Question

I know that Visual Studio under debugging options will fill memory with a known value.
Does g++ (any version, but gcc 4.1.2 is most interesting) have any options that would
fill an uninitialized local POD structure with recognizable values?

struct something{ int a; int b; };
void foo() {
    something uninitialized;
    bar(uninitialized.b);
}

I expect uninitialized.b to be unpredictable randomness; clearly a bug and easily
found if optimization and warnings are turned on. But compiled with -g only, no
warning. A colleague had a case where code similar to this worked because it
coincidentally had a valid value; when the compiler upgraded, it started failing.
He thought it was because the new compiler was inserting known values into the structure
(much the way that VS fills 0xCC). In my own experience, it was just different
random values that didn’t happen to be valid.

But now I’m curious — is there any setting of g++ that would make it fill
memory that the standard would otherwise say should be uninitialized?

Answer 1

I don’t think such option/feature exists in gcc/g++.

For instance, all global (and static) variables reside in the .bss section, which is always initialised to zeroes. However, uninitialised ones are put in a special section within the .bss, for sake of compatibility.

If you want the them to be zeroed too, you can pass -fno-common argument to the compiler. Or, if you need it on a per-variable basis, use __attribute__ ((nocommon)).

For heap, it’s possible to write your own allocator to accomplish what you described. But for stack, I don’t think there’s an easy solution.