Casting function ptrs

[John F. Haugh II]

In article <280 at marob.MASA.COM>, daveh at marob.MASA.COM (Dave Hammond) writes:
> Given a list of varying typed functions, their names and addresses:
> 
> struct funcs {
>     char *name;
>     unsigned (*func)();
> }
> char *foo();
> struct funcs f = {  "foo", (char *)foo };


the field `func' is `pointer to function returning unsigned'.  assigning
anything else to it, such as a pointer to a function returning a pointer
to a character, as you do in the example above, is wrong.

> And a routine which is passed a function name in order to lookup and return
> its associated address: 
> 
> unsigned *lookup_func(name)
> char *name;
> {
>   ...
>   return( (unsigned *) f->func );
> }

this is also incorrect.  lookup_func should have its return value the same
type as f->func.  the correct declaraction for lookup_func is

unsigned *(*lookup_func()) (name)
char	*name;
{
	...
}

> How does one declare the lookup function, the return address variable and
> call the returned function, while properly casting the function return value?
> I'm doing it like so:
> unsigned *lookup_func();
> unsigned *(*func)();
> char *retp;
> 
> if ((func = lookup_func("foo")) != (unsigned *)0) {
>     if (( retp = (char *) (*func)() ))
> 	printf("retp=%s",retp);
>     }

the return address variable has the same type as f->func, which is `pointer
to function returning pointer to unsigned', or

unsigned *(*retp) ();

> Yes? No? Gonna blow up on me at some point ?
>-- 
> Dave Hammond

the people at ANSI land will tell you can't go converting between different
types of pointers and expecting ``reasonable'' results.  so, yes, it will
blow up on you at some point.

i suggest you find a copy of the `cdecl' program.  it can come in very
handy for writing casts and function and variable declarations.  it also
comes in handy for writing USENET articles ;-)

- john.
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