[LLVMdev] Uninitialized variable - question

[Nick Lewycky]

On 11/24/2012 02:08 AM, Jakub Staszak wrote:
> Hello,
>
> I was wondering about the case below. I tried to find any information in C standard, but I found nothing.
> In this case, variable "i" is uninitialized, but it is the _same_ value passed as an argument, so only of "a" or "b" should be printed.
>
> What I found is that with -O2:
> LLVM (trunk) prints both "a" and "b"
> GCC (4.2) prints both "a" and "b"
> GCC (trunk) prints "b" only.
>
> As I said, I don't know what standard says here.
>
>
> #include<stdio.h>
>
> void f(int i) __attribute__((noinline));
> void g(int i) __attribute__((noinline));
>
> void f(int i) {
>    if (i) printf("a\n");
> }
>
> void g(int i) {
>    if (!i) printf("b\n");
> }
>
> int main() {
>    int i;
>    f(i);
>    g(i);
> }

Passing an uninitialized value as a function argument is undefined 
behaviour on the spot, regardless of what the callee does (even if it 
never references that argument).

That aside, there is no way that 'i' has the same value, since it has no 
value. This:
   int i;
   printf("%d\n", i);
   printf("%d\n", i);
is allowed to print two different values (or, applying the above rule, 
format your hard drive). The way the standard defines the behaviour that 
you see when reading a value is by stating that it must have the value 
that was last stored in it. When no value was stored, there are no rules 
to apply about what you get each time you look at it, and there is no 
other guarantee of a consistent value anywhere in the standard. This 
rule permits your program to print both 'a' and 'b', or neither.

I should mention that the above is for C++, and I don't have a copy of 
any of the standards handy, but I expect the rules to be the same for C 
and C++ here.

Nick