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From: gary@apex.UUCP (Gary Wisniewski)
Newsgroups: comp.lang.c
Subject: Re: Passing (char *) NULL to printf to match %s
Message-ID: <298@apex.UUCP>
Date: Fri, 14-Aug-87 20:16:25 EDT
Article-I.D.: apex.298
Posted: Fri Aug 14 20:16:25 1987
Date-Received: Tue, 18-Aug-87 06:18:38 EDT
References: <166@qetzal.UUCP> <157@hobbes.UUCP> <875@bsu-cs.UUCP> <1219@cognos.UUCP> <25173@sun.uucp>
Reply-To: gary@apex.UUCP (Gary Wisniewski)
Organization: Apex Software Corp., Pittsburgh, Pa.
Lines: 32
Keywords: MSC,NULL
Summary: NULL is ds:0, why would anyone put strings there?

In article <25173@sun.uucp> guy%gorodish@Sun.COM (Guy Harris) writes:
>>   Speaking of this... has anyone else noticed any problems with this in
>> small model Microsoft C?  I seem to be unable to print out a string that
>> happens to be placed at offset 0 of the data segment -- MSC's libraries
>> decide it is a null pointer and format it as "(null)".
> [ ... comments about NULLness of pointers ... ]
>Either Microsoft C allowed it to be placed there [ds:0], in which case
>*that* is the bug, or somebody got around whatever mechanism
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>Microsoft C provides to ensure that no C object gets put at that
>location, in which case they got what they deserved.
>	Guy Harris

Code at the beginning of DS is an unlikely place to find user strings.
Microsoft places a small unchanging bit of junk there which it keeps
a checksum on.  If that section is written to during program execution,
MSC prints out "Error 2000: Null Pointer Assignment" when the program
terminates (the message may not be quite right---you get the idea).

This is actually a helpful technique in lieu of hardware address fault
checking.  It is also nice that NULL pointer references don't destroy
anything of great importance.

The original sender (name not known) seems to have reorganized or rewritten
the stock MSC runtime library routines.  I can't think of any other way
to get useful code at DS:0.  However, the tone of their comments suggests
that the NULL behavior is a mystery to them (anyone who had rewritten
runtime libraries would doubtless understand the formalities of segment
usage under MSC!)---Hmmmm.

Gary Wisniewski
{allegra,bellcore,cadre}!pitt!darth!apex!gary