On Sun, 6 Sep 2020 16:48:18 -0700 Charles Mills wrote:
> I'm familiar with the use of NULL as a "special" value. I think > the C standard says that 0 may never be a valid address. The ISO/IEC 9899:20xx "C" standard cites no restriction on the value zero being an invalid address. An extraction from the standard: Topic 6.3.2.3 Pointers An integer may be converted to any pointer type. Except as previously specified, the result is implementation-defined, might not be correctly aligned, might not point to an entity of the referenced type, and might be a trap representation. An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function. In other words, the standard requires that the simple assignment of the integer value zero to a pointer causes that pointer to be assigned the implementation defined value of NULL. For example: char *somepointer = 0; A conforming compiler treats the above assignment as: char *somepointer = NULL; While on most implementations this assigns the value zero to "somepointer", it is not necessarily so; the value of NULL may be any bit pattern which satisfies the constraints stated in the standard. Most implementations define NULL (for example, in stdlib.h or stddef.h) as: #define NULL ((void *)0) An example of an implementation dependent way to get the value zero assigned to a pointer is shown below. The "trick" here is that the value being assigned is not a constant expression. volatile int intzero = 0; char *somepointer = (char *)intzero; C does not prohibit dereferencing the NULL pointer; rather it makes it undefined (and implementation dependent) behavior. Certainly, on an IBM mainframe beginning with the S/360 and continuing to the present, it would be awkward (and obnoxious) to be unable to reference the PSA via a pointer variable. Bob
