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0 pointsGlitchMr8y ago0 comments

Not really that dangerous, I think it's more of an issue of having code written before strict aliasing rule.

Essentially, what strict aliasing rule requires is that user cannot go crazy casting pointers. Casting float pointer to int pointer is completely wrong. The exception is that it's completely safe to cast values to char pointer or void pointer (and back, but only to original type of a pointer). Objects of different types cannot use the same memory area.

What this usually affects is code that parses external data into a structure. This can be dealt with by use of `memcpy` (to copy data from char array into a structure) instead of pointer casts, which is mostly safe as far C specification is concerned (mostly because C specification doesn't really define stuff like paddings, so you need to be careful about that). There is also an option of using `union` which is allowed by C11 specification (but compilers released with C11 that had strict aliasing optimization didn't break code that had aliasing through union and even documented that).

0 comments

milesrout8y ago

Not really that dangerous, except for all the code for which it is dangerous?

Of course, you've got me, it only breaks existing code, and doesn't affect code that's written perfectly according to the rules of C and not peoples' intuitions about what pointers actually mean on a hardware level.

It's not even safe to do this:

    struct a {
        int x;
    };
    struct b {
        int x;
        int y;
    };

    struct b b;
    struct a *a = &b;

You have to write b like this:

    struct b {
        struct a inner;
        int y;
    };

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