Is it dangerous to use NULL pointer dereferencing at rvalue side ?

Question

I have the following code.

int *a = NULL;
int b;

b = *a;

Is it undefined in any way? I am trying to access the location pointed by a only for reading. What wrong it can do ?

Answer 1

Is it undefined in any way?

Yes, the behavior is undefined.

I am trying to access the location pointed by a only for reading.

The location pointed to does not exist. There's nothing there. To read something, there has to be a value to read, but a null pointer points... nowhere. It doesn't point at anything.

Answer 2

Implementation chooses the physical values of null-pointers specifically to make them "point" to address(es) that contains nothing of any value on the given platform. That means that there's nothing to see there. Moreover, on a modern virtual memory platform there might not even be a specific "there" associated with that address.

If you are doing it out of pure curiosity, it still won't work on a typical modern platform: that [virtual] memory region is typically "not yours", meaning that hardware/OS protection mechanism will prevent you from reading it (by crashing your program).

Meanwhile, the language states that any attempts to dereference a null-pointer leads to undefined behavior, regardless of what you are dereferencing it for.

Answer 3

Yes, it's totally undefined behaviour. You are dereferencing a null pointer. Anything could happen.

Answer 4

Don't dereference NULL pointers. It's like crossing the streams, but worse. I'm not even sure what you're trying to accomplish? Are you trying to initialize two pointers to NULL?

Answer 5

The result is undefined and will be different under different platforms.

NULL is equal to 0. Therefore saying int* a = NULL means that you make a pointer to address 0, which should contain an integer.

By dereferecing a by b = *a you instruct the program to go to the address 0 and get the data.

Now, depending on the platform, address 0 will most probably be inaccessible to the user code. If you are not running on top of an operating system (as the case is for example on a microcontroller environment), then address 0 is normally the beginning of the internal ROM. In other cases it can be the beginning of the stack etc.

In most of the cases you are going to get a null pointer exception when you dereference a null pointer.

Answer 6

You have two alarms that pop up here. NULL and all other forms of null pointers are strickly forbidden by the standard. Not because they are often realized internally by a bit pattern of all 0 for the representation of such a pointer but because that is the one-and-single reserved value for pointers that indicate that it isn't pointing anywhere. Don't do this.

Having a pointer point to the address that corresponds to the integer value 0 is a different thing. You could only enforce such a value and interpretation of a point by some trick, eg

union over {
  int *a;
  uintptr_t b;
} ov;

ov.b = 0;
int c = *ov.a;

What then happens is completely depending on your platform. If 0 is a valid address in your address space this could work, it probably wouldn't. Don't do it unless you know better.