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    SubjectRe: [PATCH 56/62] watchdog: tangox_wdt: Convert to use device managed functions
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    On 11/01/2017 18:51, Guenter Roeck wrote:

    > However, some other unrelated undefined behavior does not mean that this
    > specific behavior is undefined.

    True :-)

    Let me just give two additional examples of UB that /have/ bitten
    Linux kernel devs.

    int i;
    for (i = 1; i > 0; ++i)
    /* do_something(); */

    => optimized into an infinite loop

    and

    void func(struct foo *p) {
    int n = p->field;
    if (!p) return;

    => null-pointer check optimized away

    > So far we have a claim that a cast to a void * may somehow be different
    > to a cast to a different pointer, if used as function argument, and that
    > the behavior with such a cast may be undefined. In other words, you claim
    > that a function implemented as, say,
    >
    > void func(int *var) {}
    >
    > might result in undefined behavior if some header file declares it as
    >
    > void func(void *);
    >
    > and it is called as
    >
    > int var;
    >
    > func(&var);
    >
    > That seems really far fetched to me.

    Thanks for giving me an opportunity to play the language lawyer :-)

    C99 6.3.2.3 sub-clause 8 states:

    "A pointer to a function of one type may be converted to a pointer to a function of another
    type and back again; the result shall compare equal to the original pointer. If a converted
    pointer is used to call a function whose type is not compatible with the pointed-to type,
    the behavior is undefined."

    So, the behavior is undefined, not when you cast clk_disable_unprepare,
    but when clk_disable_unprepare is later called through the devres->action
    function pointer.

    However, I agree that it will work as expected on typical platforms
    (where all pointers are the same size, and the calling convention
    treats all pointers the same).

    > I do get the message that you do not like this kind of cast. But that doesn't
    > mean it is not correct.

    If it's already widely used in the kernel, it seems there is no point
    fighting it ;-)

    Regards.

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