Zsh Mailing List Archive
Messages sorted by:
Re: (NULL == 0) ?
- X-seq: zsh-workers 1178
- From: Zefram <A.Main@xxxxxxxxxxxxxxxxx>
- To: coleman@xxxxxxxxxxxxxxx (Richard Coleman)
- Subject: Re: (NULL == 0) ?
- Date: Sat, 25 May 1996 23:00:23 +0100 (BST)
- Cc: zsh-workers@xxxxxxxxxxxxxxx
- In-reply-to: <199605252125.RAA21046@xxxxxxxxxxxxxxxxxxxxxxxxx> 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
>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.
Messages sorted by: