Why is address zero used for the null pointer?

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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)

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