Testing pointers for validity (C/C++)

Question

Is there any way to determine (programatically, of course) if a given pointer is "valid"? Checking for NULL is easy, but what about things like 0x00001234? When trying to dereference this kind of pointer an exception/crash occurs.

A cross-platform method is preferred, but platform-specific (for Windows and Linux) is also ok.

Update for clarification: The problem is not with stale/freed/uninitialized pointers; instead, I'm implementing an API that takes pointers from the caller (like a pointer to a string, a file handle, etc.). The caller can send (in purpose or by mistake) an invalid value as the pointer. How do I prevent a crash?

Answer 1

There are no provisions in C++ to test for the validity of a pointer as a general case. One can obviously assume that NULL (0x00000000) is bad, and various compilers and libraries like to use "special values" here and there to make debugging easier (For example, if I ever see a pointer show up as 0xCECECECE in visual studio I know I did something wrong) but the truth is that since a pointer is just an index into memory it's near impossible to tell just by looking at the pointer if it's the "right" index.

There are various tricks that you can do with dynamic_cast and RTTI such to ensure that the object pointed to is of the type that you want, but they all require that you are pointing to something valid in the first place.

If you want to ensure that you program can detect "invalid" pointers then my advice is this: Set every pointer you declare either to NULL or a valid address immediately upon creation and set it to NULL immediately after freeing the memory that it points to. If you are diligent about this practice, then checking for NULL is all you ever need.

Answer 2

There is no way to make that check in C++. What should you do if other code passes you an invalid pointer? You should crash. Why? Check out this link: http://blogs.msdn.com/oldnewthing/archive/2006/09/27/773741.aspx

Answer 3

IsBadReadPtr(), IsBadWritePtr(), IsBadCodePtr(), IsBadStringPtr() for Windows.
These take time proportional to the length of the block, so for sanity check I just check the starting address.

Answer 4

you should avoid these methods because they do not work. blogs.msdn.com/oldnewthing/archive/2006/09/27/773741.aspx – JaredPar Feb 15 '09 at 16:02

If they don't work - next windows update will fix it ? If they don't work on concept level - function will be probably removed from windows api completely.

MSDN documentation claim that they are banned, and reason for this is probably flaw of further design of application (e.g. generally you should not eat invalid pointers silently - if you're in charge of design of whole application of course), and performance/time of pointer checking.

But you should not claim that they does not work because of some blog. In my test application I've verified that they do work.