Question

I am reading Microsoft’s CRT source code, and I can come up with the following code, where the function __initstdio1 will be executed before main() routine.

The question is, how to execute some code before entering the main() routine in VC (not VC++ code)?

#include <stdio.h>  #pragma section('.CRT$XIC',long,read)  int __cdecl __initstdio1(void);  #define _CRTALLOC(x) __declspec(allocate(x))  _CRTALLOC('.CRT$XIC') static pinit = __initstdio1;  int z = 1;  int __cdecl __initstdio1(void) {     z = 10;     return 0; }  int main(void) {     printf('Some code before main!\n');     printf('z = %d\n', z);     printf('End!\n');     return 0; } 

The output will be:

Some code before main! z = 10 End! 

However, I am not able to understand the code.

I have done some google on .CRT$XIC but no luck is found. Can some expert explain above code segment to me, especially the followings:

1. What does this line _CRTALLOC('.CRT$XIC') static pinit = __initstdio1; mean? What is the significance of the variable pinit?
2. During compilation the compiler (cl.exe) throws a warning saying as below:

Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 15.00.30729.01 for 80×86 Copyright (C) Microsoft Corporation. All rights reserved.

stdmacro.c stdmacro.c(9) : warning C4047: 'initializing' : 'int' differs in levels of indirection from 'int (__ cdecl *)(void)' Microsoft (R) Incremental Linker Version 9.00.30729.01 Copyright (C) Microsoft Corporation.  All rights reserved.  /out:stdmacro.exe stdmacro.obj 

What is the corrective action needs to be done to remove the warning message?

Thanks in advance.

Added:

I have modified the code and give type to pinit as _PIFV. Now the warning message is gone.

The new code is as follows:

#include <stdio.h>  #pragma section('.CRT$XIC1',long,read)  int __cdecl __initstdio1(void);  typedef int  (__cdecl *_PIFV)(void);  #define _CRTALLOC(x) __declspec(allocate(x))  _CRTALLOC('.CRT$XIC1') static _PIFV pinit1 = __initstdio1;  int z = 1;  int __cdecl __initstdio1(void) {     z = 100;      return 0; }  int main(void) {     printf('Some code before main!\n');     printf('z = %d\n', z);     printf('End!\n');     return 0; } 

Answer 1

There’s some information here (search for CRT). The significance of variable pinit is none, it’s just a piece of data placed in the executable, where the runtime can find it. However, I would advise you to give it a type, like this:

_CRTALLOC('.CRT$XIC') static void (*pinit)()=... 

The linker warning probably just warns you you have a function that has int return type, but doesn’t return anything (probably you’d better change the return type to void).