Is it possible to write a program without using main() function?

Sample program without a visible main function.

/* 
    7050925.c 
    $ gcc -o 7050925 7050925.c
*/

#include <stdio.h>
#define decode(s,t,u,m,p,e,d) m##s##u##t
#define begin decode(a,n,i,m,a,t,e)

int begin()
{
        printf("How mainless!\n");
}

From: http://learnhacking.in/c-program-without-main-function/

main means an entry point, a point from which your code will start executing. although main is not the first function to run. There are some more code which runs before main and prepares the environment to make your code run. This code then calls main . You can change the name of the main function by recompiling the code of the startup file crt0.c and changing the name of the main function. Or you can do the following:

#include <stdio.h>

extern void _exit (register int code);

_start()
{
  int retval;
  retval = my_main ();
  _exit(retval);
}

int my_main(void)
{
  printf("Hello\n");
  return 0;
}

Compile the code with:

gcc -o no_main no_main.c -nostartfiles

The -nostartfiles will not include the default startup file. You point to the main entry file with the _start .

main is nothing but a predefined entrypoint for the user code. Therefore you can name it whatever, but at the end of the day you do need an entry point. In C/C++ and other languages the name is selected as main if you make another language or hack the sources of these language compilers then you can change the name of main to pain but it will bring pain, as it will violate the standards.

But manipulating the entry function name is useful for kernel code, the first function to run in the kernel, or code written for embedded systems.

Within standard C++ a main function is required, so the question does not make sense for standard C++.

Outside of standard C++ you can for example write a Windows specific program and use one of Microsoft's custom startup functions (wMain, winMain, wWinmain). In Windows you can also write the program as a DLL and use rundll32 to run it.

Apart from that you can make your own little runtime library. At one time that was a common sport.

Finally, you can get clever and retort that according to the standard's ODR rule main isn't "used", so any program qualifies. Bah! Although unless the interviewers have unusual good sense of humor (and they wouldn't have asked the question if they had) they'll not think that that's a good answer.

No you cannot unless you are writing a program in a freestanding environment (embedded environment OS kernel etc.) where the starting point need not be main(). As per the C++ standard main() is the starting point of any program in a hosted environment.

As per the:

C++03 standard 3.6.1 Main function

1 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. [ Note: In a freestanding environment, start-up and termination is implementation-defined; startup contains the execution of constructors for objects of namespace scope with static storage duration; termination contains the execution of destructors for objects with static storage duration.

---

What is freestanding Environment & What is Hosted Environment?
There are two kinds of conforming implementations defined in the C++ standard; hosted and freestanding.

A freestanding implementation is one that is designed for programs that are executed without the benefit of an operating system.
For Ex: An OS kernel or Embedded environment would be a freestanding environment.

A program using the facilities of an operating system would normally be in a hosted implementation.

From the C++03 Standard Section 1.4/7:

A freestanding implementation is one in which execution may take place without the benefit of an operating system, and has an implementation-defined set of libraries that includes certain language-support libraries.

Further,
Section: 17.4.1.3.2 Freestanding implementations quotes:

A freestanding implementation has an implementation-defined set of headers. This set shall include at least the following headers, as shown in Table:

18.1 Types <cstddef>   
18.2 Implementation properties <limits>   
18.3 Start and termination <cstdlib> 
18.4 Dynamic memory management <new> 
18.5 Type identification <typeinfo> 
18.6 Exception handling <exception> 
18.7 Other runtime support <cstdarg>