About the return value of the main function

Many programmers can't understand why the main function must have a integer as the return value. They have been told that the return value of the main function is helpful to check the exit state of a program. However, nobody has been told how to do the check. Today, I found that we can get the return value of a program after it finishes executing by viewing the $? enviromental variable (with "echo $?" command).

For example:
Source file (TestRV.c):
#include <stdio.h>
int main()
{
 int rt;
 scanf("%d",&rt);
 return rt;
}

Compile & Run as:
$ gcc -o TRV TestRV.c
$ ./TRV
34
$ echo $?
34
$ echo $?
0

(Enviornment: linux kernel 2.6.17, gcc 4.1.2, echo 5.96)

Can you understand this easy program? Yes, it get your input to return as the return value of the main function, and the "echo $?" command has test the last return value and output. You may ask that why "echo $?" output "0" the second time. The answer is that its output is the return value of the main function of the first time you type "echo $?".

 

But, there is still one question left: What does program "echo" do exactly?
I'm not sure of that yet, just to be continued.