main Function in C (original) (raw)

Last Updated : 07 Mar, 2025

The **main function is the entry point of a C program. It is a user-defined function where the execution of a program starts. Every C program must contain, and its return value typically indicates the success or failure of the program. In this article, we will learn more about the main function in C.

**Example:

C `

#include <stdio.h>

int main() {

printf("Hello, World!\n");
return 0;

}

**Explanation: This basic example demonstrates a simple main() function. It prints "Hello, World!" and returns 0 to indicate that the program executed successfully.

If we try create a function with another name, the program wont be able to execute:

C `

#include <stdio.h>

int func() {

printf("Hello, World!\n");
return 0;

}

**Output

/usr/lib/x86_64-linux-gnu/crt1.o: In function _start': (.text+0x20): undefined reference to main'
collect2: error: ld returned 1 exit status

Syntax of main()

The syntax of the main() function can be written in two common forms:

**Without Command-Line Arguments

**return_type main() {
// Code goes here
// Returning
****}**

We can write the main function in many ways in **C language as follows:

int **main(){} or int **main(void){}

**main(){} or void **main(){} or **main(void){} or void **main(void){}

In the above notations, int means integer return type, and void return type means that does not return any information, and ****(void) or ()** means that does not take any information.

**Note: The void return type is supported but generally not recommended as the OS may not get any notification about the program execution if no value is returned

**With Command-Line Arguments

The argument of main function is different from the rest of user defined functions. As main function is called by OS, the user has to provide arguments at the time of starting program from the command line. That is why these arguments are also called **command line arguments.

return_type **main(int argc, char *argv[]) {
// Code goes here....
}

Here,

**argc: Stands for ARGument Count, it is an integer variable that stores the number of command-line arguments passed.
**argv: Stands for ARGument Vector, it is an array of character pointers listing all the arguments.

Types of main() in C

There are 3 variations of main() in C:

main() with No Arguments and void Return Type - Not Recommended
main() with no arguments and int return type - Recommended
main() with the Command Line Arguments

1. main() with No Arguments and void Return Type - Not Recommended

**This type of main() is rare and not commonly used. It doesn't accept any command-line arguments and does not return any value. While it may be supported by some compilers, it's not a standard practice in C, as the C standard expects main() to return an int.

**Example

C `

#include <stdio.h>

// Can also be 'void main(void)' void main() {
printf("Hello Geek!"); }

**Note: The return type of main function according to C standard should be int only. Even if your compiler is able to run void main(), it is recommended to avoid it.

2. main() with No Arguments and int Return Type - Recommended

This is the most commonly used form of main() in C. It does not take any command-line arguments but returns an integer value to the operating system. By convention, returning 0 indicates successful execution, while returning a non-zero value signals an error or abnormal termination.

**Example

C `

#include <stdio.h>

// Can be 'int main(void)' or 'main(void)' // or just 'main()' int main() { printf("Hello Geek!"); return 0; }

**Explanation: This basic example demonstrates a simple main() function. It prints "Hello, Geek!" and returns 0 to indicate that the program executed successfully.

3. main() with Command Line Arguments

This version of main() is used when a program needs to accept command-line arguments. The arguments are passed through argc (argument count) and argv (argument vector). argc holds the number of arguments, and argv is an array of strings representing the arguments.

This type is useful for programs that need to handle external input, such as filenames, options, or parameters passed at the time of execution.

**Example

C `

#include <stdio.h>

int main(int argc, char* argv[]) {

// Printing the coundt of arguments
  printf("The value of argc is %d\n", argc);

// Prining each argument
for (int i = 0; i < argc; i++) {
    printf("%s \n", argv[i]);
}

return 0;

}

Now, run the programs in the command prompt or terminal as seen below screenshot and passed any arguments. **main.exe is the name of the executable file created when the program runs for the first time. We passed three arguments "geeks for geeks" and print them using a loop.

output of main with command line arguments

Output of Main with Command Line Arguments