Essential C [pdf]

Re:Essential C [pdf] by bdowling on Sunday March 06, 2022 @11:09PM (#30584141)

Here's an outline of a toy version:

    #include <stdlib.h> /* for size_t */
    static char data_storage[10000000];
    static char *nextp = data_storage;
    
    void *malloc(size_t size)
    {
        void *p = nextp;
        nextp += size;  
        return p;
    }
    void free(void *p){}

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Re:Essential C [pdf] by astrange on Monday March 07, 2022 @12:53AM (#30584604)

This does not "yield a pointer to an object disjoint from any other object". So it only works if the implementation is not visible to callers and they can pretend it's compliant.

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Re:Essential C [pdf] by bdowling on Monday March 07, 2022 @02:07AM (#30584984)

Huh? My version never returns a pointer to space that was already allocated because it never deallocates anything. So the objects are ALWAYS disjoint from every other object. Or are you concerned that they are aliased with the static 'data_storage' char array?

More context around your quote:

> The lifetime of an allocated object extends from the allocation until the deallocation. Each such allocation shall yield a pointer to an object disjoint from any other object.

https://port70.net/%7Ensz/c/c11/n1570.html#7.22.3

In context, I take that to mean that all allocated objects must be disjoint from one another throughout their lifetime. It wouldn't make sense otherwise.

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Re:Essential C [pdf] by astrange on Monday March 07, 2022 @02:33AM (#30585126)

> So the objects are ALWAYS disjoint from every other object. Or are you concerned that they are aliased with the static 'data_storage' char array?

Yes, plus if you call it twice it returns two pointers to the same object, because there's no way in C to create another "object". Of course, as long as the callers don't know this it's fine, but it would be a problem if eg you compile your custom malloc with ASAN/Valgrind and don't tell it that it's a malloc.

I think C++ partially addresses this with "placement new" but not sure how far.

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Re:Essential C [pdf] by bdowling on Monday March 07, 2022 @03:25AM (#30585415)

> if you call it twice it returns two pointers to the same object

How are you defining an object? This draft version of the C11 spec defines 'object' as a "region of data storage in the execution environment, the contents of which can represent values". https://port70.net/%7Ensz/c/c11/n1570.html#3.15

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Re:Essential C [pdf] by astrange on Monday March 07, 2022 @08:52PM (#30595611)

The drafting is not the best. The key point is that objects have bounds (even though it doesn't explicitly say it), and pointers point to a base object, and it's UB if those pointers go outside the bounds of that object.

malloc is defined to return a pointer to a new "base object". But your code doesn't do that; you can see by reading it that it returns a pointer to `data_storage`. That means the UB conditions for using that pointer don't match the spec.

You could say it's supposed to magically work if the function is named `malloc`, but my understanding of all C implementations is they don't do that.

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