A null pointer simply means "I am not allocated yet!" and "I am not pointing to anything yet!".
The C language definition states that for every available pointer type, there is a special value which is called the null pointer. It is guaranteed to compare unequal to a pointer to any object or function.
A null pointer is very different from an uninitialized pointer. A null pointer does not point to any object or function; but an uninitialized pointer can point anywhere.
There is usually a null pointer for each type of a pointer, and the internal values of these null pointers for different pointer types may be different, its up to the compiler. The & operator will never yield a null pointer, nor will a successful call to malloc() (malloc() does return a null pointer when it fails).
In this call to execl(), the last argument has been explicitly casted to force the 0 to be treated as a pointer.
Also, if ptr is a pointer then
and
are perfectly valid.
How is NULL defined?
ANSI C allows the following definition
NULL and 0 are interchangeable in pointer contexts.
Make sure you are able to distinguish between the following : the null pointer, the internal representation of a null pointer, the null pointer constant (i.e, 0), the NULL macro, the ASCII null character (NUL), the null string ("").
Examples:
What is the subtle error in the following code segment?
void fun(int n, int arr[])
{
int *p=0;
int i=0;
while(i++
p = &arr[i];
*p = 0;
}
Answer & Explanation:
If the body of the loop never executes p is assigned no address. So
p remains NULL where *p =0 may result in problem (may rise to
runtime error “NULL pointer assignment” and terminate the
program).
The C language definition states that for every available pointer type, there is a special value which is called the null pointer. It is guaranteed to compare unequal to a pointer to any object or function.
A null pointer is very different from an uninitialized pointer. A null pointer does not point to any object or function; but an uninitialized pointer can point anywhere.
There is usually a null pointer for each type of a pointer, and the internal values of these null pointers for different pointer types may be different, its up to the compiler. The & operator will never yield a null pointer, nor will a successful call to malloc() (malloc() does return a null pointer when it fails).
execl("/bin/ls", "ls", "-l", (char *)0);
In this call to execl(), the last argument has been explicitly casted to force the 0 to be treated as a pointer.
Also, if ptr is a pointer then
if(ptr){}
and
if(!ptr){}
are perfectly valid.
How is NULL defined?
ANSI C allows the following definition
#define NULL ((void *)0)
NULL and 0 are interchangeable in pointer contexts.
Make sure you are able to distinguish between the following : the null pointer, the internal representation of a null pointer, the null pointer constant (i.e, 0), the NULL macro, the ASCII null character (NUL), the null string ("").
Examples:
What is the subtle error in the following code segment?
void fun(int n, int arr[])
{
int *p=0;
int i=0;
while(i++
p = &arr[i];
*p = 0;
}
Answer & Explanation:
If the body of the loop never executes p is assigned no address. So
p remains NULL where *p =0 may result in problem (may rise to
runtime error “NULL pointer assignment” and terminate the
program).
0 comments:
Post a Comment