Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!seismo!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c,misc.jobs.misc
Subject: Re: weird c code/ c test
Message-ID: <7761@mimsy.UUCP>
Date: Thu, 30-Jul-87 07:23:06 EDT
Article-I.D.: mimsy.7761
Posted: Thu Jul 30 07:23:06 1987
Date-Received: Sat, 1-Aug-87 08:06:54 EDT
References: <1089@gilsys.UUCP>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 65
Keywords: test, employment, weird, code, pointers
Xref: mnetor comp.lang.c:3374 misc.jobs.misc:517

In article <1089@gilsys.UUCP> mc68020@gilsys.UUCP (Thomas J Keller) writes:
>... See if you can determine the output of this program without actually
>compiling it:
>
>#include	<stdio.h>
>
>char *c[] = { "ENTER", "NEW", "POINT", "FIRST" };
>char **cp[] = { c+3, c+2, c+1, c };
>char ****cpp = cp;

Error:  Illegal type combination: attempt to assign pointer to pointer to
	pointer to char (char ***) to object of type pointer to pointer
	to pointer to pointer to char (char ****).

Remember, the type of `cp' is `array 4 of pointer to pointer to
char'; when the name `cp' is used in a context other than `sizeof',
this is altered by changing `array N of' to `pointer to', which
gives `pointer to pointer to pointer to char'.  That PCC compiles
this without complaint simply means there is a bug in PCC.  Guy
Harris posted a fix not long ago.

Assuming this was a typographic error fixes up the example (which
happens to work on any machine where the sizes of (char ****),
(char ***), (char **), and (char *) are equal).

>  (actually, the program AS PRESENTED probably wouldn't have compiled, as
>  the *c declaration was as follows:
>
>  char *c[] = { "ENTER", "NEW", "POINT", "FRIST", };
>
>  )

There is nothing wrong with this.  `c' is `array 4 of pointer to char',
as before.  (I assume "FRIST" is a typographic error, though in fact
it does not affect the output.)

Back to the code, changing the last declaration to `char ***cpp':

    printf("%s", **++cpp);

++cpp changes the value of cpp from &cp[0] to &cp[1]; *&cp[1] == cp[1];
cp[1] == &c[2]; *&c[2] is "POINT".

    printf("%s ", *--*++cpp+3);

++cpp changes its value from &cp[1] to &cp[2]; *&cp[2] == cp[2].
--cp[2] changes its value from &c[1] to &c[0]; *&c[0] == c[0], which
points to the first character of "ENTER"; adding three points to
the second E, so we print "ER ".

    printf("%s", *cpp[-2]+3);

Cpp now points to &cp[2]; two back from this is &cp[0].  One
dereference gives *&cp[0] == &c[3], another gives just c[3] which
points to the F in "FIRST"; adding three we print "ST".

    printf("%s\n", cpp[-1][-1]+1);

Cpp still points to &cp[2].  cpp[-1] is cp[1] or &c[2]; (&c[2])[-1]
is the same as (c+2)[-1] or *((c+2)+(-1)) or *(c+1), which points
at the N in "NEW".  Adding 1, we get "EW".  And now I have finished
my bread and will go make some mixed vegetables to fill out breakfast.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
Domain:	chris@mimsy.umd.edu	Path:	seismo!mimsy!chris