__do_global_dtors_aux and __do_global_ctors_aux

问题:

I disassembled a simple program written in C++ and there are these two function names. I guess that ctor means constructor and dtor means destructor, and word global maybe means that they create and destroy global objects. I cannot guess the name aux. What do these two functions do?

回答1:

The addresses of constructors and destructors of static objects are each stored in a different section in ELF executable. for the constructors there is a section called .CTORS and for the destructors there is the .DTORS section.

the compiler creates two auxillary functions __do_global_ctors_aux and __do_global_dtors_aux for calling the constructors and destructors of these static objects, respectively.

__do_global_ctors_aux function simply performs a walk on the .CTORS section, while the __do_global_dtors_aux does the same job only for the .DTORS section which contains the program specified destructors functions.

回答2:

They are auxiliary functions added by the compiler to construct and destroy static objects.

e.g.

std::vector<int> some_global;

int main() { return 0; }

some_global needs to be actually constructed (and destructed) somewhere, and C++ guarantees that construction happens before main. One way to do this is to emit a function that happens before main, which constructs global objects, and another function that happens after main to destroy them.

If you stuck a breakpoint inside the std::vector constructor and ran this program, the stack trace would show you the function that it was being initialised from.