MSVCR and CRT Initialization

Question

Out of curiosity, what exactly happens when an application compiled with the MSVCR is loaded, resp. how does the loader of Windows actually initialize the CRT? For what I have gathered so far, when the program as well as all the imported libraries are loaded into memory and all relocations are done, the CRT startup code (_CRT_INIT()?) initializes all global initializers in the .CRT$XC* sections and calls the user defined main() function. I hope this is correct so far.

But let's assume, for the sake of explanation, a program that is not using the MSVCR (e.g. an application built with Cygwin GCC or other compilers) tries to load a library at runtime, requiring the CRT, using a custom loader/runtime linker, so no LoadLibrary() involved. How would the loader/linker has to handle CRT initialization? Would it have to manually initialize all "objects" in said sections, does it have to do something else to make the internal wiring of the library work properly, or would it have to just call _CRT_INIT() (which unpractically is defined in the runtime itself and not exported anywhere as far as I am aware). Would this mix-up even work in any way, assuming that the non-CRT application and the CRT-library would not pass any objects, exceptions and things the like between them?

I would be very interested in finding out, because I can't quite make out what the CRT has an effect on the actual loading process...

Any information is very appreciated, thanks!

Answer 1

Each library compiled against the C++ runtime is calling _DllMainCRTStartup when it's loaded. _DllMainCRTStartup calls _CRT_INIT, which initializes the C/C++ run-time library and invokes C++ constructors on static, non-local variables.

The PE format contains an optional header that has a slot called 'addressofentrypoint', this slot calls a function that will call _DllMainCRTStartup which fires the initialization chain.

after _DllMainCRTStartup finishes the initialization phase it will call your own implemented DllMain() function.

Answer 2

The entrypoint for an executable image is selected with the /ENTRY linker option. The defaults it uses are documented well in the MSDN Library article. They are the CRT entrypoint.

If you want to replace the CRT then either pick the same name or use the /ENTRY option explicitly when you link. You'll also need /NODEFAULTLIB to prevent it from linking the regular .lib

Answer 3

When you learn about programming, someone will tell you that "the first thing that happens is that the code runs in main. But that's a bit like when you learn about atoms in School, they are fairly well organized and operate acorrding to strict rules. If you later go to a Nuclear/Particle Physics class at university, those simple/strict rules are much more detailed and don't always apply, etc.

When you link a C or C++ program the CRT contains some code something like this:

start()
{
   CRT_init();
   ...
   Global_Object_Constructors(); 
   ... 
   exit(main());
}

So the initialization is done by the C runtime library itself, BEFORE it calls your main.

A DLL has a DllMain that is executed by LoadLibrary() - this is responsible for initializing/creating global objects in the DLL, and if you don't use LoadLibrary() [e.g. loading the DLL into memory yourself] then you would have to ensure that objects are created and initialized.