C++: When (and how) are C++ Global Static Constructors Called?

Question

I'm working on some C++ code and I've run into a question which has been nagging me for a while... Assuming I'm compiling with GCC on a Linux host for an ELF target, where are global static constructors and destructors called?

I've heard there's a function _init in crtbegin.o, and a function _fini in crtend.o. Are these called by crt0.o? Or does the dynamic linker actually detect their presence in the loaded binary and call them? If so, when does it actually call them?

I'm mainly interested to know so I can understand what's happening behind the scenes as my code is loaded, executed, and then unloaded at runtime.

Thanks in advance!

Update: I'm basically trying to figure out the general time at which the constructors are called. I don't want to make assumptions in my code based on this information, it's more or less to get a better understanding of what's happening at the lower levels when my program loads. I understand this is quite OS-specific, but I have tried to narrow it down a little in this question.

Answer 1

When talking about non-local static objects there are not many guarantees. As you already know (and it's also been mentioned here), it should not write code that depends on that. The static initialization order fiasco...

Static objects goes through a two-phase initialization: static initialization and dynamic initialization. The former happens first and performs zero-initialization or initialization by constant expressions. The latter happens after all static initialization is done. This is when constructors are called, for example.

In general, this initialization happens at some time before main(). However, as opposed to what many people think even that is not guaranteed by the C++ standard. What is in fact guaranteed is that the initialization is done before the use of any function or object defined in the same translation unit as the object being initialized. Notice that this is not OS specific. This is C++ rules. Here's a quote from the Standard:

It is implementation-defined whether or not the dynamic initialization (8.5, 9.4, 12.1, 12.6.1) of an object of namespace scope is done before the first statement of main. If the initialization is deferred to some point in time after the first statement of main, it shall occur before the first use of any function or object defined in the same translation unit as the object to be initialized

Answer 2

This depends heavy on the compiler and runtime. It's not a good idea to make any assumptions on the time global objects are constructed.

This is especially a problem if you have a static object which depends on another one being already constructed.

This is called "static initialization order fiasco". Even if thats not the case in your code, the C++Lite FAQ articles on that topic are worth a read.