From: Zefram <A.Main@dcs.warwick.ac.uk>
To: coleman@math.gatech.edu (Richard Coleman)
Cc: zsh-workers@math.gatech.edu
Subject: Re: (NULL == 0) ?
Date: Sat, 25 May 1996 23:00:23 +0100 (BST) [thread overview]
Message-ID: <15522.199605252200@stone.dcs.warwick.ac.uk> (raw)
In-Reply-To: <199605252125.RAA21046@redwood.skiles.gatech.edu> from "Richard Coleman" at May 25, 96 05:25:35 pm
To answer the subject line, (NULL == 0) *is* guaranteed. 0 is the
representation of the null pointer constant in C.
>I would be willing to bet money it's a non-unix box. Just think of
>the tremendous amount of unix code that uses the fact that calloc (in
>our case zcalloc) returns memory that is all zeros (and hence is
>NULL). Since zsh only works on unix boxes, I think zeroing out memory
>with memset is ok.
A lot of C code assumes this, but it is broken. No Unix standard
guarantees more than the C standard about the representation of null
pointers.
>I'm no expert on compilers, but I would be surprised if gcc could
>optimize such a loop to be as fast as memset. Usually memset is
>hand optimized assembler code.
True, it generally won't be as good, but GCC does produce very
efficient code. And the speed isn't really critical -- for example, in
this particular case, the code is only executed when you complete on a
command line with 16 words, and it then only needs to assign to 16
pointers. (It could only cause a crash when you reach 32 words, and on
my system it took 64.)
>The function `newhashtable' uses zcalloc to allocate hashtable memory
>and the function `emptyhashtable' uses memset to set this same memory
>to zero when it is emptied. If assuming NULL was equal to 0 would
>crash a machine, then these two pieces of code would have done it a
>long time ago. The hashtable code gets executed a lot.
I can believe that zsh is not currently being used on any system where
the null pointer is not all-bits-zero. Consequently it's not a really
important issue. But code that makes this assumption is ill-formed.
-zefram
next prev parent reply other threads:[~1996-05-25 22:14 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
1996-05-25 14:38 clwords bugfix Zefram
1996-05-25 18:02 ` Richard Coleman
1996-05-25 20:52 ` Zefram
1996-05-25 21:25 ` (NULL == 0) ? Richard Coleman
1996-05-25 22:00 ` Zefram [this message]
1996-05-27 0:00 ` Zoltan Hidvegi
1996-05-27 6:39 ` Bart Schaefer
1996-05-27 7:29 ` anthony baxter
1996-05-27 13:07 ` Zefram
1996-05-27 22:00 ` Zoltan Hidvegi
1996-05-27 22:11 ` Zefram
1996-05-27 22:41 ` Zoltan Hidvegi
1996-05-27 22:54 ` Zefram
1996-05-28 4:54 ` Bart Schaefer
1996-05-27 23:36 ` Bart Schaefer
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