Memory Management – Value of Byte Pointed by Null Pointer

memorynullpointers

I know that long time ago computer scientists decided to treat all pointers to memory cell of address 0 as NULL. However, the memory cell at that address does exists after all, right? In that case, what value is stored there? Since it's OS that decides how to allocate memory it should be able to access (and modify) value of all cells in the RAM, including the one of index 0, right?

Yes, I have already seen this question. The thing is that I don't really ask where the NULL pointer actually points to. It is more of extension of former question as I want to know what the byte value of that memory cell is or what it depends on.

Best Answer

The physical 0 address corresponds indeed in general to a memory location in which some value is stored. The value is however system and environment dependent:

if the hardware maps this address to some ROM chip, the value is obviously fixed.
if it's RAM, at power-on the content is unpredictable, and you should consider it random. It's however not really random : it could contain some previously stored value. This property is used in cold boot attacks.
The BIOS/UEFI/OS could would clear it to 0 at startup. Some process could store their variable there.

But nowadays, most CPU and OSes use virtual memory. In such a scheme, every process only sees a virtual address space. This space is organized in "pages" (blocks of contiguous memory) that are mapped to physical address, not mapped, and/or swapped to some disk space by a page handler.

So the address N of one process will not be the address N of another process. And in particular, the virtual address 0 might not be teh physical address 0. It could well remain unmapped, causing a hardware exception (page fault) for every access attempt (more explanations here).

Related Solutions

SQL: Empty String vs NULL Value – Key Differences

I would say that NULL is the correct choice for "no email address". There are many "invalid" email addresses, and "" (empty string) is just one. For example "foo" is not a valid email address, "a@b@c" is not valid and so on. So just because "" is not a valid email address is no reason to use it as the "no email address" value.

I think you're right in saying that "" is not the correct way to say "I don't have a value for this column". "" is a value.

An example of where "" might be a valid value, separate to NULL could be a person's middle name. Not every one has a middle name, so you need to differentiate between "no middle name" ("" - empty string) and "I don't know if this person has a middle name or not" (NULL). There's probably many other examples where an empty string is still a valid value for a column.

Best Way to Have a Pointer to Several Methods in C++ – Code Critique

what is the best syntax and coding style for generating a pointer to a method?

Don't. Use std::function<void()>.

Now, on to the steps. Step 1 is technically illegal, but your compiler allowed it by implicitly taking the address for you. Step 2 is how it should be done. The name of a static function (in this case, Foo::dummy) is a function lvalue and must have it's address taken to be legal C++ and yield a function pointer.

Step 3 simply doesn't make any sense at all. You're trying to assign ... a run-time function pointer ... to a type? That makes no sense whatsoever. And then you try to access ... a member ... of a function pointer? Does not compute.

There are two parts to the answer "What do I do when I want to X in C++?". The first part, for C veterans only, is "Forget whatever you think you learned in C". The second part is about doing X.

how can i write an array of pointers or a pointer to pointers structure to store methods ( static or non-static from real objects )

std::vector<std::function<void()>> arr;

le done.

Best Answer

Related Solutions

SQL: Empty String vs NULL Value – Key Differences

Best Way to Have a Pointer to Several Methods in C++ – Code Critique

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