Difference between Linux and Windows linker

What is the difference in linking on various operating system?

For example the following code produces a linker error on Windows (compiled both with Vs2010 and gcc), but compiles successfully on Linux (Ubuntu,gcc):

extern int foo

int main() {
    foo=1;
}

Gcc command:

gcc -shared fil开发者_StackOverflow中文版ename.cpp

If you are trying to compile it as a windows shared library you need something like (code stolen from Wikipedia!) :-

#include <windows.h>


// DLL entry function (called on load, unload, ...)
BOOL APIENTRY DllMain(HANDLE hModule, DWORD dwReason, LPVOID lpReserved)
{
    return TRUE;
}

// Exported function - adds two numbers
extern "C" __declspec(dllexport) double AddNumbers(double a, double b)
{
    return a + b;
}

Windows shared modules (DLLs) require a DllMain entry point (executed the first time the module is loaded) and function names need to be exported via the declspec gobledygook before they can be used by another program.