A C program is a set of declarations and function definitions; execution starts at main, and its return value becomes the process exit status.

Anatomy of a program

C

/* 1. Preprocessor directives */
#include <stdio.h>
#define GREETING "Hi"

/* 2. Global declarations */
static int counter = 0;

/* 3. Function prototype (declaration) */
int square(int x);

/* 4. main: the entry point */
int main(void) {
    counter++;
    printf("%s! square(7) = %d, counter = %d\n", GREETING, square(7), counter);
    return 0;                /* 0 = success */
}

/* 5. Function definitions */
int square(int x) {
    return x * x;
}

Output

Hi! square(7) = 49, counter = 1

Valid signatures of main

Signature	Use
`int main(void)`	No command-line arguments.
`int main(int argc, char *argv[])`	With arguments (`char **argv` is identical).
`void main()`	Not standard C. Some compilers accept it; never write it.

return 0; or return EXIT_SUCCESS; means success; EXIT_FAILURE (from <stdlib.h>) means failure.
Reaching the closing } of main without return is treated as return 0; (C99+) — but only for main.
int main() with empty parentheses means unspecified arguments in C17; prefer (void).

Comments

C

#include <stdio.h>

int main(void) {
    /* block comment — can span
       several lines, but cannot be nested */
    int x = 5;   // line comment (C99+)
    printf("%d\n", x);
    return 0;
}

Output

Declarations vs definitions

What happens before main runs?

What does the return value of main mean?