From: Keith.S.Thompson+u@gmail.com
Janis Papanagnou writes:
> On 2026-02-06 06:10, Keith Thompson wrote:
>> Bart writes:
>>
>>> If I compile this code with 'gcc -Wall -Wextra -Wpedantic':
>>>
>>> #include
>>>
>>> int main() {
>>> int a = -1;
>>> printf("%u", a);
>>> }
>>>
>>> it says nothing. The program displays 4294967295 instead of -1.
>
> Yes. You instruct 'printf' with '%u' to interpret and display it
> (the variable 'a') as unsigned. ('-1' is not an unsigned numeric
> representation.) - I wonder what you are thinking here.
>
>> The behavior is unsurprising. The lack of a warning is very mildly
>> inconvenient.
>
> Indeed unsurprising. And I even don't see any inconvenience given
> that even an initialized declaration of 'unsigned a = -1;' is not
> considered a problem in "C". I rather learned that to be a useful
> code pattern when programming in "C".
Sure, but the point is that the program has undefined behavior.
If you define `unsigned a = -1;`, the value of the initializer is
implicitly converted from int to unsigned, and the value of UINT_MAX
is stored in `a`. That's well defined.
Passing a argument of type int to printf with a "%u" format is
well defined if and only if the value of the argument is within
the ranges of both types (which is almost certainly equivalent to
it being non-negative). This is strongly implied prior to C23,
and guaranteed in C23. There is no implicit conversion; the int
is treated as if it were of type unsigned int. In practice, it's
almost certain to display the value of UINT_MAX unless the compiler
goes out of its way to do something else.
A warning would not be inappropriate -- and in fact clang version
21 does issue a warning with "-Weverything". (The warning refers to
"-Wformat", which by itself doesn't trigger the warning; that might
be a bug.)
--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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