pointers to arrays

[Wayne Throop]

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