NULL Checking and Defensive Programming

Nemanja Trifunovic

PIEBALDconsult wrote:

But in that example you didn't set the pointer to NULL and you know it.

Of course I did. Just after deleting it. I didn't set other pointers that point to the same object to NULL because it is impossible to do, and that was the point of my sample.

PIEBALDconsult wrote:

A called method should not free something that was passed in, or if it's expected to, you'll need double indirection. Find a better example, that one's a coding horror on its own.

Of course it is a horror - and you can't protect from such horrors by checking if a pointer is NULL. That's all I am trying to point here.

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Nemanja Trifunovic

Dave Kreskowiak wrote:

what's the default for a newly created, but uninitialized pointer??

Whatever it happens to be in that memory location at the time a pointer is defined :)

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PIEBALDconsult

I didn't see it in the spec and I don't have an up-to-date C++ compiler handy. But I expect that if it isn't NULL already it soon will be.

Nemanja Trifunovic

PIEBALDconsult wrote:

But I expect that if it isn't NULL already it soon will be.

No it won't. The new standard is ready to be adopted and there is nothing about it that would mandate such behavior. None of the compilers I used recently (MS and GNU) automatically initialize local variables.

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Nemanja Trifunovic

Randor wrote:

Come on man I know your not really that pedantic about verbiage. Call it whatever you want, "Access Violation" is simply an error title. Now your just trying to be argumentative. I'll call it segmentation fault[^] from now on just to get under your skin.

It is more than just verbiage. By following your rules (which btw really make sense in C90 where you have to declare all variables in the beginning of the function) you will indeed never hit a "null pointer segfault" but segfaults happen on any invalid pointer, not just NULL.

Randor wrote:

There are many techniques to making software more robust. Assigning pointers to NULL and checking for NULL is only one of them

At least now we are not claiming that by following these two simple rules we will never segfault :)

Randor wrote:

Initializing a pointer to NULL to denote an invalid memory address is the same magic number technique. It assists the programmer with validating pointers and most certainly assists with making software more robust.

And guess what? I agree. If you initialize a pointer to NULL, and then go to different code paths in which it may or may not be set to point to a valid object, checking it for NULL makes a perfect sense. However, this thread is about a guy who inserts NULL checks all around the code in the hope it would make it more robust. Well, it won't. If there is a bug in the code, chances are it will not be caught by a NULL check.

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Nemanja Trifunovic

PIEBALDconsult wrote:

C99 and C# initialize pointers (references) to NULL

Could you give a link to a reference for C99? I admit I have never heard of this.

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supercat9

Pointers should be explicitly checked for null if there is a realistic scenario by which they could be null. For example, ptr=malloc(1024); will set ptr to null if the system cann't allocate 1024 bytes for it. If the program isn't allocating much memory, such a scenario may be unlikely but not unrealistic. On the other hand, in something like:

{
int arr[5];
int *p;
int i;

p=arr;
for (i=0; i<5; i++)
*p++ = i;
}

there is no realistic way that p is ever going to be null. It simply can't happen.

PIEBALDconsult

I think it's more a question of, if you write a function (perhaps a library function) that takes one or more pointers, do you check them for null or let them blow up? And why?

mbcvamsidhar

Use Assertions as it will be useful to chack for NULL pointers.Debug.Assertin C#

Thanks and Rgds, VamsiDhar.MBC SoftwareEngineer.

supercat9

PIEBALDconsult wrote:

I think it's more a question of, if you write a function (perhaps a library function) that takes one or more pointers, do you check them for null or let them blow up? And why?

IMHO, the biggest questions would be:

Is the operation in the null-pointer situation defined by the interface standard?
Would the null-pointer situation have a logical meaning (e.g. it may be useful for a function that reads data from a stream to have an option to simply throw away some data; allowing the function to take a null pointer for such usage may be more elegant than requiring the use of a separate function)?
Are there any circumstances that could case a null pointer to be passed in accidentally?
How would the probable consequence of passing in a null pointer compare with the best result one could achieve?

Incidentally, I found myself annoyed at the design of some TCP libraries which returned the same sort of failure code when a non-blocking write attempt was done on a port whose buffer was full, as when such an attempt was performed on a port that was closed. The full-buffer case needs to be easily distinguishable from the closed-port case, since one will want to wait in the former case but not the latter. In my own libraries, I allow a write to a closed port to immediately return 'success', but then check whether the port is actually open. If the port closed unexpectedly, the data I'm sending will vanish into the aether, but the program won't crash. I may not know how much data vanished in the aether, but oftentimes (1) it won't matter, and (2) it may be impossible to know for certain if some packets gets sent but never acked. A closed port isn't quite the same thing as a null pointer, but I think some of the philosophical arguments are similar.

Yuriy Levytskyy

You are using pointers???

Wenff

WTF!!!???? :omg: I think I might be close to speechless. Has the man never had a blue screen of death and gotten annoyed? Never thought I'd hear of some genius campaigning for application failure...