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I thought “integer literal zero (in pointer contexts)” was the definition of NULL in all versions of C. As I understand it the resulting pointer doesn’t necessa

I thought “integer literal zero (in pointer contexts)” was the definition of NULL in all versions of C. As I understand it the resulting pointer doesn’t necessarily have to have a value of 0 (even though that’s what it says in the source code) so long as it behaves appropriately nullish—i.e. the compiler can translate “int *foo = 0;” as “MOV foo 0xDEADCODE” so long as that value won’t be used for any legitimate pointer.

msla 6y ago
The above-linked site about POSIX has this paragraph: https://www.austingroupbugs.net/view.php?id=940 https://www.austingroupbugs.net/view.php?id=940 > The C standard goes to great length to ensure that NULL (and for that matter, any representation of the null pointer, which is written in source code by converting a constant value 0 to a pointer) need not be represented in hardware by all 0 bits (that is, 'int p = 0;' or 'static int p;' is allowed to compile to assembly instructions that assign a non-zero bit pattern to the location named p). And in fact, there are historical machines where the most efficient null pointer in hardware was indeed a non-zero bit pattern: http://c-faq.com/null/machexamp.html http://c-faq.com/null/machexamp.html ... and the site it links to has this: > Q: Seriously, have any actual machines really used nonzero null pointers, or different representations for pointers to different types? > A: The Prime 50 series used segment 07777, offset 0 for the null pointer, at least for PL/I. Later models used segment 0, offset 0 for null pointers in C, necessitating new instructions such as TCNP (Test C Null Pointer), evidently as a sop to [footnote] all the extant poorly-written C code which made incorrect assumptions. Older, word-addressed Prime machines were also notorious for requiring larger byte pointers (char 's) than word pointers (int 's). [snip] > The Symbolics Lisp Machine, a tagged architecture, does not even have conventional numeric pointers; it uses the pair <NIL, 0> (basically a nonexistent <object, offset> handle) as a C null pointer.

msla 6y ago

The above-linked site about POSIX has this paragraph: https://www.austingroupbugs.net/view.php?id=940 https://www.austingroupbugs.net/view.php?id=940 > The C standard goes to great length to ensure that NULL (and for that matter, any representation of the null pointer, which is written in source code by converting a constant value 0 to a pointer) need not be represented in hardware by all 0 bits (that is, 'int p = 0;' or 'static int p;' is allowed to compile to assembly instructions that assign a non-zero bit pattern to the location named p). And in fact, there are historical machines where the most efficient null pointer in hardware was indeed a non-zero bit pattern: http://c-faq.com/null/machexamp.html http://c-faq.com/null/machexamp.html ... and the site it links to has this: > Q: Seriously, have any actual machines really used nonzero null pointers, or different representations for pointers to different types? > A: The Prime 50 series used segment 07777, offset 0 for the null pointer, at least for PL/I. Later models used segment 0, offset 0 for null pointers in C, necessitating new instructions such as TCNP (Test C Null Pointer), evidently as a sop to [footnote] all the extant poorly-written C code which made incorrect assumptions. Older, word-addressed Prime machines were also notorious for requiring larger byte pointers (char 's) than word pointers (int 's). [snip] > The Symbolics Lisp Machine, a tagged architecture, does not even have conventional numeric pointers; it uses the pair <NIL, 0> (basically a nonexistent <object, offset> handle) as a C null pointer.