Below C code demonstrates how to define a void(void) function to be called at program/library load time, before main executes.

For MSVC, this places a pointer to the function in the user initializer section (.CRT$XCU), basically the same thing the compiler does for the constructor calls for static C++ objects. For GCC, uses a constructor attribute.

    // Initializer/finalizer sample for MSVC and GCC/Clang.
    // 2010-2016 Joe Lowe. Released into the public domain.
#include <stdio.h>
#include <stdlib.h>

#ifdef __cplusplus
    #define INITIALIZER(f) \
        static void f(void); \
        struct f##_t_ { f##_t_(void) { f(); } }; static f##_t_ f##_; \
        static void f(void)
#elif defined(_MSC_VER)
    #pragma section(".CRT$XCU",read)
    #define INITIALIZER2_(f,p) \
        static void f(void); \
        __declspec(allocate(".CRT$XCU")) void (*f##_)(void) = f; \
        __pragma(comment(linker,"/include:" p #f "_")) \
        static void f(void)
    #ifdef _WIN64
        #define INITIALIZER(f) INITIALIZER2_(f,"")
    #else
        #define INITIALIZER(f) INITIALIZER2_(f,"_")
    #endif
#else
    #define INITIALIZER(f) \
        static void f(void) __attribute__((constructor)); \
        static void f(void)
#endif

static void finalize(void)
{
    printf( "finalize\n");
}

INITIALIZER( initialize)
{
    printf( "initialize\n");
    atexit( finalize);
}

int main( int argc, char** argv)
{
    printf( "main\n");
    return 0;
}

__attribute__((constructor)) equivalent in VC?, I was wondering if it's possible to use C constructors in VC just as it is constructor(fn) void fn(void) __attribute__((constructor)) #endif static� I've looked at __attribute__((constructor)) equivalent in VC? and CRT Initialization, which were both helpful regarding the gcc-specific __attribute__((constructor)).

I don't think there's a way to avoid using C++ features with MSVC. (MSVC's C support sucks anyways.)

Untested, but this should at least allow the same code to work in both MSVC and GCC.

#if defined(_MSC_VER)
struct construct { construct(void (*f)(void)) { f(); } };
#define constructor(fn) \
    void fn(void); static constructor constructor_##fn(fn)
#elif defined(__GNUC__)
#define constructor(fn)
    void fn(void) __attribute__((constructor))
#endif

static constructor(foo);
void foo() {
    ...
}

tenmyo/initializer.c, c - __attribute__((constructor)) equivalent in VC? - Stack Overflow. // http:// stackoverflow.com/questions/1113409/attribute-constructor-equivalent-in-vc/ 2390626#� Equivalent macro for "__attribute__" in Visual studio. OSR_Community_User Member Posts: 110,217. May 2016 in NTDEV. Hi Experts, I am working on porting device driver

You are probably interested in DllMain.

Function Attributes, In GNU C, you declare certain things about functions called in your program which help The keyword __attribute__ allows you to specify special attributes when making a declaration. Similarly, the destructor attribute causes the function to be called __attribute__ ((sentinel)) is equivalent to __attribute__ (( sentinel(0))). I was wondering what the best way to deal with code containing GCC's __attribute__ extension when using MSVC. Is the following a safe way of dealing with this: #define __attribute__(x) /* blank - should simply ignore thanks to C preprocessor */ Thanks!

I tried the last answer in MSVC like

#ifdef _MSC_VER
#pragma section(".CRT$XCU",read)
#define INITIALIZER2_(f,p) \
        static void f(void); \
        __declspec(allocate(".CRT$XCU")) void (*f##_)(void) = f; \
        __pragma(comment(linker,"/include:" p #f "_")) \
        static void f(void)
#ifdef _WIN64
#define INITIALIZER(f) INITIALIZER2_(f,"")
#else
#define INITIALIZER(f) INITIALIZER2_(f,"_")
#endif
#else
#define INITIALIZER(f) \
        static void f(void) __attribute__((constructor)); \
        static void f(void)
#endif

but INITIALIZER(f) can't appear in 2 different files with the same function name passed to INITIALIZER, the following definition will allow that

#ifdef _MSC_VER
#define INITIALIZER(f) \
    static void f();\
    static int __f1(){f();return 0;}\
    __pragma(data_seg(".CRT$XIU"))\
    static int(*__f2) () = __f1;\
    __pragma(data_seg())\
    static void f()
#else
#define INITIALIZER(f) \
    __attribute__((constructor)) static void f()
#endif

This page provides great understanding about the constructor and destructor attribute implementation and the sections within within ELF that allow them to work. After digesting the information provided here, I compiled a bit of additional information and (borrowing the section example from Michael Ambrus above) created an example to illustrate the concepts and help my learning.

__attribute__ ((sentinel)) is equivalent to __attribute__ ((sentinel(0))) The attribute is automatically set with a position of 0 for the built-in functions execl and execlp. The built-in function execle has the attribute set with a position of 1. A valid NULL in this context is defined as zero with any pointer type.

I am using Visual c++ 6.0 for the first time in my life. My new Windows machine cannot find the "attribute" mechanism. Apparently this is because __attribute__ is gcc specific. I am trying to compile some headers that I created on my Linux box using gcc. How can I force Visual C++ to use gcc's __attribute__ mechanism? Is there a special header

__attribute__ ((sentinel)) is equivalent to __attribute__ ((sentinel(0))) The attribute is automatically set with a position of 0 for the built-in functions execl and execlp . The built-in function execle has the attribute set with a position of 1.

Comments

• for those of us who haven't used it, what does it do? (And what do you need it for?)
• Any function marked as constructor is run by the dynamic linker as it loads the object.
• @:Jalf: How exactly does attribute constructor work?
• Hi Joe: great post! I really need the tricks for working with VC. It's critical to auto-register a unit test function in C(not C++). BTW: There should be a tailing \ in the second line of macro INITIALIZER
• Such constuctors will be optimized out in new Visual Studio release builds. Its rerely known and unfortunately unsolved bug. The only workaround I found so far: Project properties > C / C++ > Optimization > Whole Program Optimization(/ GL) must be disabled.
• I'm experiencing the same problem as described above so don't just copy paste above code assuming it will just work.
• GLib recently switched from using .CRT$XCU section to using DllMain for this reason: bugzilla.gnome.org/show_bug.cgi?id=752837
• @user3042599. Thanks for the heads up on the issues with MSVC 2015 link-time optimizations. I have edited the code sample and it should not have issues, though it now requires MSVC 2008 or later due to use of __pragma() .
• As far as I can tell this does not work in plain C on MSVC 2013 or 2015. Seems like C compiler mode doesnt support constructors in a struct. I tried to make it work in various ways.
• It must be executed before entering main(), that's for dlls, how's that related ? :/
• About the only real use people have for __attribute__((constructor)) is to use them in shared libraries similar to emulate a DllMain :)
• __attribute__((constructor)) is useful even in a single program image; for example, inserting global hooks around library and system calls, or registering built-in "plugins", or initializing data structures that dynamically linked modules will need in their "DllMain"-alikes.
• The linker pragma in my answer is needed, and does have the effect that initializer function names must be unique for the whole exe/dll. See the comments under my example by user3042599, and my response. I am unaware if the change of section name is an alternate solution to the linker pragma, but it will have other side effects.