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Implementation of dynamic initialization for global variables and static member variables in C++
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I have some questions about the implementation of variable initialization in C++ in regards to the linking and executable module loading process. My main concern is with dynamic initialization of global variables and static member variables, wherein the initialization process involves the execution of code. I'm looking for the answer to address my questions for both Windows and Linux.

I already understand that in the case of static initialization:

-the initial value is placed into its own section during compilation

-the sections are mapped into memory by the OS module loader

-the variable is assigned the location of the initial value's memory address with the application of a DIR32 type relocation

Here are my questions.

  1. What information does the compiler place into a generated object file relating to dynamic initialization of global variables for the linker to use? Please go into as much detail as possible about related sections and generated symbols. What differences are there for static member variables compared to non-static global variables?

  2. What information does the linker place into the final linked module during the linking process so that the OS module loader is able to correctly initialize all variables (including dynamically initialized global/static member variables that make calls to functions as part of initialization)?

  3. How is the function that needs to be executed during dynamic variable initialization mapped to the particular variable that needs to be initialized with that code?

  4. When an executable or dynamically linked module is loaded, how is dynamic initialization of variables performed?

  5. Does the implementation of C++11 constant expressions (marked by the constexpr specifier) involve any special considerations compared to the implementation of regular static member variables and functions?

I have a specific example case I'm hoping the answer could refer to within the framework of the above questions, since I felt that having a concrete example of taking an object file, identifying the relevant sections/symbols and how this particular code would be linked and loaded so that successful initialization of the static variable could be carried out would make the answer easier to understand. This example is for Windows using MSVC as the compiler; please mention specific differences for gcc/linux wherever they exist.

Here is a simple example of C++ code involving a regular variable and a static member variable which from my understanding needs to be dynamically initialized by the OS loader before main because it makes a call to a function as part of its initialization:

class Test
{
public:

    static int testFunction()
    {
        return 10;
    }

    static int memberVar;
};

int Test::memberVar = Test::testFunction();

int foo()
{
    return 5;
}

int var = foo();


int main(int argc, char* argv[])
{
    var;

    Test::memberVar;

    return 0;
}

And here is a dump of the sections and symbols for the object file produced by MSVC using the above code compiled in debug mode (the dump was created with llvm-readobj, a utility which comes with llvm/clang):

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Thank you very much for considering my question; a thorough answer would be greatly appreciated.

Onym answered 30/6, 2015 at 11:40 Comment(7)
I'm not sure but I think you misunderstand static functions. Static functions are functions that are only visible to other functions in the same file (translation unit). They have no relationship to static data other than the keyword "static". – Sinapism
var needs dynamic initialization because its initializer is not a constant expression (it calls a non-constexpr function). As the comment above this one says, whether that function is static or not is completely irrelevant. It is also completely irrelevant whether the variable is static. Your question is about variables with static storage duration but all globals (except thread-locals) have static storage duration, not only ones marked static. – Misdeal
What you're asking in your first four questions is implementation specific. Do you want to know about these details for both Windows and Linux? You're basically asking two separate questions: what are all these details for Windows and what are all these details for Linux. – Scholarship
Jonathan Wakely, you're right and I've adjusted the question to reflect your comments. – Onym
CrazyCasta, I'm looking for the details for both Linux and Windows. Modified the question to make that clearer. – Onym
Why do you need to know all that? The compiler generates some executable code that initializes the variable. This code magically gets executed, hopefully before the variable is accessed for the first time. That's all normal people need to know. If you are doing something special, like implementing a compiler, you need to know more than that. But then you need to know quite a bit more than what can fit in an answer on this site. – Heine
n.m.: I'm working on a small linker for fun and learning and this is the last feature I need to implement. I'll try the ld linker mailing list as well and if I get the details there I'll answer my own question for anyone who might be interested. – Onym
B
2

This is less a linker problem than a compiler/run-time problem. The complete answer varies from system to system of course, but for gcc/clang on Linux goes something like this. Any specific symbols or sections that I'll mention are for the ARM, other processors may be different.

I copied you little example program into the ELLCC demo, which is a clang based distribution , and compiled it for ARM. I had to turn off optimization to see anything interesting, because your initialized variables aren't used.

What you'll see by looking at the assembly language is that the compiler will generate code to do any initialization that needs to be done in the source file. As you pointed out, things initialized to link-time constant values can be initialized by putting the appropriate symbol and it's initial value in a section (usually called .data for writable stuff and .const for read only stuff.) Values that can't be calculated at compile or link time are initialized by a a compiler generated function that is executed before main() is entered. If you compile your example and look at the assembly, near the end there are a few lines that look like this:

.section    .init_array,"aw",%init_array
.align  2
.long   _GLOBAL__sub_I__6873_0.cc(target1)

The magic here is that the section .init_array is a section with special meaning to the compiler and run-time system. the compiler has placed the address of an internally generated function in the .init_array section. The function does any initialization that this source files needs. If you had static C++ constructors in the source they'd also be called from here. There is a similar section for x86 processors called .ctors which has similar semantics.

Now comes the run-time part. When the program starts up, the run-time system gets control first. It does things like initialize the library, maybe load dynamic libraries, etc. and the takes each function pointer in the .init_array and executes it. This gets your variables initialized.

Notice that the linker didn't really have to do anything except gather all the function pointers in .init_array in one place so the run-time system could fund them.

As you might be guessing by now, there is also another section called .fini_array (or .dtors in the x86 world) that us used when the program tries to exit do handle global destructors.

Burschenschaft answered 5/7, 2015 at 20:9 Comment(1)
Thank you so much Richard, I was able to get dynamic initialization working thanks to your information. Following on from the information you provided I also found additional information about dynamic initialization for MSVC here msdn.microsoft.com/en-us/library/bb918180.aspx and here gonwan.com/?p=9. These articles explain how the CRT calls the array of function pointers before main (as you described). Under MSVC the section .CRT$XCU contains the functions responsible for initialization. Your post was instrumental in putting me on the right track, I really appreciate it! – Onym

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