JTC1/SC22/WG14 N1046

From:Subject: Validity of constant in unsigned long long range


Consider a constant 9223372036854775808 in a C99 implementation that
has 64-bit two's complement long long, and no extended integer types.

6.4.4.1 says that an unsuffixed decimal constant has the first of the
types on the following list into which its value will fit: int, long
int, long long int.  In this case, the value does not fit into any of
those types, and there are no extended integer types to try.  (The
value would fit into unsigned long long, but that's not on the list.)

So I conclude that this constant is invalid, just as a grossly
too-large constant (say, one consisting of a 1 followed by 1,000
zeroes) would be invalid. (And I think that's a good thing, because
otherwise this constant could be unsigned on some implementations and
signed on others that have larger extended integer types.)

However, I'm not sure 6.4.4.1 (or 6.4.4) says anything that requires
an error, or even gives meaning to this constant. It doesn't say what
happens if the constant doesn't fit in any type on its list and there
are no extended integer types.

Is this a defect, or was this intentionally worded vaguely to allow
latitude to implementations?

A related issue comes up with UINT64_C(9223372036854775808). One
plausible implementation for the macro UINT64_C would seem to be to
cast the constant to the proper type. However, that does not work in
this particular case, because the constant before casting is the same
invalid constant discussed above. Another plausible implementation
(and suggested by 7.18.4.1p2) is to concatenate a suffix to the
constant, e.g., a "U" in this case. Sounds good, but 7.18.4p2 doesn't
say that the argument to the macro must be an unsuffixed constant;
indeed it says that the syntax must match 6.4.4.1, which implies that 
a suffix is allowed.

So if 9223372036854775808 is an invalid constant, it seems that an
implementation must rely on compiler magic to get UINT64_C right; the
tricks available with standard macros don't work.