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From: johnhlee@CS.Cornell.EDU (John H. Lee)
Newsgroups: comp.sys.amiga.programmer
Subject: Re: ANOTHER SAS C BUG
Message-ID: <1991Apr2.194634.27075@cs.cornell.edu>
Date: 2 Apr 91 19:46:34 GMT
References: <1991Mar31.035009.13183@csis.dit.csiro.au> <1991Apr1.050544.13878@cs.cornell.edu> <98@hdwr1.medar.com>
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Reply-To: johnhlee@cs.cornell.edu (John H. Lee)
Organization: Cornell Univ. CS Dept, Ithaca NY 14853
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In article <98@hdwr1.medar.com> jseymour@medar.com (James Seymour) writes:
>John, you are correct in that what the original poster was complaining about
>is, indeed, a portability "bug".  You are incorrect regarding your comment
>on how the string should be declared however (does this belong in the C
>newsgroup?).
>
>char *str = "hello";		/* declare a pointer to a char array,
>				   initialize pointer to a string */
>
>char str[] = "hello";		/* declare a character array, initialized
>				   in length and contents */
>
>BIG difference.  Especially if a function in another function is told
>that str *contains* a pointer to char, when in reality it *is* the start
>of the character array itself. [I've bitten myself with that one more times
>than I'd care to count :-)].  Lastly, the amount of memory consumed is
>not the same for each.  The first declaration takes up <pointer_sized> more
>memory than the second (although generally this is an insignificant
>point).  Which way it should be declared depends on what the program[mer]
>needs.

Yikes!  Let me try again...

I meant to say that the declaration should have been something like this:

	char buf[] = "hello";
	char *str = buf;

My point is that the declaration

	char *str = "hello";

declares a pointer to a string and sets its inital value to point to a string
in memory.  The string might be considered a "reusable constant" by the
compiler and a use of the string elsewhere in the module like this:

	printf("hello");

could have it that the same string is used.  Thus if you execute this:

	*str = 'i';
	printf("hello");

you'll get "iello" printed out!  This is by no means a certainty;  page 104
of the second edition of K&R C says that the effect is undefined.  By
declaring a character array instead of a pointer, you are guarenteed to
receive a private copy.

As for my comment about the memory allocation being the same, I was off-base
in the stated context, but should be right in the corrected context.

-------------------------------------------------------------------------------
The DiskDoctor threatens the crew!  Next time on AmigaDos: The Next Generation.
	John Lee		Internet: johnhlee@cs.cornell.edu
The above opinions of those of the user, and not of this machine.