Redefining NULL

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The C standard does not require null pointers to be at the machine’s address zero. HOWEVER, casting a 0 constant to a pointer value must result in a NULL pointer (§6.3.2.3/3), and evaluating the null pointer as a boolean must be false. This can be a bit awkward if you really do want a zero address, and NULL is not the zero address.

Nevertheless, with (heavy) modifications to the compiler and standard library, it’s not impossible to have NULL be represented with an alternate bit pattern while still remaining strictly conformant to the standard library. It is not sufficient to simply change the definition of NULL itself however, as then NULL would evaluate to true.

Specifically, you would need to:

  • Arrange for literal zeros in assignments to pointers (or casts to pointers) to be converted into some other magic value such as -1.
  • Arrange for equality tests between pointers and a constant integer 0 to check for the magic value instead (§6.5.9/6)
  • Arrange for all contexts in which a pointer type is evaluated as a boolean to check for equality to the magic value instead of checking for zero. This follows from the equality testing semantics, but the compiler may implement it differently internally. See §6.5.13/3, §6.5.14/3, §6.5.15/4, §6.5.3.3/5, §6.8.4.1/2, §6.8.5/4
  • As caf pointed out, update the semantics for initialization of static objects (§6.7.8/10) and partial compound initializers (§6.7.8/21) to reflect the new null pointer representation.
  • Create an alternate way to access true address zero.

There are some things you do not have to handle. For example:

int x = 0;
void *p = (void*)x;

After this, p is NOT guaranteed to be a null pointer. Only constant assignments need be handled (this is a good approach for accessing true address zero). Likewise:

int x = 0;
assert(x == (void*)0); // CAN BE FALSE

Also:

void *p = NULL;
int x = (int)p;

x is not guaranteed to be 0.

In short, this very condition was apparently considered by the C language committee, and considerations made for those who would choose an alternate representation for NULL. All you have to do now is make major changes to your compiler, and hey presto you’re done 🙂

As a side note, it may be possible to implement these changes with a source code transformation stage before the compiler proper. That is, instead of the normal flow of preprocessor -> compiler -> assembler -> linker, you’d add a preprocessor -> NULL transformation -> compiler -> assembler -> linker. Then you could do transformations like:

p = 0;
if (p) { ... }
/* becomes */
p = (void*)-1;
if ((void*)(p) != (void*)(-1)) { ... }

This would require a full C parser, as well as a type parser and analysis of typedefs and variable declarations to determine which identifiers correspond to pointers. However, by doing this you could avoid having to make changes to the code generation portions of the compiler proper. clang may be useful for implementing this – I understand it was designed with transformations like this in mind. You would still likely need to make changes to the standard library as well of course.

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