return

[aewarnick]

I am reading this book and it is using return to do certain things like putting return x * y (x-1); at the bottom of the program. While a function prototype is at the top outside all braces.

But it does not explain what return is actually doing? And I know that return 0 terminates the program but why?

[Jop]

return does actually what it says it does
it returns a value.

lets try it by an example, let's say you have a function that is used to increment a number you pass it by 5.

int Inc(int iNr){
return (iNr + 5);
}

The function expects an integer as a parameter, and returns an integer too.

so if you call this function like

int iTmp = 5; //This is the value we're gonna send to the function
iTmp = Inc(iTmp); //This will pass 5 to the function

iTmp will be 10 now, since we passed 5, and the function added 5 to it.

I hope you understand it now?

By the way, with returning from the Main function, you can specify error codes. Like if you return 0 it means no error occurred, and if you return 1 you can specify that something went wrong. This is used if your app is being launched from another app, and the numbers you return can mean anything, it's just the way you implement it.
usually you just return 0

[parksie]

If your program returns other than 0, most environments will assume it failed to do what was necessary. For example:

$ myprog --arg=val && somecommand --dependent=myprog

If myprog fails, then somecommand cannot operate correctly. If you return 0 from myprog, the shell will interpret that as non-zero, i.e. success.

[Jop]

Oops I messed things up, sorry I rarely make console apps, my bad...

[aewarnick]

Jop, did you erase your post?

[CornedBee]

The DOS choice app used return values from the app too. You used it in batch files. You call it and it waits for the user to type one of several characters. Then it returns the 0-based index of this character in the parameter list. You could then test this value using the ERRORLEVEL.

if ERRORLEVEL 3 goto three
if ERRORLEVEL 2 goto two
...

retrun actually means "return from the function". You can simply write
return;
if the function has no return type. This will exit the function.
If the function has a return type you have to specify what should be returned:
return 0;

[aewarnick]

I think that I understand return now. Return is only useful if another function or statement calls it, right? Except for the return 0 to end the program.

[CornedBee]

return isn't called. return is a statement.

return is useful in many places. It ends the function you're in and gives control back to the function that called it. It can optionally return a value.

The return 0; at the end of your app gives control back to the function that called your main function. This is some windows-internal function which will then end the program.

[Yonatan]

Originally posted by CornedBee
This is some windows-internal function

Not a Windows function but a CRT function, mainCRTStartup, assuming you use the MSVCRT.

Check out: crt\src\crt0.c
It's one big function that's actually four functions all in one.
The function (of the four) that will be compiled depends on the preprocessor macros defined.

#ifdef _WINMAIN_

#ifdef WPRFLAG
int wWinMainCRTStartup(
#else  /* WPRFLAG */
int WinMainCRTStartup(
#endif  /* WPRFLAG */

#else  /* _WINMAIN_ */

#ifdef WPRFLAG
int wmainCRTStartup(
#else  /* WPRFLAG */
int mainCRTStartup(
#endif  /* WPRFLAG */

#endif  /* _WINMAIN_ */

        void
        )

{

This is the function Windows calls on startup (the entry point).
Looking further down...

mainret = main(__argc, __argv, _environ);

is the actual call to main, and

exit(mainret);

is the call that terminates the process after main returns.

[Sastraxi]

Return codes and && save you a hell of a lot of grief in UNIX command lines.

[CornedBee]

Originally posted by Yonatan
Not a Windows function but a CRT function, mainCRTStartup, assuming you use the MSVCRT.

I know, but it doesn't really matter, does it? Let's not make it overcomplicated. I regretted even mentioning the startup function when I had posted this.

[parksie]

Anything outside main() should never normally be considered, except by compiler/library implementors.

For example, on here I don't have a mainCRTStartup function...

[CornedBee]

Yep, that's why I regretted it.

[aewarnick]

"It ends the function you're in and gives control back to the function that called it."

What if I put return; at the end of a function but no funcion has called it. For example:

{int main()
cout<<"fjlakjsdlkfjdjflkajsd";
int funk (){
int c; cin>>c; cout<<"c";

return;}

return 0;}

[aewarnick]

"It ends the function you're in and gives control back to the function that called it."

What if I put return; at the end of a function but no funcion has called it. Will it just be ignored and move on down? For example:

{int main()
cout<<"fjlakjsdlkfjdjflkajsd";
int funk (){
int c; cin>>c; cout<<"c";

return;}

return 0;}

[CornedBee]

You may not put a function inside another function.

[aewarnick]

I can't put any function inside int main()? So, for, while, do/while, and if are not functions? How do I know what is a function and what is not?

[Yonatan]

Nor inside any other, no.

Note that you can put a function declaration inside a function:

int a()
{
  int b();
  return b() + 17;
}

int b()
{
  return 42;
}

But this is deprecated, and if it isn't, it should be.

[aewarnick]

So, for, while, do/while, and if are not functions? How do I know what is a function and what is not?

[parksie]

A function is defined as a function. I.E.:

type name(args) {
    /* statements */
}

A function looks like a special kind of block, which is any code between braces.

[aewarnick]

Got it! Thank you.