Why do C and C++ compilers place explicitly initialized and default initialized global variables in different segments?

Question

I was reading this great post about memory layout of C programs. It says that default initialized global variables resides in the BSS segment, and if you explicitly provide a value to a global variable then it will reside in the data segment.

I've tested the following programs in C and C++ to examine this behaviour.

#include <iostream>
// Both i and s are having static storage duration
int i;     // i will be kept in the BSS segment, default initialized variable, default value=0
int s(5);  // s will be kept in the data segment, explicitly initialized variable,
int main()
{
    std::cout<<&i<<' '<<&s;
}

Output:

0x488020 0x478004

So, from the output it clearly looks like both variable i & s resides in completely different segments. But if I remove the initializer (initial value 5 in this program) from the variable S and then run the program, it gives me the below output.

Output:

0x488020 0x488024

So, from the output it clearly looks like both variables i and s resides in the same (in this case BSS) segment.

This behaviour is also the same in C.

#include <stdio.h>
int i;      // i will be kept in the BSS segment, default initialized variable, default value=0
int s=5;    // s will be kept in the data segment, explicitly initialized variable,
int main(void)
{
    printf("%p %p\n",(void*)&i,(void*)&s);
}

Output:

004053D0 00403004

So, again we can say by looking at the output (means examining the address of variables), both variable i and s resides in completely different segments. But again if I remove the initializer (initial value 5 in this program) from the variable S and then run the program it gives me the below output.

Output:

004053D0 004053D4

So, from the output it clearly looks like both variables i and s resides in the same (in this case BSS) segment.

Why do C and C++ compilers place explicitly initialized and default initialized global variables in different segments? Why is there a distinction about where the global variable resides between default initialized and explicitly initialized variables? If I am not wrong, the C and C++ standards never talk about the stack, heap, data segment, code segment, BSS segment and all such things which are implementation-specific. So, is it possible for a C++ implementation to store explicitly initialized and default initialized variables in the same segments instead of keeping it in different segments?

Answer 1

Neither language C or C++ has any notion of "segments", and not all OSs do either, so your question is inevitably dependent on the platform and compiler.

That said, common implementations will treat initialized vs. uninitialized variables differently. The main difference is that uninitialized (or default 0-initialized) data does not have to be actually saved with the compiled module, but only declared/reserved for later use at run time. In practical "segment" terms, initialized data is saved to disk as part of the binary, while uninitialized data is not, instead it's allocated at startup to satisfy the declared "reservations".