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From: torek@elf.ee.lbl.gov (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: Is this ok??
Message-ID: <10784@dog.ee.lbl.gov>
Date: 10 Mar 91 03:49:50 GMT
References: <DAVIS.91Mar6213546@pacific.mps.ohio-state.edu> <1991Mar09.092611.24821@pilikia.pegasus.com>
Reply-To: torek@elf.ee.lbl.gov (Chris Torek)
Distribution: comp
Organization: Lawrence Berkeley Laboratory, Berkeley CA
Lines: 69
X-Local-Date: Sat, 9 Mar 91 19:49:50 PST

In article <1991Mar09.092611.24821@pilikia.pegasus.com>
art@pilikia.pegasus.com (Art Neilson) writes:
>After seeing lots of replies to John Davis I went back and examined the
>program he posted again and realized I was wrong regarding his pointer
>usage, I sorta jumped the gun when I saw the &'s and ** stuff, I was
>wrong.

Indeed.  (Incidentally, the reason his program `failed' on VMS was that
he was testing a different program, an easy enough mistake to make; when
he tested the one he posted, it worked.)

>I still don't think it's OK to assign the quoted string "Hello\n"
>to *s in fm2() as shown below.  Where does *s point to ?  Where in storage
>would "Hello\n" reside ?  Does the compiler assign some scratch storage or 
>something for it ??

If you do not know the answer to the latter question, you should not
be posting definitive followups to comp.lang.c.

A double quoted string in C is, with one exception%, an anonymous
object of type `array N of char', where N is one more than the number
of characters enclosed in quotes (after escape interpolation).  The
value of an array object is of course determined by The Rule, hence in

>>#include <stdio.h>
>>void fm2(s)
>>char **s;
>>{
>>    *s = "Hello\n";
>>}

the object at `*s' (which will have to be an <object, pointer to char>
for the assignment to succeed) is made to point to the first character
of this anonymous array of (in this case) 7 characters.  That array
itself resides in some sort of system-allocated space which has static
storage duration; a typical system puts it in a code or initialized
data segment.  (There are atypical systems; a C compiler for Xerox
D-machines once kept the original strings in Mesa `string descriptor'
format and allocated the C versions on the heap.  Perhaps it still does,
although Xerox D-machines have largely been relegated to the Dustbin
of History....)

In the original example in question, `s' happened in this case to
point to a single `char *' object declared in fm1():

	fm1(ss) char **ss; { char *s; fm2(&s); *ss = s; }

(I may have some of the original names mixed up), so the clause `which
will have to be an <object, pointer to char> for the assignment to
succeed' is satisfied.  The object at fm2's `*s' is fm1's `s', so s is
made to point to the `H' in `Hello\n\0'.  Fm1 then sets the object at
`*ss' (which has the exact same restriction) to point to that `H' as
well, and fm1's *ss in the original example was main's `s', so
everything was fine, if overly convoluted.
-----
% The exception occurs when a double quoted string is used as an
  initializer for an object of type `array N of char' or `array N
  of const char', including when the size N is given as `the size
  of the initializer':

	char xyz[] = "abcd";

  makes xyz an <object, array 5 of char, `abcd\0'>, and need not
  leave a copy of the sequence `abcd\0' anywhere else.  (If it does
  leave a copy somewhere else, the compiler is wasting space, unless
  this copy exists for, e.g., debugging purposes.)
-- 
In-Real-Life: Chris Torek, Lawrence Berkeley Lab EE div (+1 415 486 5427)
Berkeley, CA		Domain:	torek@ee.lbl.gov