Why is address zero used for the null pointer?

Question

In C (or C++ for that matter), pointers are special if they have the value zero: I am adviced to set pointers to zero after freeing their memory, because it means freeing the pointer again isn't dangerous; when I call malloc it returns a pointer with the value zero if it can't get me memory; I use if (p != 0) all the time to make sure passed pointers are valid, etc.

But since memory addressing starts at 0, isn't 0 just as a valid address as any other? How can 0 be used for handling null pointers if that is the case? Why isn't a negative number null instead?

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Edit:

A bunch of good answers. I'll summarize what has been said in the answers expressed as my own mind interprets it and hope that the community will correct me if I misunderstand.

• Like everything else in programming it's an abstraction. Just a constant, not really related to the address 0. C++0x emphasizes this by adding the keyword nullptr.

• It's not even an address abstraction, it's the constant specified by the C standard and the compiler can translate it to some other number as long as it makes sure it never equals a "real" address, and equals other null pointers if 0 is not the best value to use for the platform.

• In case it's not an abstraction, which was the case in the early days, the address 0 is used by the system and off limits to the programmer.

• My negative number suggestion was a little wild brainstorming, I admit. Using a signed integer for addresses is a little wasteful if it means that apart from the null pointer (-1 or whatever) the value space is split evenly between positive integers that make valid addresses and negative numbers that are just wasted.

• If any number is always representable by a datatype, it's 0. (Probably 1 is too. I think of the one-bit integer which would be 0 or 1 if unsigned, or just the signed bit if signed, or the two bit integer which would be [-2, 1]. But then you could just go for 0 being null and 1 being the only accessible byte in memory.)

Still there is something that is unresolved in my mind. The Stack Overflow question Pointer to a specific fixed address tells me that even if 0 for null pointer is an abstraction, other pointer values aren't necessarily. This leads me to post another Stack Overflow question, Could I ever want to access the address zero?.

Answer 1

2 points:

• only the constant value 0 in the source code is the null pointer - the compiler implementation can use whatever value it wants or needs in the running code. Some platforms have a special pointer value that's 'invalid' that the implementation might use as the null pointer. The C FAQ has a question, "Seriously, have any actual machines really used nonzero null pointers, or different representations for pointers to different types?", that points out several platforms that used this property of 0 being the null pointer in C source while represented differently at runtime. The C++ standard has a note that makes clear that converting "an integral constant expression with value zero always yields a null pointer, but converting other expressions that happen to have value zero need not yield a null pointer".

• a negative value might be just as usable by the platform as an address - the C standard simply had to chose something to use to indicate a null pointer, and zero was chosen. I'm honestly not sure if other sentinel values were considered.

The only requirements for a null pointer are:

• it's guaranteed to compare unequal to a pointer to an actual object
• any two null pointers will compare equal (C++ refines this such that this only needs to hold for pointers to the same type)

Answer 2

Historically, the address space starting at 0 was always ROM, used for some operating system or low level interrupt handling routines, nowadays, since everything is virtual (including address space), the operating system can map any allocation to any address, so it can specifically NOT allocate anything at address 0.

Answer 3

IIRC, the "null pointer" value isn't guaranteed to be zero. The compiler translates 0 into whatever "null" value is appropriate for the system (which in practice is probably always zero, but not necessarily). The same translation is applied whenever you compare a pointer against zero. Because you can only compare pointers against each other and against this special-value-0, it insulates the programmer from knowing anything about the memory representation of the system. As for why they chose 0 instead of 42 or somesuch, I'm going to guess it's because most programmers start counting at 0 :) (Also, on most systems 0 is the first memory address and they wanted it to be convenient, since in practice translations like I'm describing rarely actually take place; the language just allows for them).

Answer 4

You must be misunderstanding the meaning of constant zero in pointer context.

Neither in C nor in C++ pointers can "have value zero". Pointers are not arithmetic objects. They canot have numerical values like "zero" or "negative" or anything of that nature. So your statement about "pointers ... have the value zero" simply makes no sense.

In C & C++ pointers can have the reserved null-pointer value. The actual representation of null-pointer value has nothing to do with any "zeros". It can be absolutely anything appropriate for a given platform. It is true that on most plaforms null-pointer value is represented physically by an actual zero address value. However, if on some platform address 0 is actually used for some purpose (i.e. you might need to create objects at address 0), the null-pointer value on such platform will most likely be different. It could be physically represented as 0xFFFFFFFF address value or as 0xBAADBAAD address value, for example.

