the multiplication is the last operation to affect the overflow flag
so the overflowing sub-expression escapes.
/Leigh
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