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by mjevans 849 days ago
I've forgotten if 'address of' ( & ) is signed or unsigned. Worse, in a quick search online I can't seem to find anyplace that mentions what the return type of address of is, just a bunch of basic working with pointers pages.

The pointer math line should still be legal, even if it would be 'unsafe' in some popular other languages. Useful for determining where to write to memory mapped files or IO. Not so useful in the above case.

The warnings/errors I'd expect would be: *p = 7; // out of known bounds write
1 comments
nlewycky 848 days ago
> I've forgotten if 'address of' ( & ) is signed or unsigned.

Neither, pointers are their own types which have no sign, only integral types come in signed and unsigned. There exist intptr_t (signed) and uintptr_t (unsigned) which are integral types you can losslessly cast a pointer to. Also the difference of two pointers is a ptrdiff_t which is a signed integral type.

> The warnings/errors I'd expect would be: *p = 7; // out of known bounds write

In that regard I picked a bad example, this code always executes undefined behaviour, so this could indeed be detected by a compiler warning at the cost of doing a simulated execution of the code. Most real problems come about where the code is UB only for certain runtime inputs.

The reason I chose that example was to make a point about how the compiler doesn't realize it's doing anything to "change" the program, and isn't going out of its way to optimize based on undefined behavior. It assumes that a local variable's value can't change with it being assigned to, so how could it know whether to issue the warning?

Another way to put it is:

    void written_in_asm_in_another_file();
    void test() {
        int x = 4;
        written_in_asm_in_another_file();
        printf("%d", x);
    }
The function written in asm might go up the stack to find caller's stack frame and search for the 0x00000004 and change it to a different value. Is the compiler forbidden to replace printf("%d", x); with printf("%d", 4);? Is the compiler allowed to, but required to emit a warning? Are we required to have a 4 on the stack as opposed to keeping it only in registers?

> The pointer math line should still be legal, even if it would be 'unsafe' in some popular other languages.

I'm not sure what you mean by "should", you might be suggesting a change to C or you might be stating what you think the code currently does. Right now in C, the very creation of an invalid pointer is UB whether you use the thing or not. I'm told this is because of very old CPU designs that had distinct pointer and integer registers and loading an invalid address into the pointer register would trap.

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