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

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?

In standard C (and, I believe, C++ as well), you can't, at least not for a hosted environment (but see below). The standard specifies that the starting point for the C code is main. The standard (c99) doesn't leave much scope for argument:

5.1.2.2.1 Program startup: (1) The function called at program startup is named main.

That's it. It then waffles on a bit about parameters and return values but there's really no leeway there for changing the name.

That's for a hosted environment. The standard also allows for a freestanding environment (i.e., no OS, for things like embedded systems). For a freestanding environment:

In a freestanding environment (in which C program execution may take place without any benefit of an operating system), the name and type of the function called at program startup are implementation-defined. Any library facilities available to a freestanding program, other than the minimal set required by clause 4, are implementation-defined.

You can use "trickery" in C implementations so that you can make it look like main isn't the entry point. This is in fact what early Windows compliers did to mark WinMain as the start point.

First way: a linker may include some pre-main startup code in a file like start.o and it is this piece of code which runs to set up the C environment then call main. There's nothing to stop you replacing that with something that calls bob instead.

Second way: some linkers provide that very option with a command-line switch so that you can change it without recompiling the startup code.

Third way: you can link with this piece of code:

int main (int c, char *v[]) { return bob (c, v); }

and then your entry point for your code is seemingly bob rather than main.

However, all this, while of possibly academic interest, doesn't change the fact that I can't think of one single solitary situation in my many decades of cutting code, where this would be either necessary or desirable.

I would be asking the interviewer: why would you want to do this?

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...

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.

If you are on VS2010, this could give you some idea

As it is easy to understand, this is not mandated by the C++ standard and falls in the domain of 'implemenation specific behavior'.