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

I think it is easy to remove the undesired main() symbol from the object before linking.

Unfortunately the entry point option for g++ is not working for me(the binary crashes before entering the entry point). So I strip undesired entry-point from object file.

Suppose we have two sources that contain entry point function.

1. target.c contains the main() we do not want.
2. our_code.c contains the testmain() we want to be the entry point.

After compiling(g++ -c option) we can get the following object files.

1. target.o, that contains the main() we do not want.
2. our_code.o that contains the testmain() we want to be the entry point.

So we can use the objcopy to strip undesired main() function.

objcopy --strip-symbol=main target.o

We can redefine testmain() to main() using objcopy too.

objcopy --redefine-sym testmain=main our_code.o

And then we can link both of them into binary.

g++ target.o our_code.o -o our_binary.bin

This works for me. Now when we run our_binary.bin the entry point is our_code.o:main() symbol which refers to our_code.c::testmain() function.