[tz] Minor (unimportant really) technical UB bug in strftime() ?

Paul Eggert eggert at cs.ucla.edu
Fri Nov 11 21:14:24 UTC 2022

On 2022-11-11 03:55, Clive D.W. Feather wrote:
>      int x;
>      int y = x;
> is undefined behaviour because it might assign a trap representation to y.

It's undefined even on implementations that don't have trap 
representations. Draft C23 "Lvalues, arrays, and function 
designators" says, "If the lvalue designates an object of automatic 
storage duration that could have been declared with the register storage 
class (never had its address taken), and that object is uninitialized 
(not declared with an initializer and no assignment to it has been 
performed prior to use), the behavior is undefined."

Hence, in the typical case where there are no trap representations, this 

     time_t x, y = x;
     return !y;

has undefined behavior, whereas this code:

     time_t x, y = x;
     return time(&x) == (time_t)-1 ? !y : !!y;

must return 0 or 1 and the implementation cannot reject the first line's 
use of the uninitialized variable x. This part of the standard is 
strange indeed, and I can't imagine debugging implementation wanting to 
conform to the standard as written.

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