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From: tim@nucleus.amd.com (Tim Olson)
Newsgroups: comp.lang.c++,comp.std.c
Subject: Re: references to dereferenced null pointers
Message-ID: <29472@amdcad.AMD.COM>
Date: 12 Mar 90 15:01:04 GMT
References: <51083@microsoft.UUCP> <25EB8EE8.8462@paris.ics.uci.edu> <52081@microsoft.UUCP> <25F8D2FB.10981@paris.ics.uci.edu> <1990Mar11.015305.28264@utzoo.uucp> <1990Mar11.222634.2701@almira.uucp>
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In article <1990Mar11.222634.2701@almira.uucp> haug@Columbia.NCR.COM (Brian Haug) writes:
| In article <1990Mar11.015305.28264@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
| >This code is illegal in C, and always has been.  There is no general
| >relationship between the integer value 0 and the null pointer.  An
| >integer *constant* value equal to zero in a pointer context is automatically
| >converted to a null pointer; this may be a non-trivial conversion, and is
| >guaranteed valid only at compile time.  The result of converting the
| >integer value 0 to a pointer at run time is implementation-defined.
| 
| Henry,
|     I beg to differ.  I see where you are coming from with the wording:
| "an integral constant expression with the value 0", but in K&R First edition,
| page 97, there is an explicit example which 0 is returned by a function
| returning a pointer to a character.

That is a correct use.  Just as in an assignment to a pointer type,
the compiler knows that it is to coerce the integral constant 0 to the
machine-specific nil representation for that pointer type.

This differs from the case that was being discussed, in that the
return value is specifically declared to be a pointer type.  In the
previous discussion, the return value was declared "int".


|     Also, from Appendix A, section 7.7 (Equality operators) "A pointer may be
| compared to an integer, but the result is machine dependent unless the integer
| is the constant 0.  A pointer to which 0 has been assigned is guaranteed no to
	 ^^^^^^^^^^
| point to any object, and will appear to be equal to 0; in conventional usage,
| such a pointer is considered to be null."  This implies to me that I can
| compare a pointer to any arbitrary expression which evaluates to 0 and have
| machine independent behavior.

Not unless it is a constant expression that evaluates to 0.  Constant
expressions are a subset of expressions that involve only integral
constants and certain operators -- function calls are not allowed.


	-- Tim Olson
	Advanced Micro Devices
	(tim@amd.com)