How to write a running C code without main()? Last Updated : 11 Sep, 2023 Comments Improve Suggest changes Like Article Like Report 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:- 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: GeeksforgeeksCompilation 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) Comment More infoAdvertise with us Next Article How to write a running C code without main()? A Abhay Rathi Improve Article Tags : C Language c-puzzle Similar Reads Print "Hello World" in C/C++ without using any header file Write a C/C++ program that prints Hello World without including any header file. Conceptually it's seems impractical to write a C/C++ program that print Hello World without using a header file of "stdio.h". Since the declaration of printf() function contains in the "stdio.h" header file. But we can 2 min read How to Change the Output of printf() in main() in C? To change the output of printf() in main(), we can use Macro Arguments. #define macro can be used for this task. This macro is defined inside the function. Although, #define can be used without declaring it in the function, in that case always the printf() will be changed. The function needs to be c 2 min read How to write your own header file in C? As we all know that files with .h extension are called header files in C. These header files generally contain function declarations which we can be used in our main C program, like for e.g. there is need to include stdio.h in our C program to use function printf() in the program. So the question ar 4 min read What does main() return in C and C++? C According to coding standards, a good return program must exit the main function with 0. Although we are using void main() in C, In which we have not suppose to write any kind of return statement but that doesn't mean that C code doesn't require 0 as exit code. Let's see one example to clear our t 3 min read How to print "GeeksforGeeks" with empty main() in C, C++ and Java? Write a program that prints "GeeksforGeeks" with empty main() function. You are not allowed to write anything in main(). C language One way of doing this is to apply GCC constructor attribute to a function so that it executes before main() (See this for details). C #include <stdio.h> /* Apply 2 min read Like