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

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  • Difficulty Level : Easy
  • Last Updated : 21 Jun, 2022
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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:-up

  1. Using a macro that defines main 

C




#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) 

C




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

Output: Geeksforgeeks

        3. 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

       4. 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)

This article is contributed by Abhay Rathi and improved by Shubham Bansal. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.


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