Andres Freund <and...@anarazel.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