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Date: Thu, 1 Aug 2013 08:47:14 -0400
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From: DJ Delorie <dj AT delorie DOT com>
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Subject: Re: _CRT0_FLAG_NULLOK
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> A NULL pointer can and does point to accessable memory for most C  
> compilers.

It's not the compiler that decides this, it's the operating system.
Many operating systems do not map the first page of memory, so a NULL
pointer may not point to accessible memory.

>  Typically, this is pointer or address to memory location zero, but
> it's not required to be.  It can be *any* value as long as no other
> C object has that same address.

"An integer constant expression with the value 0, or such an
 expression cast to type void *, is called a null pointer constant."

So, it's zero now.

> If NULL's type is a void pointer, it is (losslessly) converted to a char
> pointer when passed into a function for a char pointer argument.  I.e.,
> after the conversion, it functions as a char pointer in the function.

 "A string is a contiguous sequence of characters terminated by and
  including the first null character."

So there's a difference between "char pointer" and "pointer to a
string".  atoi() takes a pointer to a string.  Not all char pointers
are pointers to strings.

"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."

Since a pointer to any valid string cannot be equal to the null
pointer, a null pointer cannot point to a string.