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From: throopw@dg_rtp.UUCP (Wayne Throop)
Newsgroups: comp.lang.c
Subject: Re: pointers to arrays
Message-ID: <717@dg_rtp.UUCP>
Date: Tue, 25-Nov-86 18:12:51 EST
Article-I.D.: dg_rtp.717
Posted: Tue Nov 25 18:12:51 1986
Date-Received: Wed, 26-Nov-86 19:59:33 EST
References: <273@bms-at.UUCP> <1138@genrad.UUCP> <274@bms-at.UUCP>
Lines: 48
Summary: Subtle note about &array under X3J11, and another method

Stuart coded an example that "ought to work" under X3J11 C like so:

> #include <setjmp.h>
> jmp_buf *envptr;
> void f(){
>   jmp_buf local_env;
>   envptr = &local_env;
>   if (!setjmp(local_env)) {
>     /* . . . */
>   }
> }

Couple of points.  A longjmp to the envptr, coded like so:

                longjmp( envptr, 1 );

would be type incorrect, since envptr has type (jmp_buf *) (that is,
type (int (*)[N])), and longjmp expects type a promoted jmp_buf type
(that is, type (int *)).  The longjmp ought to be coded:

                longjmp( *envptr, 1 );

Note that Stuart wanted a "better way" to code all this under current,
pre X3J11 C.  I note that if you don't mind the sleaziness of knowing
that jmp_buf is really an array, and what element type jmp_buf has (int,
in most cases), you can code it like so:

        #include <setjmp.h>
        int *envptr;
        void f(){
          jmp_buf local_env;
          envptr = local_env;
          if (!setjmp(local_env)) {
            /* . . . */
          }
        }

And then, the longjmp becomes just

                longjmp( envptr, 1 );

I hope everybody sees why this works.

--
A program without a loop and a structured variable isn't worth writing.
                        --- Alan J. Perlis
-- 
Wayne Throop      <the-known-world>!mcnc!rti-sel!dg_rtp!throopw