use of return 0; in main() in C

Question

I'm reading Programming in C by Brian Kernighnan an Dennis Ritchie in that(pg.25 bottom)

Here the author quotes:

The value that power computes is returned to main by the return statement. Any expression may follow return: return expression ;

But in the code he provided above:

#include <stdio.h>
int power(int m , int n);

main()
{
    int i;
    for (i = 0; i < 10; ++i)
    printf ( " %d \t %d \t %d \n ", i, power( 2 , i), power( -3 , i));
    return 0;//no. 1
}

int power(int m , int n)
{
    int i, p ;
    p = 1;
    for (i = 1; i <= n; ++i)
    p = p * m;
    return p; //no. 2
}

Here I understood why he used 2. return p; in function power (i.e to get the value of p) but why does he use 1. return 0; ?? I tried removing the line return 0; And it still worked as I had thought, is there something I'm missing??

[UPDATE] I'm sorry for not including this before but I already knew this much:
quoted in the book:

You may have noticed that there is a return statement at the end of main. Since main is a function like any other, it may return a value to its caller, which is in effect the environment in which the program was executed. Typi- cally, a return value of zero implies normal termination; non-zero values signal unusual or erroneous termination conditions. In the interests of simplicity, we have omitted return statements from our main functions up to this point, but we will include them hereafter, as a reminder that programs should return status to their environment.

Thanks to @Pascal I was able to understand the difference in both the return p; and return 0; However my intention was never to know what return 0; was but why return was used also to know the difference between both the return statment......

Answer 1

In a function other than the entry point of the program, return e; indicates the result that will be sent to the caller.

---

A return i; statement in the main() function is a way instead for the program to communicate an exit code to the operating system. In POSIX, return 0; indicates success.

The return 0; at the end of function main() is so perfunctory that in C99, it was made optional. The } ending function main() implicitly behaves like return 0; according to the standard:

5.1.2.2.3 Program termination

1 If the return type of the main function is a type compatible with int, a return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument; reaching the } that terminates the main function returns a value of 0[…]

However, in C90, there is no such exception, and it is technically undefined behavior to have an int-returning function such as main() reach the final } without a return statement. The worst that seems to happen in practice is that the program returns with an indeterminate value in the register or stack slot reserved for function results, so that the Operating System may think that the program failed when it actually succeeded.

Answer 2

main()'s return code is conventionally a way for programs to give the operating system an indication on whether they completed successfully or no. Conventionally a return value of zero means success, while other values are used to indicate errors.

OS's like UNIX/Linux and Windows provide ways to check such return code; in sh or bash the $? holds the return code of the last command executed, in Windows' cmd you can obtain it as %ERRORLEVEL%.

Answer 3

The main function of every C program should return 0 if the program worked correctly. It's a convention which you should always follow.

Answer 4

According to the ISO/IEC C++ Standard (14882), section 3.6.1 states that the function main should have one of the two signatures:

int main(void) { .... }
or
int main(int argc, char** argv) { .... }

Most compilers accept variants of this, such as just main() { .... } for backwards comparability, such as your example code. The return at the end of main is the value that is returned to the calling environment (typically the command shell) and it can be used to represent successful completion of the program (by return 0), or return an error code to the calling environment in case of program error.

If you leave the return off at the end of main, a return 0; is assumed.