How global pointer variables are stored in memory?

Question

Suppose we have a simple code :

int* q = new int(13);

int main() {
    return 0;
}

Clearly, variable q is global and initialized. From this answer, we expect q variable to be stored in initialized data segment (.data) within program file but it is a pointer, so it's value (which is an address in heap segment) is determined at run time. So what's the value stored in data segment within program file ?

My try:
In my thinking, compiler allocates some space for variable q (typically 8 bytes for 64 bit address) in data segment with no meaningful value. Then, puts some initialization code in text segment before main function code to initialize q variable at run time. Something like this in assembly :

     ....
     mov  edi, 4
     call operator new(unsigned long)
     mov  DWORD PTR [rax], 13  // rax: 64 bit address (pointer value)

     // offset : q variable offset in data segment, calculated by compiler
     mov  QWORD PTR [ds+offset], rax // store address in data segment
     ....
main:
     ....

Any idea?

Answer 1

Yes, that is essentially how it works.

Note that in ELF .data, .bss, and .text are actually sections, not segments. You can look at the assembly yourself by running your compiler:

c++ -S -O2 test.cpp

You will typically see a main function, and some kind of initialization code outside that function. The program entry point (part of your C++ runtime) will call the initialization code and then call main. The initialization code is also responsible for running things like constructors.