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Fredrik Eriksson a écrit :
> On Thu, 24 Jul 2008 14:24:51 +0200, Vincent Le Chevalier
> <gal### [at] libertyallsurfsp amfr> wrote:
>> Here's what I ended up with:
> [...snipped...]
>
> The "right" way to do it, assuming C++:
> http://www.gotw.ca/gotw/057.htm
>
Thanks for that!
--
Vincent
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Invisible <voi### [at] devnull> wrote:
> More exactly: It's a workaround for C's lack of lack of GOTO.
Except that C does have goto.
--
- Warp
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Chris Cason <del### [at] deletethistoopovrayorg> wrote:
> 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.
Actually there are other ways for the loop to end. For instance, the
function being called could execute an exit() call.
(Another less fancy way would be to return a null pointer from the
function, in which case the program will end with a segmentation fault.)
--
- Warp
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>> More exactly: It's a workaround for C's lack of lack of GOTO.
>
> Except that C does have goto.
Ooo, really?
In that case, I have *no clue* why they did it this way...
Oh, wait, maybe I do... Can you use GOTO to jump to a *pointer*? Or only
a literal label?
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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Warp wrote:
> Actually there are other ways for the loop to end. For instance, the
> function being called could execute an exit() call.
I believe that is in fact what's meant to happen, yes.
> (Another less fancy way would be to return a null pointer from the
> function, in which case the program will end with a segmentation fault.)
...or have count() perform any other illegal program-halting action.
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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Invisible <voi### [at] devnull> wrote:
> >> More exactly: It's a workaround for C's lack of lack of GOTO.
> >
> > Except that C does have goto.
> Ooo, really?
> In that case, I have *no clue* why they did it this way...
Because "while(1)" is a common idiom, and avoids the ugly goto.
> Oh, wait, maybe I do... Can you use GOTO to jump to a *pointer*? Or only
> a literal label?
Jumping to a pointer is normally called "function call".
Goto works only with labels.
--
- Warp
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>>> Except that C does have goto.
>
>> Ooo, really?
>
>> In that case, I have *no clue* why they did it this way...
>
> Because "while(1)" is a common idiom, and avoids the ugly goto.
>
>> Oh, wait, maybe I do... Can you use GOTO to jump to a *pointer*? Or only
>> a literal label?
>
> Jumping to a pointer is normally called "function call".
>
> Goto works only with labels.
Yes, but jumping to a function pushes a return address onto the stack.
And they specifically don't want to do that. (Because it would use an
unbounded amount of stack.)
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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Invisible <voi### [at] devnull> wrote:
> Yes, but jumping to a function pushes a return address onto the stack.
> And they specifically don't want to do that. (Because it would use an
> unbounded amount of stack.)
Functions in C don't need to return anything.
--
- Warp
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Warp wrote:
> Invisible <voi### [at] devnull> wrote:
>> More exactly: It's a workaround for C's lack of lack of GOTO.
>
> Except that C does have goto.
I thought that's what "lack of lack of GOTO" meant.<G>
--
I almost had a psychic girlfriend, but she left me before I met her.
/\ /\ /\ /
/ \/ \ u e e n / \/ a w a z
>>>>>>mue### [at] nawazorg<<<<<<
anl
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>> Yes, but jumping to a function pushes a return address onto the stack.
>> And they specifically don't want to do that. (Because it would use an
>> unbounded amount of stack.)
>
> Functions in C don't need to return anything.
The idea here is that we jump to one label, then jump to another, then
jump to another, never returning to the "caller".
If we implement this using C functions that never return, an unbounded
amount of stack gets eaten.
Hence, they arranged for each function to *return* the function it would
like to jump to next, rather than actually jumping to it, and the loop I
posted is apparently the magic incantation that implements the jumping.
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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Chris Cason wrote:
> In the real world, no programmer would ever do something
> that mean with a macro called TRUE. Right? :-)
There was an entry over at the Daily WTF in which someone posted code,
written by another person, containing this:
#define TRUE 0
#define FALSE 1
I swear that I am not making this up.
Regards,
John
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Vincent Le Chevalier wrote:
> 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)
Casting back and forth from void* is considered exceptionally unwise in
C++, but it's considered a great way of writing tighter code in C.
Regards,
John
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Warp wrote:
> Chris Cason <del### [at] deletethistoopovrayorg> wrote:
>> 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.
>
> Actually there are other ways for the loop to end. For instance, the
> function being called could execute an exit() call.
>
> (Another less fancy way would be to return a null pointer from the
> function, in which case the program will end with a segmentation fault.)
Or return a pointer to the exit() function, which is the same as above.
Regards,
John
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John VanSickle <evi### [at] hotmailcom> wrote:
> Casting back and forth from void* is considered exceptionally unwise in
> C++, but it's considered a great way of writing tighter code in C.
