C main() Function

C main()

The main() function is the entry point of every C program. It is where execution begins. The C compiler looks for the main() function to start program execution. Every C program must have exactly one main() function.

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Importance of main() Function

• Entry Point: Execution of a C program begins from the main() function.
• Standard Requirement: The C standard mandates that every C program must have a main() function.
• Return Value: The main() function typically returns an integer value to indicate the success or failure of the program.
• Arguments: It can take command-line arguments argc and argv to pass inputs from the command line.

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Syntax of main() Function

The syntax of main() function is:

int main() {
    // Code execution starts here
    return 0;
}

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Examples of main() Function

1. Basic main() Function

In this example, we will define a simple main() function that prints “Hello, World!” to the console.

main.c

#include <stdio.h>

int main() {
    printf("Hello, World!\n");
    return 0;
}

Explanation:

1. #include <stdio.h>: The standard input-output library is included to use printf().
2. int main(): The program execution starts from the main() function.
3. printf("Hello, World!\n");: The printf() function prints the text to the console.
4. return 0;: The program returns 0 to indicate successful execution.

Output:

Hello, World!

2. Returning an Exit Code from main()

In this example, we will return a non-zero exit code from the main() function to indicate an error.

main.c

#include <stdio.h>

int main() {
    printf("Exiting with error code 1\n");
    return 1; // Non-zero value indicates an error
}

Explanation:

1. printf("Exiting with error code 1\n");: Prints a message indicating an error.
2. return 1;: Returns a non-zero exit code to signal an error.

Output:

Exiting with error code 1

3. Using main() with Command-Line Arguments

In this example, we will modify the main() function to accept command-line arguments and print them.

main.c

#include <stdio.h>

int main(int argc, char *argv[]) {
    printf("Number of arguments: %d\n", argc);
    for (int i = 0; i < argc; i++) {
        printf("Argument %d: %s\n", i, argv[i]);
    }
    return 0;
}

Explanation:

1. int argc, char *argv[]: The main() function accepts command-line arguments.
2. printf("Number of arguments: %d\n", argc);: Prints the number of command-line arguments.
3. for (int i = 0; i < argc; i++): Iterates through all the arguments.
4. printf("Argument %d: %s\n", i, argv[i]);: Prints each argument passed to the program.

Output (Example Run in Terminal):

$ ./a.out Hello World
Number of arguments: 3
Argument 0: ./a.out
Argument 1: Hello
Argument 2: World

Conclusion

In this tutorial, we covered the main() function in C:

1. We covered the importance of the main() function as the entry point of a C program.
2. We demonstrated a simple main() function printing “Hello, World!”.
3. We showed how main() can return an exit code.
4. We implemented main() with command-line arguments.