PATCH: Bad signed int to long long conversion
vincent at vinc17.net
Fri Feb 10 11:30:05 CET 2012
On 2012-02-09 18:17:17 +0100, Torbjorn Granlund wrote:
> Vincent Lefevre <vincent at vinc17.net> writes:
> On 2012-02-09 17:01:45 +0100, Torbjorn Granlund wrote:
> > Strictly speaking, I don't think things like (unsigned long)(MIN_LONG)
> > is well-defined C.
> It is well-defined:
> 220.127.116.11 Signed and unsigned integers
> 1 When a value with integer type is converted to another integer
> type other than _Bool, if the value can be represented by the
> new type, it is unchanged.
> 2 Otherwise, if the new type is unsigned, the value is converted
> by repeatedly adding or subtracting one more than the maximum
> value that can be represented in the new type until the value
> is in the range of the new type.60)
> Rule 2 surely covers this. Is this from C99?
This is from (the new) C11, but C99 had the same rule.
> IIRC, this was not well-defined in C89. Am I wrong?
The rationale (for both C89 and C99) says:
18.104.22.168 Signed and unsigned integers
Precise rules are now provided for converting to and from unsigned
integers. On a two's-complement machine, the operation is still
virtual (no change of representation is required), but the rules
are now stated independent of representation.
So, there was a change either before or after C89, but if I understand
correctly, machines with a two's-complement are not affected. So, the
number of affected still-used implementations should be close to 0.
Vincent Lefèvre <vincent at vinc17.net> - Web: <http://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
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