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CRT Initialization

This topic describes how the CRT initializes global states in native code.

By default, the linker includes the CRT library, which provides its own startup code. This startup code initializes the CRT library, calls global initializers, and then calls the user-provided main function for console applications.

Consider the following code:

int func(void)
{
    return 3;
}

int gi = func();

int main()
{
    return gi;
}

According to the C/C++ standard, func() must be called before main() is executed. But who calls it?

One way to determine this is to set a breakpoint in func(), debug the application, and examine the stack. This is possible because the CRT source code is included with Visual Studio.

When you browse the functions on the stack, you will find that the CRT is looping through a list of function pointers and calling each one as it encounters them. These functions are either similar to func() or constructors for class instances.

The CRT obtains the list of function pointers from the Visual C++ compiler. When the compiler sees a global initializer, it generates a dynamic initializer in the .CRT$XCU section (where CRT is the section name and XCU is the group name). To obtain a list of those dynamic initializers run the command dumpbin /all main.obj, and then search the .CRT$XCU section (when main.cpp is compiled as a C++ file, not a C file). It will be similar to the following:

SECTION HEADER #6
.CRT$XCU name
       0 physical address
       0 virtual address
       4 size of raw data
     1F2 file pointer to raw data (000001F2 to 000001F5)
     1F6 file pointer to relocation table
       0 file pointer to line numbers
       1 number of relocations
       0 number of line numbers
40300040 flags
         Initialized Data
         4 byte align
         Read Only

RAW DATA #6
  00000000: 00 00 00 00                                      ....

RELOCATIONS #6
                                                Symbol    Symbol
 Offset    Type              Applied To         Index     Name
 --------  ----------------  -----------------  --------  ------
 00000000  DIR32                      00000000         C  ??__Egi@@YAXXZ (void __cdecl `dynamic initializer for 'gi''(void))

The CRT defines two pointers:

  • __xc_a in .CRT$XCA

  • __xc_z in .CRT$XCZ

Both groups do not have any other symbols defined except __xc_a and __xc_z.

Now, when the linker reads various .CRT groups, it combines them in one section and orders them alphabetically. This means that the user-defined global initializers (which the Visual C++ compiler puts in .CRT$XCU) will always come after .CRT$XCA and before .CRT$XCZ.

The section will resemble the following:

.CRT$XCA
            __xc_a
.CRT$XCU
            Pointer to Global Initializer 1
            Pointer to Global Initializer 2
.CRT$XCZ
            __xc_z

So, the CRT library uses both __xc_a and __xc_z to determine the start and end of the global initializers list because of the way in which they are laid out in memory after the image is loaded.

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