sys: Remove redundant type casting to sysinit_cfunc_t

[zlei]

Summary

The definition of macro SYSINIT has already done the type cast.

No functional change intended.

MFC after: 1 week

[kib]

BTW, why does SYSINIT() enforces the cast? Isn't it more correct to not cast in the macro, and allow a compiler to check that the function passed matches the type?

[zlei]

In D47940#1093320, @kib wrote:

BTW, why does SYSINIT() enforces the cast? Isn't it more correct to not cast in the macro, and allow a compiler to check that the function passed matches the type?

typedef void (*sysinit_cfunc_t)(const void *);

struct sysinit {
        enum sysinit_sub_id     subsystem;      /* subsystem identifier */
        enum sysinit_elem_order order;          /* init order within subsystem */
        STAILQ_ENTRY(sysinit)   next;           /* singly-linked list */
        sysinit_cfunc_t func;                   /* function */
        const void      *udata;                 /* multiplexer/argument */
};

#define C_SYSUNINIT(uniquifier, subsystem, order, func, ident)  \
        static struct sysinit uniquifier ## _sys_uninit = {     \
                subsystem,                                      \
                order,                                          \
                { NULL },                                       \
                func,                                           \
                (ident)                                         \
        }; 
..

#define SYSINIT(uniquifier, subsystem, order, func, ident)      \
        C_SYSINIT(uniquifier, subsystem, order,                 \
        (sysinit_cfunc_t)(sysinit_nfunc_t)func, (void *)(ident))

If no cast in the macro, when assign a function pointer, e.g. void func(char *) to sysinit member func, the compiler would generate warning ( clang 's -Wincompatible-function-pointer-types or gcc's -Wincompatible-pointer-types ).

The last commit that defines C_SYSINIT and SYSINIT is 12cb7f73a61e, which is committed at 1999 . I believe those days the compilers are not smart enough and do not have mechanisms such as C11's _Generic. An explicit cast eliminate the warning and should relief developers to do cast everywhere.

I'm planing to employ _Generic to enforce valid function pointers. With that then this explicit casting should also be avoided.

[kib]

Let me explain my PoV.

There should be no cast in the SYSINIT() macros at all. Consumer must pass void (*)(void *) function pointer and void *-compatible arg to the macro. The initialization function shall do the required casts.

How many const void * cases do we have in the tree?

[zlei]

In D47940#1094028, @kib wrote:

Let me explain my PoV.

There should be no cast in the SYSINIT() macros at all. Consumer must pass void (*)(void *) function pointer and void *-compatible arg to the macro. The initialization function shall do the required casts.

Well, I'm experimenting the opposite, a more safe way, to avoid void (*)(void *) function pointer and void * arg. So initialization function does not have to cast.

How many const void * cases do we have in the tree?

Indeed only small amount, all are in sys/kern/init_main.c, introduce by my previous work on constify some runtime readonly global variables. I have plans to constify more, but I'd like to improve SYSINIT / C_SYSINIT firstly so I'll get convinced I'm doing the right. TUNABLE_INT is a good start, struct tunable_int s are runtime readonly.

# grep C_SYSINIT sys/kern/init_main.c | wc -l
9