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From: jimp@cognos.uucp (Jim Patterson)
Newsgroups: comp.lang.c
Subject: Re: Query: Implementation with non-zero NULL
Message-ID: <836@cognos.UUCP>
Date: Thu, 4-Jun-87 12:49:20 EDT
Article-I.D.: cognos.836
Posted: Thu Jun  4 12:49:20 1987
Date-Received: Fri, 5-Jun-87 04:42:11 EDT
References: <158@delftcc.UUCP> <1070@viper.Lynx.MN.ORG>
Reply-To: jimp@cognos.UUCP (Jim Patterson)
Organization: Cognos Incorporated, Ottawa, Canada
Lines: 50

In article <1070@viper.Lynx.MN.ORG> john@viper.UUCP (John Stanley) writes:
>In article <158@delftcc.UUCP> henry@delftcc.UUCP (Henry Rabinowitz) writes:
> >
> >Are static pointers initialized to all bits zero or to the null value
> >(as in ANSI spec)?
>
>  If, you're refering to an initialized static, it's whatever you
>specify.  If, on the other hand, you're refering to an UN-initialized
>static pointer, the answer is a resounding "Neither!"....  

I won't deny that there may be some (non-conforming) C compilers that
don't always initialize statics, or that in some environments it's
costly to require that they be initialized, but it's quite clear in
both K&R and in the ANSI C draft that ALL statics be initialized.

Kierneghan & Ritchie, in "The C Programming Language", 1978
(generally regarded as the definitive specification of the C
language prior to ANSI's involvement) state in section 4.9 (page 82)
that "In the absence of explicit initialization, external and static
variables are guaranteed to be initialized to 0".  Section 8.6 of
the reference manual (page 198) back this up; "Static and external
variables which are not initialized are guaranteed to start off as 0".

ANSI C has maintained this guarantee; in section 3.7.2 of the May
15, 1987 draft (page 73) an object defined outside of a function of
either static or with no storage class specifier and no initializer
is termed a "tentative definition". The section goes on to state "If
no subsequent definition is encountered, the first tentative
definition is taken to be a definition with initializer equal to 0".

That, however, doesn't answer Mr. Rabinowitz's question. There is some
additional clarification in the ANSI draft (this time from July 6, 1986;
I'm afraid I don't have a more current copy of the relevant pages).
This is in section 3.5.6 on Initialization (page 61):

  "If an object that has static storage duration is not initialized
  explicitly, it is initialized implicitly as if every member
  that has arithmetic type were assigned 0 and every member that has 
  pointer type were assigned a null pointer constant".

(If someone has a more up to date complete copy of the draft, perhaps
you could confirm if this has changed; I don't believe it has).

So, if the compiler is ANSI-conforming it should initialize pointers
to null pointer constants, not 0.  If it isn't ANSI-conforming, who
knows? If the static pointer is in a union, then obviously there are
problems.
-- 
Jim Patterson
Cognos Inc.