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31/10/2007, 09h36

santosh <santosh.k83@gmail.com> writes:
[...]
> The point was that different pointer types might differ in size. That
> doesn't mean that the memory pool from which malloc gets space has to
> be different for each differing pointer types.

Right. Pointers can differ; that doesn't mean that the memory they
point to can differ.

> It may be that the implementation, for example, decides to append extra
> information with pointers to floating types, but not to other types. In
> this case such pointers might conceivably be larger than others.

Right. Perhaps more realistically, on a system where machine pointers
point to words (where a word is several byte), an int* might be, say,
4 bytes, but a char* might be larger, with extra information added to
resolve which byte within the word it points to.

> Also an implementation might decide to allocate objects of a certain
> type in lower address spaces than others. In such a case, pointers to
> that type may be of smaller width, though the memory for the entire
> gamut is the same.

I don't believe a conforming implementation can do this, at least not
consistently. A pointer returned by malloc must be able to point to
any type of object; similarly, it's possible to declare a union of any
two or more object types. Restricting objects of a certain type to be
allocated only at low addresses might be possible, but other objects
would be dragged in as well.

--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

31/10/2007, 09h36	#60
Keith Thompson Messages: n/a Hébergeur:	Re: sizeof(ptr) = ? santosh <santosh.k83@gmail.com> writes: [...] > The point was that different pointer types might differ in size. That > doesn't mean that the memory pool from which malloc gets space has to > be different for each differing pointer types. Right. Pointers can differ; that doesn't mean that the memory they point to can differ. > It may be that the implementation, for example, decides to append extra > information with pointers to floating types, but not to other types. In > this case such pointers might conceivably be larger than others. Right. Perhaps more realistically, on a system where machine pointers point to words (where a word is several byte), an int* might be, say, 4 bytes, but a char* might be larger, with extra information added to resolve which byte within the word it points to. > Also an implementation might decide to allocate objects of a certain > type in lower address spaces than others. In such a case, pointers to > that type may be of smaller width, though the memory for the entire > gamut is the same. I don't believe a conforming implementation can do this, at least not consistently. A pointer returned by malloc must be able to point to any type of object; similarly, it's possible to declare a union of any two or more object types. Restricting objects of a certain type to be allocated only at low addresses might be possible, but other objects would be dragged in as well. -- Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst> San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst> "We must do something. This is something. Therefore, we must do this." -- Antony Jay and Jonathan Lynn, "Yes Minister"