In fact, for example casting method pointers to void* and back is
undefined behavior and in no way guaranteed to work. (And, in fact,
in many compilers method pointers are as large as two regular pointers,
and thus casting to void* would drop information.)
--
- Warp
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Invisible <voi### [at] devnull> wrote:
> The idea here is that we jump to one label, then jump to another, then
> jump to another, never returning to the "caller".
>
> If we implement this using C functions that never return, an unbounded
> amount of stack gets eaten.
>
> Hence, they arranged for each function to *return* the function it would
> like to jump to next, rather than actually jumping to it, and the loop I
> posted is apparently the magic incantation that implements the jumping.
Yes, indeed.
http://en.wikipedia.org/wiki/Tail_recursion#Implementation_methods
I take it that you were looking at the C output of GHC, right? Haskell relies
on tail-recursion as much as any other functional language, as Scheme. The
method it is implemented underneath is as a trampoline.
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John VanSickle wrote:
> There was an entry over at the Daily WTF in which someone posted code,
> written by another person, containing this:
>
> #define TRUE 0
> #define FALSE 1
>
> I swear that I am not making this up.
I remember it well...
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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On 26-Jul-08 21:39, John VanSickle wrote:
> Chris Cason wrote:
>
>> In the real world, no programmer would ever do something
>> that mean with a macro called TRUE. Right? :-)
>
> There was an entry over at the Daily WTF in which someone posted code,
> written by another person, containing this:
>
> #define TRUE 0
> #define FALSE 1
>
> I swear that I am not making this up.
Probably Unix convention?
http://en.wikipedia.org/wiki/True_(Unix)
http://en.wikipedia.org/wiki/False_(Unix)
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>> There was an entry over at the Daily WTF in which someone posted code,
>> written by another person, containing this:
>>
>> #define TRUE 0
>> #define FALSE 1
>>
>> I swear that I am not making this up.
>
> Probably Unix convention?
Ah yes - return code 0 = OK, any other code = failure.
What have we learned here? That using integers to represent Booleans is
a Very Bad Thing. :-P
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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Orchid XP v8 <voi### [at] devnull> wrote:
> >> There was an entry over at the Daily WTF in which someone posted code,
> >> written by another person, containing this:
> >>
> >> #define TRUE 0
> >> #define FALSE 1
> >>
> >> I swear that I am not making this up.
> >
> > Probably Unix convention?
> Ah yes - return code 0 = OK, any other code = failure.
C actually distinguishes between TRUE/FALSE and EXIT_SUCESS/EXIT_FAILURE.
They shouldn't be confused nor mixed.
--
- Warp
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Warp wrote:
>
> Because "while(1)" is a common idiom, and avoids the ugly goto.
>
Either that or for(;;) { ... }
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>> Because "while(1)" is a common idiom, and avoids the ugly goto.
>
> Either that or for(;;) { ... }
...and to think people complain when a Haskell programmer writes
fix ((1:) . scanl (+) 1)
or something... :-P
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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Warp wrote:
> (Another less fancy way would be to return a null pointer from the
> function, in which case the program will end with a segmentation fault.)
Well, maybe if you're lucky. :-) Otherwise, it just goes off happily
executing whatever is at memory address zero (or whatever NULL maps to).
--
Darren New / San Diego, CA, USA (PST)
Helpful housekeeping hints:
Check your feather pillows for holes
before putting them in the washing machine.
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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> > (Another less fancy way would be to return a null pointer from the
> > function, in which case the program will end with a segmentation fault.)
> Well, maybe if you're lucky. :-) Otherwise, it just goes off happily
> executing whatever is at memory address zero (or whatever NULL maps to).
I'm pretty sure POSIX or some other type of standard states that jumping
to the null pointer will always cause a segmentation fault.
--
- Warp
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Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> Warp wrote:
>>> (Another less fancy way would be to return a null pointer from the
>>> function, in which case the program will end with a segmentation fault.)
>
>> Well, maybe if you're lucky. :-) Otherwise, it just goes off happily
>> executing whatever is at memory address zero (or whatever NULL maps to).
>
> I'm pretty sure POSIX or some other type of standard states that jumping
> to the null pointer will always cause a segmentation fault.
But not the C standard. Try it on MS-DOS, Win 3.1, AmigaDOS, a credit
card terminal, a cell phone, etc. :-) Basically, you're saying you
can't have a POSIX-compliant system without memory mapping, methinks.
But C is used in particular because of its primitiveness and lack of
reliance on a particular complex bit of hardware.
"The operating system is there to provide the facilities the language
designer left out."
--
Darren New / San Diego, CA, USA (PST)
Helpful housekeeping hints:
Check your feather pillows for holes
before putting them in the washing machine.
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