Function pointers and NULL

[Richard Heathfield]

Is it legal to assign NULL to a function pointer?

I believe it is, because of {3.3(C89)|6.5(C99)}.16.1, Constraint #5:

* the left operand is a pointer and the right is a null pointer
constant.

(it doesn't say "object pointer", just "pointer"), but nevertheless I am
mildly nervous.

If necessary, I could do this:

void null(void);

#define FUNCTION_NULL null

but I'd rather avoid that if I can, because I'd have to add a cast to
just about every usage thereof.

 

[Harald van =?UTF-8?B?RMSzaw==?=]

Is it legal to assign NULL to a function pointer?

Yes, that is legal for the reason you have stated. Regardless of whether
NULL is of an integer type or of a void pointer type, it can be converted
implicitly to any function pointer type.

 

[Richard Heathfield]

Harald van D?k said:

Yes, that is legal for the reason you have stated. Regardless of
whether NULL is of an integer type or of a void pointer type, it can
be converted implicitly to any function pointer type.

Thanks. That's what I figured, too, but it's good to have it confirmed
by a bright bunny.

 

[Malcolm McLean]

Is it legal to assign NULL to a function pointer?

I believe it is, because of {3.3(C89)|6.5(C99)}.16.1, Constraint #5:

* the left operand is a pointer and the right is a null pointer
constant.

(it doesn't say "object pointer", just "pointer"), but nevertheless I am
mildly nervous.

If necessary, I could do this:

void null(void);

#define FUNCTION_NULL null

but I'd rather avoid that if I can, because I'd have to add a cast to
just about every usage thereof.

It's a glitch.
NULL is allowed to be defined as (void *) 0, which the compiler is then
allowed to reject as a function pointer. However fptr = NULL has to be
accepted. So a compiler needs a little patch if it enforces strict type
checking.

 

[Harald van =?UTF-8?B?RMSzaw==?=]

It's a glitch.
NULL is allowed to be defined as (void *) 0, which the compiler is then
allowed to reject as a function pointer.

(void *) 0 is a null pointer constant, and any null pointer constant can be
converted to any pointer type, including function pointer types.

 

[Malcolm McLean]

(void *) 0 is a null pointer constant, and any null pointer constant can
be
converted to any pointer type, including function pointer types.

That's the patch, in words rather than in code.