in c++ main function is the entry point to program how i can change it to an other function?

Question

I was asked an interview question to change the entry point of a C or C++ program from main() to any other function. How is it possible?

Answer 1

This is highly speculative, but you might have a static initializer instead of main:

#include <iostream>

int mymain()
{
    std::cout << "mymain";
    exit(0);
}

static int sRetVal = mymain();

int main()
{
    std::cout << "never get here";
}

You might even make it 'Java-like', by putting the stuff in a constructor:

#include <iostream>

class MyApplication
{
public:
    MyApplication()
    {
        std::cout << "mymain";
        exit(0);
    }
};

static MyApplication sMyApplication;

int main()
{
    std::cout << "never get here";
}

Now. The interviewer might have thought about these, but I'd personally never use them. The reasons are:

• It's non-conventional. People won't understand it, it's nontrivial to find the entry point.
• Static initialization order is nondeterministic. Put in another static variable and you'll never now if it gets initialized.

That said, I've seen it being used in production instead of init() for library initializers. The caveat is, on windows, (from experience) your statics in a DLL might or might not get initialized based on usage.

Answer 2

It's very simple:

As you should know when you use constants in c, the compiler execute a kind of 'macro' changing the name of the constant for the respective value.

just include a #define argument in the beginning of your code with the name of start-up function followed by the name main:

Example:

#define my_start-up_function (main)

Answer 3

Changing a value in Linker Settings will override the entry point. i.e., MFC applications use a value of 'Windows (/SUBSYSTEM:WINDOWS)' to change entry point from main() to CWinApp::WinMain().

Right clicking on solution > Properties > Linker > System > Subsystem > Windows (/SUBSYSTEM:WINDOWS)

...

Very practical benefit to modifying entry point:

MFC is a framework we take advantage of to write Windows applications in C++. I know it's ancient, but my company maintains one for legacy reasons! You will not find a main() in MFC code. MSDN says the entry point is WinMain(), instead. Thus, you can override the WinMain() of your base CWinApp object. Or, most people override CWinApp::InitInstance() because the base WinMain() will call it.

Disclaimer: I use empty parentheses to denote a method, without caring how many arguments.

Answer 4

From C++ standard docs 3.6.1 Main Function,

A program shall contain a global function called main, which is the designated start of the program. It is implementation-defined whether a program in a freestanding environment is required to define a main function.

So, it does depend on your compiler/linker...

Answer 5

With gcc, declare the function with attribute((constructor)) and gcc will execute this function before any other code including main.

Answer 6

The entry point is actually the _start function (implemented in crt1.o) .

The _start function prepares the command line arguments and then calls main(int argc,char* argv[], char* env[]), you can change the entry point from _start to mystart by setting a linker parameter:

g++ file.o -Wl,-emystart -o runme

Of course, this is a replacement for the entry point _start so you won't get the command line arguments:

void mystart(){

}

Note that global/static variables that have constructors or destructors must be initialized at the beginning of the application and destroyed at the end. Keep that in mind if you are planning on bypassing the default entry point which does it automatically.