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From: am@cl.cam.ac.uk (Alan Mycroft)
Newsgroups: comp.arch,comp.lang.c
Subject: Re: String Processing Instruction
Message-ID: <699@jenny.cl.cam.ac.uk>
Date: Mon, 27-Apr-87 07:31:14 EDT
Article-I.D.: jenny.699
Posted: Mon Apr 27 07:31:14 1987
Date-Received: Sun, 3-May-87 00:50:14 EDT
References: <15292@amdcad.UUCP> <693@jenny.cl.cam.ac.uk> <7349@boring.mcvax.cwi.nl> <4605@utcsri.UUCP>
Reply-To: am@cl.cam.ac.uk (Alan Mycroft)
Organization: U of Cambridge Comp Lab, UK
Lines: 12
Xref: mnetor comp.arch:1152 comp.lang.c:2013

In article <4605@utcsri.UUCP> greg@utcsri.UUCP (Gregory Smith) writes:
>In article <693@jenny.cl.cam.ac.uk> am@cl.cam.ac.uk (Alan Mycroft) writes:
>>   #define has_nullbyte_(x) ((x - 0x01010101) & ~x & 0x80808080)
>>Then if e is an expression without side effects (e.g. variable)
>>   has_nullbyte_(e)
>>is nonzero iff the value of e has a null byte.
>
>If one of the bytes contains 0x80, then has_nullbyte() will
>say 'yes'. This can be circumvented by a more thorough test
>after this one to see if there really is a null there.

Untrue.  It would be true if I had mistakenly used ^ instead of &~.