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How to find size of an array in C without sizeof

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Re: How to find size of an array in C without sizeof

#41

Post by quotemstr »

flamedoge wrote: post #29
ycmbntrthrwaway wrote: post #5

The result you get with this trick is signed, while the result you get with sizeof is unsigned. Edit: Just to clarify, what you get is ptrdiff_t instead of size_t. So if array size is greater than PTRDIFF_MAX, you get undefined behavior [1]. [1] http://en.cppreference.com/w/c/types/ptrdiff_t

How likely do you run into array bigger than 2gb?

The "how likely is it, really?" response to questions of technical correctness has always bothered me. It takes a mindset completely alien to mine to say "Here's a race condition. Sure, it's undefined behavior, but the race is narrow, so it's rare" or to say "Sure, memory allocation can theoretically fail, but in practice almost never does" or to say "fsync is too slow and most computers have batteries these days".

Software is unreliable enough as it is due to problems beneath our notice. It seems reckless to avoid fixing problems that we do notice. Sure, you could argue that rare problems are rare and that users probably won't notice them --- this attitude is penny-wise and pound-foolish, because you can't meaningfully reason about a system that's only probably correct.

Re: How to find size of an array in C without sizeof

#42

Post by Tempest1981 »

babuskov wrote: post #26
Stratoscope wrote: post #21

Whether you use this method of getting the number of elements in an array or the more traditional sizeof method, please encapsulate the logic in a macro. Instead of writing either of these: size_t length = sizeof array / sizeof array[0]; size_t length = (&array)[1] - array; Define this macro instead: #define countof( array ) ( sizeof(array) / sizeof((array)[0]) ) Or if you must: #define countof( array ) ( (&(array))[…

> please encapsulate the logic in a macro. Why? When reading such code, it means I would have to go and lookup a macro definition. So, there's a clear drawback. What's the benefit that makes it worthwhile?

Faster to read, and keeps the reader's mind at a semantically higher level.

Re: How to find size of an array in C without sizeof

#43

Post by mjburgess »

quotemstr wrote: post #41
flamedoge wrote: post #29

Earlier quoted context omitted.

How likely do you run into array bigger than 2gb?

The "how likely is it, really?" response to questions of technical correctness has always bothered me. It takes a mindset completely alien to mine to say "Here's a race condition. Sure, it's undefined behavior, but the race is narrow, so it's rare" or to say "Sure, memory allocation can theoretically fail, but in practice almost never does" or to say "fsync is too slow and most computers have batteries these days". S…

The problem you're latching on to I think is how the context for caculating a probability can vary.

If it were really as likely as, say, the sun exploding that X happened then it would be of no use to expend time on X.

BUT very often people speaking about the probability of events given suspicious constraints. While a memory allocation might not fail in most situations it will fail often in some situations. And a one-in-a-million chance is almost guaranteed when there are millions of uses.

Re: How to find size of an array in C without sizeof

#45

Post by vram22 »

flamedoge wrote: post #29
ycmbntrthrwaway wrote: post #5

The result you get with this trick is signed, while the result you get with sizeof is unsigned. Edit: Just to clarify, what you get is ptrdiff_t instead of size_t. So if array size is greater than PTRDIFF_MAX, you get undefined behavior [1]. [1] http://en.cppreference.com/w/c/types/ptrdiff_t

How likely do you run into array bigger than 2gb?

Not likely, but possible. This reminds me of the bug that was found in the binary search algorithm a few years ago, IIRC, in Java. The interesting thing is that binary search is probably one of the earliest-invented algorithms. Yet, in the book Writing Efficient Programs by Jon Bentley (which I mentioned in a recent HN comment), he says that in a class he taught to several industrial programmers with many years of experience, some had bugs in their implementations of binary search that he set them as an exercise. Not sure but I think I remember reading in the article about the Java binary search issue, that even his algorithm had the bug that was found in the Java version. Why it was not found earlier is (maybe) because it only occurred with an extremely large array, IIRC. Don't have a link right now, but it can probably be found by searching for the right phrase.

Re: How to find size of an array in C without sizeof

#46

Post by cperciva »

halayli wrote: post #31
cperciva wrote: post #25

Earlier quoted context omitted.

this is undefined behavior. &arr + 1 can overflow No. From 6.5.6 Additive operators: 7 For the purposes of these operators, a pointer to an object that is not an element of an array behaves the same as a pointer to the first element of an array of length one with the type of the object as its element type. 8 [...] if the expression P points to the last element of an array object, the expression (P)+1 points one past…

But arr != &arr even though they have the same value. #8 applies to arr (P), but in the post OP is using &arr which is a ptr to array[x] and doesn't apply to it.

That's why I quoted paragraph 7: arr is not an element of an array, so &arr (being a pointer to an object which is not an element of an array) behaves like a pointer to the first element of an array of length one with the type of the arr as its element type.

So &arr behaves like it's a pointer to the start of int[5][1].

Re: How to find size of an array in C without sizeof

#48

Post by cperciva »

flamedoge wrote: post #29
ycmbntrthrwaway wrote: post #5

The result you get with this trick is signed, while the result you get with sizeof is unsigned. Edit: Just to clarify, what you get is ptrdiff_t instead of size_t. So if array size is greater than PTRDIFF_MAX, you get undefined behavior [1]. [1] http://en.cppreference.com/w/c/types/ptrdiff_t

How likely do you run into array bigger than 2gb?

A few months ago I was doing FFTs on arrays larger than 4GB. Amusingly, this uncovered a bug in the LLVM optimizer: It was looking at stride lengths to figure out if accesses were independent, and truncated a 4GB stride down to 0.

Re: How to find size of an array in C without sizeof

#49

Post by icedchai »

flamedoge wrote: post #29
ycmbntrthrwaway wrote: post #5

The result you get with this trick is signed, while the result you get with sizeof is unsigned. Edit: Just to clarify, what you get is ptrdiff_t instead of size_t. So if array size is greater than PTRDIFF_MAX, you get undefined behavior [1]. [1] http://en.cppreference.com/w/c/types/ptrdiff_t

How likely do you run into array bigger than 2gb?

On a 64-bit platform (anything modern), ptrdiff_t is going to be 64-bit so this will not be an issue (ok, 63-bit... but you get my point.)

Re: How to find size of an array in C without sizeof

#50

Post by vram22 »

vram22 wrote: post #45
flamedoge wrote: post #29

Earlier quoted context omitted.

How likely do you run into array bigger than 2gb?

Not likely, but possible. This reminds me of the bug that was found in the binary search algorithm a few years ago, IIRC, in Java. The interesting thing is that binary search is probably one of the earliest-invented algorithms. Yet, in the book Writing Efficient Programs by Jon Bentley (which I mentioned in a recent HN comment), he says that in a class he taught to several industrial programmers with many years of ex…

Just did a google search, and it even partially auto-completed this search for me:

bug in java binary search

and showed a related search in the drop-down, 'programming pearls ...', a book by Jon Bentley, which seems to confirm what I said above (though I saw it in his other book, "Efficient Programs", IIRC - he might have mentioned the same issue in the Programming Pearls book too).

Edit: and the Wikipedia article confirms it too:

https://en.wikipedia.org/wiki/Binary_search_algorithm#Implem...

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