Nevertheless, regardless of how the null-pointer value is respresented on a given platform, in your code you will still continue to designate null-pointers by constant 0. In order to assign a null-pointer value to a given pointer, you will continue to use expressions like p = 0. It is the compiler's responsibility to realize what you want and translate it into the proper null-pointer value representation, i.e. to translate it into the code that will put the address value of 0xFFFFFFFF into the pointer p, for example.

In short, the fact that you use 0 in your sorce code to generate null-pointer values does not mean that the null-pointer value is somehow tied to address 0. The 0 that you use in your source code is just "syntactic sugar" that has absolutely no relation to the actual physical address the null-pointer value is "pointing" to.

Answer 5

But since memory addressing starts at 0, isn't 0 just as a valid address as any other?

On some/many/all operating systems, memory address 0 is special in some way. For example, it's often mapped to invalid/non-existent memory, which causes an exception if you try to access it.

Why isn't a negative number null instead?

I think that pointer values are typically treated as unsigned numbers: otherwise for example a 32-bit pointer would only be able to address 2 GB of memory, instead of 4 GB.

Answer 6

My guess would be that the magic value 0 was picked to define an invalid pointer since it could be tested for with less instructions. Some machine languages automatically set the zero and sign flags according to the data when loading registers so you could test for a null pointer with a simple load then and branch instructions without doing a separate compare instruction.

(Most ISAs only set flags on ALU instructions, not loads, though. And usually you aren't producing pointers via computation, except in the compiler when parsing C source. But at least you don't need an arbitrary pointer-width constant to compare against.)

On the Commodore Pet, Vic20, and C64 which were the first machines I worked on, RAM started at location 0 so it was totally valid to read and write using a null pointer if you really wanted to.

Answer 7

I think it's just a convention. There must be some value to mark an invalid pointer.

You just lose one byte of address space, that should rarely be a problem.

There are no negative pointers. Pointers are always unsigned. Also if they could be negative your convention would mean that you lose half the address space.

Answer 8

Although C uses 0 to represent the null pointer, do keep in mind that the value of the pointer itself may not be a zero. However, most programmers will only ever use systems where the null pointer is, in fact, 0.

But why zero? Well, it's one address that every system shares. And oftentimes the low addresses are reserved for operating system purposes thus the value works well as being off-limits to application programs. Accidental assignment of an integer value to a pointer is as likely to end up zero as anything else.

Answer 9

Historically the low memory of an application was occupied by system resources. It was in those days that zero became the default null value.

While this is not necessarily true for modern systems, it is still a bad idea to set pointer values to anything but what memory allocation has handed you.

Answer 10

Regarding the argument about not setting a pointer to null after deleting it so that future deletes "expose errors"...

If you're really, really worried about this then a better approach, one that is guaranteed to work, is to leverage assert():

...
assert(ptr && "You're deleting this pointer twice, look for a bug?");
delete ptr;
ptr = 0;
...

This requires some extra typing, and one extra check during debug builds, but it is certain to give you what you want: notice when ptr is deleted 'twice'. The alternative given in the comment discussion, not setting the pointer to null so you'll get a crash, is simply not guaranteed to be successful. Worse, unlike the above, it can cause a crash (or much worse!) on a user if one of these "bugs" gets through to the shelf. Finally, this version lets you continue to run the program to see what actually happens.

I realize this does not answer the question asked, but I was worried that someone reading the comments might come to the conclusion that it is considered 'good practice' to NOT set pointers to 0 if it is possible they get sent to free() or delete twice. In those few cases when it is possible it is NEVER a good practice to use Undefined Behavior as a debugging tool. Nobody that's ever had to hunt down a bug that was ultimately caused by deleting an invalid pointer would propose this. These kinds of errors take hours to hunt down and nearly alway effect the program in a totally unexpected way that is hard to impossible to track back to the original problem.

Answer 11

An important reason why many operating systems use all-bits-zero for the null pointer representation, is that this means memset(struct_with_pointers, 0, sizeof struct_with_pointers) and similar will set all of the pointers inside struct_with_pointers to null pointers. This is not guaranteed by the C standard, but many, many programs assume it.

Answer 12

In one of the old DEC machines (PDP-8, I think), the C runtime would memory protect the first page of memory so that any attempt to access memory in that block would cause an exception to be raised.

