Re: pointer doubt (1)
On 05/08/2013 02:46 PM, rashan wrote:
> Can ne1 explain me the difference between.... a NULL pointer ... and a
> VOID pointer.
C is a case sensitive language; the term you're looking for is "null
pointer". NULL is the name of a standard macro. For the same reason,
there's no such thing as a VOID pointer (unless VOID has been given a
meaning by something else, and you'll have to tell us what that meaning
is before we can tell you what "VOID pointer" means). What you're
looking for is "void pointer", or better, "pointer to void".
"void" is a C keyword. It is used in contexts where it could be
interpreted as a type, but it's used exclusively for the purpose of
indicating that no actual type is relevant. In the case of a "pointer to
void", what it means is a pointer to an unspecified type.
A "null pointer constant" is a special phrase in the C language that
does not mean what you might reasonably assume it means by looking at
the individual words that make up the phrase. Not all constant
expressions with a null pointer value qualify; and some of the things
that do qualify are not null pointers at all, but rather integer
expressions. They are, however, constant.
What a "null pointer constant" actually is, is an integer constant
expression (ICE) with a value of 0, or such an expression converted to
the type void* (22.214.171.124p3). Again, ICE is a term that doesn't mean what
you might think by looking at the individual words. It would seem to
mean "an expression with an integer type and a constant value", but in
reality there are a lot of restrictions on what kinds of expressions can
qualify as ICEs. See section 6.6p6 for details on those restrictions.
A null pointer is what you get when a null pointer constant is converted
to a pointer type. Note that "(void*)0" is both a null pointer constant
in it's own right, and a null pointer constant (0) converted to a
pointer type; it is therefore a null pointer. Null pointer constants get
implicitly converted to pointers, and therefore null pointers, whenever
used in certain kinds of operations where a pointer expression could
also be used. For instance, &x==0, where x identifies an object or a
function, causes the null pointer constant 0 to be implicitly converted
to a null pointer of the same type as &x.
The key important facts about null pointers are that all null pointers
compare equal to each other, and that no null pointer compares equal to
a pointer to any object or function. Therefore, &x==0 is guaranteed to
be false. They are normally used as special pointer values - they
indicate that the pointer does NOT, in fact, point at anything.
NULL is a standard macro which is required to expand into a null pointer
constant. In principle, it could be any null pointer constant: '\0', 0s,
0, 0L, 0U, 0UL, 0LL, (3LL-'\03'), (2/3). However, in practice, I don't
know any reason why any implementation would ever define it to be
anything other than 0 or ((void*)0). The second form has an advantage,
in that it can never be misused as an integer.
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