How to write a running C code without main()?

Write a C language code that prints GeeksforGeeks without any main function. Logically it's seems impossible to write a C program without using a main() function. Since every program must have a main() function because:-

• It's an entry point of every C/C++ program.
• All Predefined and User-defined Functions are called directly or indirectly through the main.

Therefore we will use preprocessor(a program which processes the source code before compilation) directive #define with arguments to give an impression that the program runs without main. But in reality it runs with a hidden main function. Let's see how the preprocessor doing their job:-  

Hence it can be solved in following ways:-

1. Using a macro that defines main 

#include<stdio.h> 
#define fun main 
int fun(void) 
{ 
    printf("Geeksforgeeks"); 
    return 0; 
}

Output: Geeksforgeeks

         2. Using Token-Pasting Operator The above solution has word 'main' in it. If we are not allowed to even write main, we can use token-pasting operator (see this for details) 

#include<stdio.h> 
#define fun m##a##i##n 
int fun() 
{ 
    printf("Geeksforgeeks"); 
    return 0; 
}

Output: Geeksforgeeks

        3. Using Argumented Macro 

#include<stdio.h> 
#define begin(m,a,i,n) m##a##i##n 
#define start begin(m,a,i,n) 

void start() { 
printf("Geeksforgeeks"); 
} 

Output: Geeksforgeeks

       4. Modify the entry point during compilation 

#include<stdio.h> 
#include<stdlib.h> 

// entry point function 
int nomain(); 

void _start(){ 

    // calling entry point 
    nomain(); 
    exit(0); 
} 

int nomain() 
{ 
    puts("Geeksforgeeks"); 
    return 0; 
} 

Output: 
Geeksforgeeks

Refer Executing main() in C – behind the scene for another solution. References: Macros and Preprocessors in C 

Time Complexity: O(1)

Auxiliary Space: O(1)