Registered: Oct 2000
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Wisdom

The tao that can be tared
is not the entire Tao.
The path that can be specified
is not the Full Path.

We declare the names
of all variables and functions.
Yet the Tao has no type specifier.

Dynamically binding, you realize the magic.
Statically binding, you see only the hierarchy.

Yet magic and hierarchy
arise from the same source,
and this source has a null pointer.

Reference the NULL within NULL,
it is the gateway to all wizardry.

(Taken from Linux MotD)

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wow....

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EEEEEEEEEEEEE!!!!!

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Registered: Dec 2001
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Commandment 2, from the C programmers bible.

Thou shalt not follow the NULL pointer, for chaos and madness await thee at its end.

Clearly the holy scriptures were mis-transcribed here, as the words should have been "null pointer", to minimize confusion between the concept of null pointers and the macro NULL (of which more anon).
Otherwise, the meaning is plain. A null pointer points to regions filled with dragons, demons, core dumps, and numberless other foul creatures, all of which delight in frolicing in thy program if thou disturb their sleep. A null pointer doth not point to a 0 of any type, despite some blasphemous old code which impiously assumes this.

Last edited by torque on 06-10-2005 at 09:12 PM

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Registered: Aug 2001
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A null pointer doesn't point to anything, I thought (indeed, isn't that the point of them?), regardless of what the compiler uses to represent them internally.

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I think it's referring to the fact that NULL and a null pointer aren't interchangeble, ie. I can't use NULL when calling a fuction that uses a null pointer as it's terminater, as the compiler treats NULL as an unadorned 0.

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I got the bit about null pointer/NULL, but I was talking about :

quote:

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...Otherwise, the meaning is plain. A null pointer points to regions filled with dragons, demons, core dumps, and numberless other foul creatures...

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Registered: Nov 2000
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It's function dependant, but as an example: If I was using a particularly braindead malloc implementation and called free(void *ptr), where ptr is a null pointer, I could get some truely interesting behaviour(free() is normally used to free up a segment of malloc()'d memory, using the the pointer that malloc() returns after being called).

Thankfully free() in most versions of malloc checks for a NULL pointer and just returns if one is found. But you get the idea.

Last edited by macker on 06-10-2005 at 10:12 PM

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Registered: Nov 2000
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To clarify a bit, how null pointers behave when read or written to is also architecture dependant.

edit: Actually, I don't think Java and C# allow pointer operations in general.

Last edited by macker on 06-10-2005 at 10:14 PM

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They still don't point to anything at all, though, surely? Even if they are misinterpreted to appear as if they do?

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Well, it's a pointer. Saying it points to nothing is, well, inaccurate. As such, what it points to is undefined. On some architectures it's 0(which might be protected(and will either throw an exception of some sort or just silently ignore the operation)), or it could the value of some data that *used* to sit the same place as what the null pointer resides in, or it could be a random bit of information from one of the cache lines in the CPU, etc.

edit: further clarification

Last edited by macker on 06-10-2005 at 10:27 PM

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I was under the impression that it really didn't point to anything, although it can be later assigned to point to something (a pointer is something that can point rather than something that does point, in that sense). Also that the internal representation of a null pointer was unimportant, just that it was guaranteed not to be the same as any pointer that does point to something.

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Or I should perhaps say that a whatever a null pointer points to, it doesn't point to a memory location (this is what is guaranteed by the compiler)? In which case, perhaps one could make the pointer is pointing to dragon. Or kittens. Or anything that isn't a memory location.

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Registered: Nov 2000
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It points to undefined. Saying it doesn't point to a memory location would indicate that it is actually defined. Which it isn't.

Dunno how else to explain it.

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Oh, I agree with you, I was just trying to see the logic/humour in the piece saying that they pointed to dragons, etc.

I don't think that saying that 'it doesn't point to a memory location' is the same as saying that it is defined, though, in logic. It means, in effect 'either it is undefined OR it does point to something defined, that isn't a memory location'.

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Saying a pointer doesn't point to a memory location is like saying water isn't wet. All pointers point to a memory location, however null pointers don't necessarily point to any *explicit* location until you actually use them. Then for that specific operation they actually *do* have an explicit location. It might seem like stupid semantics, but it's important stupid semantics.

As for the humour, I think it refers to the fact that null pointers can create havoc far from where you expect them to and in ways that you cannot immediately explain. I might have seen where you were going except they included "core dumps" and "numberless" in the list, the former being a common symptom of null pointers, the latter being a reference to its nature.

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Pointers hold a number that represents a memory location, surely? If that number (or whatever it is) doesn't correspond to a memory location that actually exists, then the pointer doesn't point to a memory location. As null pointers are promised not to be the same as any pointer that does point to a memory location, however they are internally represented, they aren't pointing to a memory location.

Picking on null pointers seems odd when stray pointers are more dangerous.

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quote:

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Originally posted by Smug Git
Pointers hold a number that represents a memory location, surely?

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Last edited by macker on 06-11-2005 at 02:29 AM

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I read a bit more on this, and it is certainly said that pointers are guaranteed not to point to anything 'real'; I guess that it depends on what you mean by 'point to'. As we can't say anything general about the internal representation of the pointer (as it varies) I'd take the definition that it doesn't, in fact, point to anything real. That it can cause problems if it is treated as pointing to something real doesn't, to me, mean that it is pointing to anything real anymore than a map of Middle Earth maps a real place, even though if you treat it as real and use it to navigate, it's going to cause you problems.

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And on the idea that the idea that a pointer, in general, has to point to something, if there exists a number I can put into a pointer that doesn't represent anything real (regardless of whether, on my machine, a null pointer fits this bill) then we can have a pointer that doesn't point to anything. Whether or not a null pointer is that is a different matter, but I'm not at all convinced by your statement that it is 'inaccurate' to say that a pointer points to nothing ('nothing' as meaning 'no-thing').

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quote:

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Originally posted by Smug Git
I read a bit more on this, and it is certainly said that pointers are guaranteed not to point to anything 'real'

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Are you saying that every possible number that can be the pointer 'is something'? I can accept that outside of my executable it might point to something (depending on the machine, etc) but is that guaranteed? In any case, I was talking about the context of the executable in any case.

If a null pointer pointed to nothing, I don't see why that would mean that you couldn't check against it, or change it (redirect it, from pointing at nothing to pointing at something). The null pointer is itself real, as a map of Middle Earth is real. What it depicts/points to might not be real, though.

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quote:

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Originally posted by Smug Git
Are you saying that every possible number that can be the pointer 'is something'?

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