What Should main() Return in C? Understanding Return Values

Last update on December 20 2024 13:15:26 (UTC/GMT +8 hours)

In C Language, the main() function is the entry point of any program, where execution starts. According to the C standard, main() should return an integer, indicating the program's exit status to the operating system. This exit status allows the operating system or calling process to understand if the program executed successfully or encountered an error.

Standard Return Values:

return 0;:

Returning 0 indicates successful program execution.
By convention, 0 means the program ran without any errors.

return 1; or another non-zero integer:

Returning any non-zero value signals an error or unsuccessful execution.
Commonly, 1 or other specific values are used to indicate different types of errors.

return EXIT_SUCCESS; or return EXIT_FAILURE;:

The stdlib.h library defines these constants:

EXIT_SUCCESS generally equals 0 and signifies success.
EXIT_FAILURE is typically non-zero and signifies an error.

Using these constants improves code readability and portability.

Example: Using Different Return Values in main()

The following code demonstrates different return values in main():

Code:

#include <stdio.h> 
#include <stdlib.h>  // Required for EXIT_SUCCESS and EXIT_FAILURE

int main() {
    int success = 1; // Simulate a condition for success or failure

    if (success) {
        printf("Program completed successfully.\n");
        return 0; // Return 0 for success
    } else {
        printf("Program encountered an error.\n");
        return 1; // Return non-zero for failure
    }
}

Output:

Program completed successfully.

Explanation:

return 0; signals the operating system that the program completed successfully.
return 1; (or any non-zero value) signals that an error occurred.

Following examples show how EXIT_SUCCESS and EXIT_FAILURE communicate the status of a program’s execution to the operating system.

Example: Successful program execution using EXIT_SUCCESS

This example demonstrates a program that checks if a file exists. If the file is found, the program returns EXIT_SUCCESS to indicate successful execution.

Code:

#include <stdio.h> 
#include <stdlib.h>

int main() {
    FILE *file = fopen("example1.txt", "r");  // Try to open a file

    if (file) {
        printf("File found and opened successfully.\n");
        fclose(file);
        return EXIT_SUCCESS;  // Return success
    } else {
        printf("File not found.\n");
        return EXIT_FAILURE;  // Return failure
    }
}

Expected Output:

If the file example.txt exists:
File found and opened successfully.

If the file example.txt doesn’t exist:
File not found.

Explanation:

If example.txt exists, the program prints File found and opened successfully. and returns EXIT_SUCCESS.
If example.txt doesn’t exist, it prints File not found. and returns EXIT_FAILURE.

Example: Unsuccessful program execution using EXIT_FAILURE

In this example, the program performs a simple division operation but checks for a possible error (division by zero). If an invalid operation is detected, the program returns EXIT_FAILURE.

Code:

#include <stdio.h> 
#include <stdlib.h>

int main() {
    int numerator = 10;
    int denominator = 0;  // Setting to 0 to simulate an error

    if (denominator == 0) {
        printf("Error: Division by zero is not allowed.\n");
        return EXIT_FAILURE;  // Return failure
    } else {
        int result = numerator / denominator;
        printf("Result: %d\n", result);
        return EXIT_SUCCESS;  // Return success
    }
}

Expected Output:

Since denominator is set to 0, the output will be:
Error: Division by zero is not allowed.
If denominator were a non-zero value (e.g., 2), the output would be:
Result: 5

Explanation:

The program checks if denominator is zero before performing the division.
If denominator is zero, it prints an error message and returns EXIT_FAILURE.
If denominator is non-zero, it performs the division and returns EXIT_SUCCESS.