typedef gripe

[Kee Hinckley]

...let's hear it for cut and paste...

>     "setjmp foo, *tmp; tmp = &foo;" is accepted with no complaints by my C
> compiler (based on the Dennis Ritchie V7 PDP11 "cc"), as it should be, because
> "tmp" and "&foo" have exactly the same (defined) type.  Either your C compiler
> has a bug in this respect, or your <setjmp.h> is #defining jmp_buf, instead of
> typedefing it.  The thing to do is not gripe about the language, but gripe to
> whomever maintains your compiler, to get this bug fixed.
> -- 
> 			    Morris M. Keesan
> 			    {decvax,linus,ihnp4,wivax,wjh12,ima}!bbncca!keesan
> 			    keesan @ BBN-UNIX.ARPA

Consider the following:
    char    c[10], *cptr;

    cptr = c;

    Clearly "c", without any subscripts is equivlent to "*cptr", I've yet to
see a C compiler complain about THAT line.  This means that "c" is a "pointer
to character", NOT a pointer to an array of characters.  Therefore taking
its address is equivilent to "&cptr", or asking for the address of the actual
pointer.
    Conceptually, "typedef" ought (my definition of ought) make the type into
an abstract object, which no longer depends upon the fact that it used to be an
array.  In fact it does not, and thus:

    typedef char jmp_buf[10];
    jmp_buf c, *cptr;

    cptr = &c;

ends up being incorrect.

Now, just in case you think my compiler is strange (Apollo CC).  Here is
the Berkeley 4.2 example.

    %8% cat foo.c
    typedef char jmp_buf[10];
    main()
    {
            jmp_buf c, *cptr;
    
            cptr = c;
            cptr = &c;
            cptr = (jmp_buf *) &c;
            cptr = (jmp_buf *) c;
    }
    %9% cc foo.c
    "foo.c", line 6: warning: illegal pointer combination
    "foo.c", line 7: warning: & before array or function: ignored
    "foo.c", line 7: warning: illegal pointer combination
    "foo.c", line 8: warning: & before array or function: ignored
    %10% 
----------------------
Apollo CC
----------------------
-=> catf foo.c
    typedef char jmp_buf[10];
    main()
    {
            jmp_buf c, *cptr;
    
            cptr = c;
            cptr = &c;
            cptr = (jmp_buf *) &c;
            cptr = (jmp_buf *) c;
    }
-=> cc foo.c

 (0006)             cptr = c;
******** Line 6: Warning:  Illegal pointer combination: incompatible types.

 (0007)             cptr = &c;
******** Line 7: Warning:  Illegal pointer combination: incompatible types.
No errors, 2 warnings, C Compiler, Rev 2.40

--------------------------------------------------
Now I'm not saying that Berkeley has the right error messages either,
in particular the complaints about the '&'s are completely out of place
given that typedef is *supposed* to make things more data independant.
I might note however, that our version of PCC also complains about 
incompatable types (although not the '&').

What's the correct answer?  I don't know.  Probably the best thing to do
is avoid typedef'ing arrays completely.


                                            -kee
                                ...decvax!wivax!apollo!nazgul