Re: NULL
  By: Torfs Tom to All on Mon Sep 08 2003 07:54 pm

 > > I'm not sure we're actually in any sort of disagreement
 > > here... If the issue is
 > > whether `if (ptr)' is portable, then the answer has to be no
 > > since ptr can be
 > > any pointer and not necessarily NULL.
 > 
 > Incorrect, this is actually perfectly OK code. In fact I use it all the time
 > Here's the relevant question from the comp.lang.c FAQ:
 > 
 > Question 5.3
 > Is the abbreviated pointer comparison ``if(p)'' to test for non-null pointer
 > valid? What if the internal representation for null pointers is nonzero?
 > 
 > ----------------------------------------------------------------------------
 > 
 > When C requires the Boolean value of an expression (in the if, while, for, a
 > do statements, and with the &&, ||, !, and ?: operators), a
false value is
 > inferred when the expression compares equal to zero, and a true value
 > otherwise. That is, whenever
 > one writes
 > 
 > 	if(expr)
 > 
 > where ``expr'' is any expression at all, the compiler essentially acts as if
 > had been written as
 > 	if((expr) != 0)
 > 
 > Substituting the trivial pointer expression ``p'' for ``expr,'' we have
 > 	if(p)	is equivalent to	if(p != 0)
 > 
 > and this is a comparison context, so the compiler can tell that the (implici
 > 0 is actually a null pointer constant, and use the correct null pointer valu
 > There is no trickery involved here; compilers do work this way, and generate
 > identical code for
 > both constructs. The internal representation of a null pointer does not matt
 > 
 > The boolean negation operator, !, can be described as follows:
 > 
 > 	!expr	is essentially equivalent to	(expr)?0:1
 > 		or to	((expr) == 0)
 > 
 > which leads to the conclusion that
 > 	if(!p)	is equivalent to	if(p == 0)

The FAQ is wrong on this question. There is no "internal representation of a
null pointer" as far as the compiler is concerned.

The C Library (and the stddef.h header file) determine the value of NULL. So if
for example, the C Library used -1 for NULL:

        if(p)

is *not* equivalent to:

        if(p!=NULL)

                                                  digital man

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