Execute main function in C++

Purpose of the _start() function

The input inputs for the next function, _libc start main(), are prepared by the _start() function. This is the _libc start main() function's prototype. The arguments supplied by the _start() function can be seen here.

int __libc_start_main(int (*main) (int, char **, char **), /*This is the address of main function*/
int argc, /*It contains number of command line args*/
char ** ubp_av, /* The command line arg array*/
void (*init) (void), /* It contains address of init function*/
void (*fini) (void), /* address of fini function*/
void (*rtld_fini) (void), /* address of dynamic linker fini function is loaded here*/
void (* stack_end) /*The end of the stack address*/
);

You can also try this code with Online C++ Compiler

Run Code

Also Read - C++ Interview Questions

Purpose of the _libc_start_main() function

The function _libc_start_main() serves the following purposes:

• Getting the environment variables ready for the program's execution
• The _init() method is called before the main() function is started, and it performs initialization.
• After the programme is finished, use the _fini() and _rtld_fini() routines to clean up.

_libc start main() calls the main() function when all of the prerequisite activities have been accomplished.

Also read -  Decimal to Binary c++

Executing program without main() function

To be clear, main() is simply a standard word for startup code. Startup code can be given any name; it is not required to be called "main." Because the _start() function calls main() by default, we must update it if we wish to run our own startup code. We can call our custom startup code instead of main by overriding the _start() function(). Consider the following program saved with the file named ninja.cpp

#include<stdio.h>
#include<stdlib.h>
void _start()
{
    int x = Coding_Ninjas(); //User-Defined main function
    exit(x);
}

int Coding_Ninjas() // User-Defined main function
{
    printf("Happy Coding!!");
    return 0;
}

You can also try this code with Online C++ Compiler

Run Code

Now we have to inform the compiler not to utilise its own _start implementation (). We can achieve this with GCC's -nostartfiles option.

gcc -nostartfiles -o ninja ninja.cpp

Executing the executable file

./ninja

Output:

Happy Coding!!

You can try by yourself with the help of online C++ Compiler .

Also check out this article - Pair in C++

FAQs

1. Can you execute a program without the main() function in C++? 
   Yes, a program can be executed without the main() function in C++.
    
2. What is the purpose of the _start() function?
   The input inputs for the next function, _libc start main(), are prepared by the _start() function.
    
3. What is the purpose of the _libc_start_main() function?
   The purpose of the _libc_start_main() function varies from setting up the environment variable , calling the _init() method and use of _fini() and _rtld_fini() routines to clean up.

Key Takeaways

In this article, we have extensively discussed executing the main function in C++. The article explains the details about the main() function in C++ and executing programs without the main() function. The major roles of _start() function and _libc_start_main() are also explained with suitable examples.
We hope that this blog has helped you enhance your knowledge regarding the main() function in C++ and if you would like to learn more, check out our articles on C++. Do upvote our blog to help other ninjas grow. 

Happy Coding!