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

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.

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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.

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

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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.

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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?