Re: This is undefined, but is it legal?
On Jul 25, 6:04 pm, jl_p...@hotmail.com wrote:
I've heard that if you've declared a variable (such as a
double or an int) and not initialize it, then the result of
printing out its value is undefined.
Anything you do with its *value* is undefined behavior. (You
can still take its address, or assign to it.) With the
exception of unsigned char and char.
I've also heard that "undefined behavior" can mean just about
anything, such as "flying monkeys shooting out of your nose."
Sure, that's an exaggeration, but normally I interpret that to
mean that the program can crash (or cease running)
erratically, or even corrupt data.
In non-priviledged mode, under a modern general purpose OS,
that's what it normally means. On systems without a priviledged
mode (or in kernel code)... I have seen it require the disk to
be reformatted.
So my question is: Although I can never safely predict the
printed output of an uninitialized int or double, is it still
safe (or legal) to do so?
No. It's undefined behavior.
In other words, if I run this program:
#include <iostream>
int main(int argc, char ** argv)
{
int i;
double d;
std::cout << "i = " << i << std::endl; // safe?
std::cout << "d = " << d << std::endl; // safe?
return 0;
}
I may not be able to predict what will print out, but is there
any chance that the program can crash because of those lines?
Of course. It's not likely with int, on most modern machines
(but there is at least one where it is a distinct possibility).
With double, it's possible on every Windows or Unix machine I
know.
If you're curious why I ask this, it's because in some code
I'm working through there is a structure with ints -- some of
which are never used nor initialized. However, this structure
(will all its ints) is getting written out to disk (and later
read back in). But at no time are the values of these
uninitialized ints used for logic in the code.
Because the code is writing out uninitialized values (and
later reading them in), is there a possibility that the
program can self- destruct (or corrupt anything) just because
those ints weren't initialized?
Formally, yes, and any good debugging system will complain. (I
know Purify does, because I've had to deal with the same
problem.) Why don't you just initialize the structs.
And how are you writing them out? If you're just copying the
bits of a struct to disk, then you have no guarantee of being
able to read the data in the future.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orient=E9e objet/
Beratung in objektorientierter Datenverarbeitung
9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, France, +33 (0)1 30 23 00 34