Answer 13

The choice of sentinel value is arbitrary, and this is in fact being addressed by the next version of C++ (informally known as "C++0x", most likely to be known in the future as ISO C++ 2011) with the introduction of the keyword nullptr to represent a null valued pointer. In C++, a value of 0 may be used as an initializing expression for any POD and for any object with a default constructor, and it has the special meaning of assigning the sentinel value in the case of a pointer initialization. As for why a negative value was not chosen, addresses usually range from 0 to 2^N-1 for some value N. In other words, addresses are usually treated as unsigned values. If the maximum value were used as the sentinel value, then it would have to vary from system to system depending on the size of memory whereas 0 is always a representable address. It is also used for historical reasons, as memory address 0 was typically unusable in programs, and nowadays most OSs have parts of the kernel loaded into the lower page(s) of memory, and such pages are typically protected in such a way that if touched (dereferenced) by a program (save the kernel) will cause a fault.

Answer 14

It has to have some value. Obviously you don't want to step on values the user might legitimately want to use. I would speculate that since the C runtime provides the BSS segment for zero-initialized data, it makes a certain degree of sense to interpret zero as an un-initialized pointer value.

Answer 15

Rarely does an OS allow you to write to address 0. It's common to stick OS-specific stuff down in low memory; namely, IDTs, page tables, etc. (The tables have to be in RAM, and it's easier to stick them at the bottom than to try and determine where the top of RAM is.) And no OS in its right mind will let you edit system tables willy-nilly.

This may not have been on K&R's minds when they made C, but it (along with the fact that 0==null is pretty easy to remember) makes 0 a popular choice.

Answer 16

The value 0 is a special value that takes on various meanings in specific expressions. In the case of pointers, as has been pointed out many many times, it is used probably because at the time it was the most convenient way of saying "insert the default sentinel value here." As a constant expression, it does not have the same meaning as bitwise zero (i.e., all bits set to zero) in the context of a pointer expression. In C++, there are several types that do not have a bitwise zero representation of NULL such as pointer member and pointer to member function.

Thankfully, C++0x has a new keyword for "expression that means a known invalid pointer that does not also map to bitwise zero for integral expressions": nullptr. Although there are a few systems that you can target with C++ that allow dereferencing of address 0 without barfing, so programmer beware.

Answer 17

There are already a lot of good answers in this thread; there are probably many different reasons for preferring the value 0 for null pointers, but I'm going to add two more:

• In C++, zero-initializing a pointer will set it to null.
• On many processors it is more efficient to set a value to 0 or to test for it equal/not equal to 0 than for any other constant.

Answer 18

This is dependent on the implementation of pointers in C/C++. There is no specific reason why NULL is equivalent in assignments to a pointer.

Answer 19

There are historic reasons for this, but there are also optimization reasons for it.

It is common for the OS to provide a process with memory pages initialized to 0. If a program wants to interpret part of that memory page as a pointer then it is 0, so it is easy enough for the program to determine that that pointer is not initialized. (this doesn't work so well when applied to uninitialized flash pages)

Another reason is that on many many processors it is very very easy to test a value's equivalence to 0. It is sometimes a free comparison done without any extra instructions needed, and usually can be done without needing to provide a zero value in another register or as a literal in the instruction stream to compare to.

The cheap comparisons for most processors are the signed less than 0, and equal to 0. (signed greater than 0 and not equal to 0 are implied by both of these)

Since 1 value out of all of possible values needs to be reserved as bad or uninitialized then you might as well make it the one that has the cheapest test for equivalence to the bad value. This is also true for '\0' terminated character strings.

If you were to try to use greater or less than 0 for this purpose then you would end up chopping your range of addresses in half.

Answer 20

The constant 0 is used instead of NULL because C was made by some cavemen trillions of years ago, NULL, NIL, ZIP, or NADDA would have all made much more sense than 0.

But since memory addressing starts at 0, isn't 0 just as a valid address as any other?

Indeed. Although a lot of operating systems disallow you from mapping anything at address zero, even in a virtual address space (people realized C is an insecure language, and reflecting that null pointer dereference bugs are very common, decided to "fix" them by dissallowing the userspace code to map to page 0; Thus, if you call a callback but the callback pointer is NULL, you wont end up executing some arbitrary code).

How can 0 be used for handling null pointers if that is the case?

Because 0 used in comparison to a pointer will be replaced with some implementation specific value, which is the return value of malloc on a malloc failure.

Why isn't a negative number null instead?

This would be even more confusing.