Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!unhd!rg
From: rg@uunet!unhd (Roger Gonzalez )
Newsgroups: comp.lang.c
Subject: extern char *foo vs. extern char foo[]
Message-ID: <1990May30.001219.23564@uunet!unhd>
Date: 30 May 90 00:12:19 GMT
Reply-To: rg@unhd.unh.edu.UUCP (Roger Gonzalez )
Distribution: comp
Organization: Marine Systems Engineering Lab
Lines: 61

A little history: I know C inside and out, and don't need any "I think"s.
I have been aware of this "feature" for quite a while, and know how to
program around it.  I am posting this to find out *why* things are done this
way, since it seems to violate what K&R say. 

According to K&R, there should be no difference between the two extern
defs in my subject header.  In fact, the second form (char foo[]) should
get translated into the first.  Unfortunately, it doesn't.  Here's a
more detailed example of what I mean:

FILE #1:

     char hello[100];
     main()
     {
         strcpy(hello, "Hello, world.\n");
         printf("In main(), hello is at %06X\n", hello);
         foo();
     }

FILE #2:

     extern char *hello;
     foo()
     {
         printf("In foo(), hello is at %06X\n", hello);
     }

Now, theoretically, since C is not strongly typed, the two values printed
should be the same.  They aren't.  Try it if you don't believe me.  If
you change file #2 so that the extern declaration is

     extern hello[];

it works.  It also will work properly if in file #1, hello is declared
to be a pointer to char, and is malloc-ated to 100 bytes.  Well, I poked
around a little further; I stopped the output at the assembler stage, and
looked at the .s files for file #2 (using the first method of correction,
namely declaring extern char hello[]), did a diff, and guess what I found?

>
mov hello, (sp%)              I know, I know. This isn't *quite* what I
<                             found, but I'm typing it from memory, and its
mov &hello, (sp%)             close enough for government work.

Aha!  Zees eez very wrong!

This is the same output I got on 3 different machines, running Unisoft
and Green Hills compilers.  So.  Why are my compilers so stupid?  Do others
behave the same way, or am I just unfortunate enough to have lousy software?
If this is more widespread, WHY?  Is this a manifestation of some other
C rule that takes precedence over what the "proper" behavior?  Please clue
me in.  Please.


-Roger
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