What is setjmp... Clarification and typedef gripe

[Henry Spencer]

>>     jmp_buf foo, *tmp;
>> 	...
>>     tmp = &foo;
>> $ cc foo
>> 
>>  (0008)     tmp = &foo;
>> ******** Line 8: Warning:  Illegal pointer combination: incompatible types.

>     "setjmp foo, *tmp; tmp = &foo;" is accepted with no complaints by my C
> compiler (based on the Ritchie V7 PDP11 "cc"), as it should be, because
> "tmp" and "&foo" have exactly the same (defined) type...

The problem, alas, goes deeper, and the Ritchie compiler is covering it up
rather than solving it.  The *real* problem is that many implementations
typedef jmp_buf as an array, and &array is not a legal use of &.  The old
Ritchie compiler lets you get away with it, but more recent ones don't.
I'm not sure just when &array was officially declared illegal; a quick
glance at K&R doesn't find a statement to that effect, but there definitely
is one in the recent ANSI draft.  And pcc, even the old V7 one, does object.

Note that casts don't really solve the problem; &array is *illegal*, and
a strict compiler will reject it completely.

This is a place where typedef isn't as opaque as it should be, alas.  If
jmp_buf is a struct, you must write "tmp = &foo;", whereas if it's an
array you must write "tmp = foo;".  Not good.

Probably the best solution is to lean on your implementor to typedef
jmp_buf as a struct instead of an array; that's probably a better way
to do things all around.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry