Need explanations...

Could you please explain why sometimes in the example the code is in this form

Code:

---

void main()
{
  printf( "Welcome to C Programming.\n" );
}

---

sometimes

Code:

---

void main(void)
{
  printf( "Welcome to C Programming.\n" );
}

---

sometimes

Code:

---

int main()
{
  printf( "Welcome to C Programming.\n" );
}

---

and

Code:

---

main()
{
  printf( "Welcome to C Programming.\n" );
}

---

Because example code isn't perfect.

Whenever you see any of the other three example types, convert it to the third form. Even safer would be to convert it to:

Code:

---

int main(void)
{
    /* ... */
    return 0;
}

---

The main should always return an integer, this is what the C/C++ standard says. void main is not correct in this sense.