main() return value is the program's C Exit status, communicated to the shell via the $? variable. Returning 0 indicates success (EXIT_SUCCESS); non-zero indicates failure (EXIT_FAILURE, typically 1). The shell can check this status with echo $? or use it in conditionals and pipelines.

Always explicitly return EXIT_SUCCESS or EXIT_FAILURE from main to make intent clear.

This example demonstrates returning different exit statuses based on program logic.

// compile: gcc -o mainreturn mainreturn.c
// run: ./mainreturn 5; echo $?
// description: return exit status from main to signal success or failure
 
#include <stdio.h>
#include <stdlib.h>
 
int main(int argc, char *argv[]) {
    if (argc != 2) {
        fprintf(stderr, "Usage: %s <number>\n", argv[0]);
        return EXIT_FAILURE;
    }
 
    int num = atoi(argv[1]);
    if (num > 0) {
        printf("Positive number: %d\n", num);
        return EXIT_SUCCESS;
    } else {
        fprintf(stderr, "Error: number must be positive\n");
        return EXIT_FAILURE;
    }
}