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

   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() defination 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()