int

[RoSsIaCrIiLoIA]

if
{
unsigned u = -INT_MIN;
int i, sign = -1;
i = sign * u;

then i == INT_MIN or not?

 

[Malcolm]

if
{
unsigned u = -INT_MIN;
int i, sign = -1;
i = sign * u;

then i == INT_MIN or not?

The calculation is allowed to overflow, producing implementation-defined (I
think) behaviour.
On most platforms, INT_MIN = -INT_MAX - 1. Taking the negative produces
INT_MAX + 1, which will overflow to INT_MIN. Multiplying by -1 will further
overflow to INT_MIN, for by a contorted route you will actually get the
answer you suggest.

 

[Peter Nilsson]

if
{
unsigned u = -INT_MIN;

-INT_MIN is performed using signed int arithmetic (before the assignment), and it can
overflow (undefined behaviour).

Note that even if you do...

unsigned u = INT_MIN;
u = -u;

You have no guarantee that u will be the magnitude of INT_MIN. Since UINT_MAX can equal
INT_MAX, you might have u == 0.

int i, sign = -1;
i = sign * u;

then i == INT_MIN or not?

sign * u will be performed in unsigned int arithmetic, so the result will always be
positive. The conversion on assignment to i will be implementation defined if the value is
outside the range [0..INT_MAX]. So, it could theoretically set i to anything. C99 even
allows an implementation defined signal to be raised.

Even C99 (optional) intN_t types, whilst necessarily unpadded twos complement integers,
are not immune from implementation defined conversions and potential signals if assigned a
value that is outside their range.

 

[RoSsIaCrIiLoIA]

-INT_MIN is performed using signed int arithmetic (before the assignment), and it can
overflow (undefined behaviour).

Note that even if you do...

unsigned u = INT_MIN;
u = -u;

what do you say for
unsigned u = -(INT_MIN + 1);

u += 1;
?
Thanks

 

[Peter Nilsson]

what do you say for
unsigned u = -(INT_MIN + 1);
u += 1;

Or just: u = 0u - INT_MIN;

This is well defined [although one comp.std.c regular might disagree about
-(INT_MIN+1)] however...

This can still leave you with u == 0 on theoretical twos complement machines
where UINT_MAX == INT_MAX.