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From:	"Alexei A. Frounze" <alexfru AT chat DOT ru>
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Subject:	Re: gcc bug?
Date:	Tue, 17 Apr 2007 02:15:49 -0700
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Rod Pemberton wrote:
> "Alexei A. Frounze" <alexfru AT chat DOT ru> wrote in message
> news:LbadnVsa3_H-ornbnZ2dnUVZ_sudnZ2d AT comcast DOT com...
>> I was checking my understanding of volatile's effect and encountered
>> an odd thing.
>>
>
> I'm _really_ unsure about this...  The "warning flag" for me are the
> additional qualifiers on a type that's already been typedef'd (Is that
> legal?  Seems to work...except for...).  I.e., are your volatile's in
> the correct location?  You want the pointer to be volatile not the
> struct, right?

In C in some cases there're at least two ways to express the same thing with 
the qualifiers. And yes, you can make volatile/constant a variable (of any 
type), a pointer to any variable/struct, and a variable/struct a pointer 
points to. And you can even have both the pointer and what it points to 
volatile or constant. Double volatile doesn't seem to make much sense, while 
double const is probably a good thing.

>> Consider the following code:
>> ----8<----
>> typedef struct {char x;} T, *PT;
>>
>> void f1 ()
>> {
>>   volatile T* pt = 0;
>
> T* volatile pt = 0;

These are different.

Let's add another typedef and see what we get:

typedef volatile T* PVT;

volatile T* - non-volatile ptr to volatile var
volatile PT - volatile ptr to non-volatile var
PVT - non-volatile ptr to volatile var
T* volatile - volatile ptr to non-volatile var

The catch is that it's easy to confuse volatile T* and volatile PT because 
they look pretty much the same and it's easy to get fooled and think that PT 
is merely a macro substitution for T*, which it's not.

At any rate, since I don't see a pointer in f1()'s generated asm code, this 
is a gcc bug. At least in version 3.3.4, at least in DJGPP.

Alex

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