Re: warnings for invalid function casts

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2020-06-30 10:15:05 -0400, Tom Lane wrote:
>> I feel like what you propose to do here is just shifting the problem
>> around: we're still casting from a function pointer that describes one
>> concrete call ABI to a function pointer that describes some other concrete
>> call ABI. That is, "void (*ptr) (void)" is *not* disclaiming knowledge
>> of the function's signature, in the way that "void *ptr" disclaims
>> knowledge of what a data pointer points to. So if current gcc fails to
>> warn about that, that's just a random and indeed obviously wrong decision
>> that they might change someday.

> ISTM that it's unlikely that they'd warn about casting from one
> signature to another?

Uh, what? Isn't that *exactly* what this warning class does?
If it doesn't do that, what good is it? I mean, I can definitely
see the point of warning when you cast a function pointer to some
other not-ABI-compatible function pointer type, because that might
be a mistake, just like assigning "int *" to "double *" might be.

gcc 8's manual says

'-Wcast-function-type'
Warn when a function pointer is cast to an incompatible function
pointer. In a cast involving function types with a variable
argument list only the types of initial arguments that are provided
are considered. Any parameter of pointer-type matches any other
pointer-type. Any benign differences in integral types are
ignored, like 'int' vs. 'long' on ILP32 targets. Likewise type
qualifiers are ignored. The function type 'void (*) (void)' is
special and matches everything, which can be used to suppress this
warning. In a cast involving pointer to member types this warning
warns whenever the type cast is changing the pointer to member
type. This warning is enabled by '-Wextra'.

so it seems like they've already mostly crippled the type-safety of the
warning with the provision about "all pointer types are interchangeable"
:-(. But they certainly are warning about *some* cases of casting one
signature to another.

In any case, I think the issue here is what is the escape hatch for saying
that "I know this cast is okay, don't warn about it, thanks". Treating
"void (*) (void)" as special for that purpose is nothing more nor less
than a kluge, so another compiler might do it differently. Given the
POSIX restriction, I think we could reasonably use "void *" instead.

regards, tom lane