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

Last Updated : 23 Jul, 2025

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 C `

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

`

Output: Geeksforgeeks

1. 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)

C `

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

`

Output: Geeksforgeeks

1. Using Argumented Macro

C `

#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

1. Modify the entry point during compilation

C `

#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

Compilation using command : gcc filename.c -nostartfiles (nostartfiles option tells the compiler to avoid standard linking) Explanation: Under normal compilation the body of _start() will contain a function call to main() [ this _start() will be appended to every code during normal compilation], so if that main() definition is not present it will result in error like "In function `_start': (.text+0x20): undefined reference to `main'. In the above code what we have done is that we have defined our own _start() and defined our own entry point i.e nomain()

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)