Can somebody interpret this for me?

   while (TRUE) { cont = (*cont)(); }

What on earth...?

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Invisible wrote:
> Can somebody interpret this for me?
> 
>   while (TRUE) { cont = (*cont)(); }
> 
> What on earth...?

cont is a pointer to a function that is expected to return a pointer to 
a function.  The code here causes a series of functions to be called, 
each one specifying another function to be called afterwards by 
returning the address of the function to be called.

Regards,
John

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>> Can somebody interpret this for me?
>>
>>   while (TRUE) { cont = (*cont)(); }
>>
>> What on earth...?
> 
> cont is a pointer to a function that is expected to return a pointer to 
> a function.  The code here causes a series of functions to be called, 
> each one specifying another function to be called afterwards by 
> returning the address of the function to be called.

Ah, so it *is* possible to read C? :-)

So cont points to a zero-argument function that returns a zero-argument 
function? And this loop just calls it repeatedly?

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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John VanSickle a écrit :
> Invisible wrote:
>> Can somebody interpret this for me?
>>
>>   while (TRUE) { cont = (*cont)(); }
>>
>> What on earth...?
> 
> cont is a pointer to a function that is expected to return a pointer to 
> a function.  The code here causes a series of functions to be called, 
> each one specifying another function to be called afterwards by 
> returning the address of the function to be called.
> 

I was thinking the same, but I can't seem to find the right type 
definition to make it work without casting to and from void*... Which is 
apparently illegal 
(http://www.parashift.com/c++-faq-lite/pointers-to-members.html#faq-33.8)

Here's what I ended up with:

typedef void* (*pfunc)();

void* test1()
{
     return (void*)test1;
}

int main()
{
     pfunc cont = test1;
     while (true) { cont = (pfunc)(*cont)(); }
     return 0;
}

I'd like to write:

typedef pfunc (*pfunc)();

pfunc test1()
{
     return test1;
}

int main()
{
     pfunc cont = test1;
     while (true) { cont = (*cont)(); }
     return 0;
}

But the typedef here does not seem to work. I can't say I blame the 
compiler for choking on that one either :-)

I'm not too comfortable with function pointers though, so it's likely 
that I'm missing something here... Any hints?

-- 
Vincent

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>> cont is a pointer to a function that is expected to return a pointer 
>> to a function.  The code here causes a series of functions to be 
>> called, each one specifying another function to be called afterwards 
>> by returning the address of the function to be called.
>>
> 
> I was thinking the same, but I can't seem to find the right type 
> definition to make it work without casting to and from void*... Which is 
> apparently illegal 

This is example code, so it might not be literally runnable... (It might 
be simplified somewhat.)

> I'm not too comfortable with function pointers though, so it's likely 
> that I'm missing something here... Any hints?

Don't look at me! ;-)

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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Invisible a écrit :
>>> cont is a pointer to a function that is expected to return a pointer 
>>> to a function.  The code here causes a series of functions to be 
>>> called, each one specifying another function to be called afterwards 
>>> by returning the address of the function to be called.
>>>
>>
>> I was thinking the same, but I can't seem to find the right type 
>> definition to make it work without casting to and from void*... Which 
>> is apparently illegal 
> 
> This is example code, so it might not be literally runnable... (It might 
> be simplified somewhat.)
> 

Does it still provide the declaration of cont, by any chance?

-- 
Vincent

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On Thu, 24 Jul 2008 14:24:51 +0200, Vincent Le Chevalier  
<gal### [at] libertyallsurfspamfr> wrote:
> I was thinking the same, but I can't seem to find the right type  
> definition to make it work without casting to and from void*... Which is  
> apparently illegal  
> (http://www.parashift.com/c++-faq-lite/pointers-to-members.html#faq-33.8)

It is illegal because a 'void *' is a data pointer, not a function pointer.

On a related note, we do not know if the original code was supposed to be  
standards-compliant or not, nor if it was supposed to be C or C++.


> Here's what I ended up with:
[...snipped...]

The "right" way to do it, assuming C++:
http://www.gotw.ca/gotw/057.htm



-- 
FE

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Fredrik Eriksson wrote:

> On a related note, we do not know if the original code was supposed to 
> be standards-compliant or not, nor if it was supposed to be C or C++.

Ment to be ANSI C (not C++).

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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Invisible wrote:
> So cont points to a zero-argument function that returns a zero-argument 
> function? And this loop just calls it repeatedly?

Assuming TRUE is a value that always returns non-zero, yes. The code itself
appears to implement a state machine.

Of course, this being C, 'TRUE' could be a macro that expands to a function
 that returns a value and thus the loop need not be infinite. It could even
expand to a variable with a decrement or increment operator. Or really
anything - if the code were presented to someone as part of an computer
science exam, for example, they would probably be expected to pick that
possibility up. In the real world, no programmer would ever do something
that mean with a macro called TRUE. Right? :-)

-- Chris

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>> So cont points to a zero-argument function that returns a zero-argument 
>> function? And this loop just calls it repeatedly?
> 
> Assuming TRUE is a value that always returns non-zero, yes. The code itself
> appears to implement a state machine.

More exactly: It's a workaround for C's lack of lack of GOTO.

[This fragment appears in the output of a compiler that's using C as 
"portable assembly".]

> In the real world, no programmer would ever do something
> that mean with a macro called TRUE. Right? :-)

Hey, I found this cool site, it's called The Daily WTF... ;-)

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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