著者: Emiliano Marini 日付: To: KatolaZ CC: dng 題目: Re: [DNG] Making sense of C pointer syntax.
+1
On Wed, Mar 30, 2016 at 5:04 AM, KatolaZ <katolaz@???> wrote:
> On Wed, Mar 30, 2016 at 09:16:22AM +0200, Edward Bartolo wrote:
> > Hi and many thanks for the replies,
> >
> > I can understand that a pointer being an address depends heavily on
> > machine architecture which means, on 32 bit machines it is 4 bytes
> > long and on 64 bit machines it is 8 bytes long. I also understand that
> > a pointer variable is essentially made of two parts as illustrated
> > below:
> >
> > address [always allocated] ------------------> data [not allocated
> > automatically]
> >
> > The address does depend on architecture but the data? And what else
> > can enter into a pointer's definition other than what I illustrated?
> >
>
> Hi Edward,
>
> sorry but your description is incorrect. A pointer in C is just a
> variable large enough to contain a memory address. Period. There is no
> explicit or implicit linking between a pointer (which is a variable
> large enough to contain a memory address) and the area of memory it
> points to. You may have several pointers pointing to the same memory
> area. You can have the same pointer (i.e., the same named variable
> able to contain a memory address) pointing to different memory areas
> at different times. You can use a pointer to wander around an
> allocated memory area at your will, changing its value by using the
> powerful pointer arithmetic provided by C. You can also have some
> allocated memory area for which you don't have any pointer at all (and
> this is what is called a "memory leak", and you should avoid it).
>
> Also, memory areas are not "typed" in C, meaning that you can in
> principle access a correctly allocated memory area with pointers of
> any type, the only problem being the semantics of pointer arithmetics,
> which is entirely left to the programmer.
>
> But please forget any implicit linkage between a pointer (the
> variable) and the memory area it points to (i.e., the address
> contained in that variable), as you don't assume any intrinsic link
> between an integer variable and the possible values that the variable
> can contain. If you don't break this spell, you will never get around
> with C pointers.
>
> Pointers are just variables which are able to contain memory
> addresses. What you put in those variables does not bother them in any
> discernible way. And shouldn't bother you either, as long as you be
> careful in using them to refer to correctly allocated memory. SIGSEGV
> is the alternative.
>
> My2Cents
>
> KatolaZ
>
> --
> [ Enzo Nicosia aka KatolaZ --- GLUG Catania -- Freaknet Medialab ]
> [ me [at] katolaz.homeunix.net -- http://katolaz.homeunix.net -- ]
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Re: [DNG] Making sense of C pointer…
