Why isn't "Return" necessary? - c

Question

The following code is for the lab https://cs50.harvard.edu/x/2021/labs/1/population/.
#include <cs50.h>
#include <stdio.h>
int main(void)
{
//Prompt for start size
int startPop;
do
{
startPop = get_int("Starting population: ");
}
while (startPop < 9);
//Prompt for end size
int endPop;
do
{
endPop = get_int("Ending population: ");
}
while (endPop < startPop);
int Years = 0;
while (startPop < endPop)
{
startPop = startPop + (startPop/3) - (startPop/4);
Years++;
}
printf("Total Years: %i", Years);
}
Why isn't return used after each integer is received? Like this
int startPop;
do
{
startPop = get_int("Starting population: ");
}
while (startPop < 9);
return startPop;
How do I know when and where to use it? What is the purpose of return?
It seems like every time I try to solve a problem I am completely off base and don't even know where/how to begin, even after hours of thinking about it and then the solution also mystifies me.

Answer 1

return will exit the function that you are currently in, and the remainder of the function will not execute. It may also return a value from that function, if it is a non-void function. Here is an example:
int main(void)
{
foo()
}
void foo()
{
//...some code...
return;
//..some more code...
}
In this case foo will be called and will run until it hits the return, at which point the execution will return to main, and all the code in foo after main will not execute.
In your program, there is only one function (main), and calling return from main will exit the program (not what we want in this case).

Answer 2

The return XX at the end of main is not necessary because the C standard says so:
C18 §5.1.2.2.3 Program termination
... reaching the } that terminates the main function returns a value of 0.
In other words, if there is no return xx the int main() function is presumed to return 0.
For all other functions that return a value, the function must explicitly return a value.
For void functions, the return statement is optional.

Answer 3

Return is the C syntax to tell the assembler to create a ret instruction.
What is a ret instruction? From Intel's x86 manual:
RET — Return from Procedure
Transfers program control to a return address located on the top of the stack. The address is usually placed on the stack by a CALL instruction, and the return is made to the instruction that follows the CALL instruction.
When you call and return from a function (here called a procedure), in reality you are executing the following code at assembly level:
push rbp // push rbp (base pointer) to the stack
mov rbp, rsp // store rsp (stack pointer) into rbp.
...
mov rsp, rbp // get back the value of rsp
pop rbp // pop rbp from the stack
ret
"The caller" is the original function in which the call to the other function, "the callee" happens.
Note that by default the C language will asssume (rightfully) that every function has a return point, and that is why for void function, as well as for main, a written return is not necessary.
The return along with a value (let say 20) is translated as (intel syntax):
mov eax, 20
ret
What is the relationship with your question?
In your example, there is no call to a callee function, you are executing code inside your function (the same as what would happen if you add "inline") so the the variable you are currently calculating startPop doesn't need a return.
PS: On a side note, having a do while with = does not make a lot of sense.