Jakob Ƙstergaard Hegelund

Tech stuff of all kinds
Archive for October 2016

Affinity to signedness


It continues to amaze me when I look at other peoples code, how many good developers think in signed integers. This is a short example of such code and a walk-through of why it is mis-guided.

Signedness in the world of computers

Almost nothing in the world of computers is signed; you cannot send a negative number of bytes, you cannot have a negative amount of memory, you cannot put a negative number of elements in a vector and so on and so forth. We will frequently subtract numbers, but we will usually never have negative results.

For example: I may want to send a buffer using a send routine that does not necessarily send all data at once - subtacting the sent number of bytes from the full number of bytes can never be negative (in other words, the routine can never send more than it was given).

Good code like the STL realizes this and typically uses the size_t type for sizes in general (basically it is used for everything you can count that you can keep in memory). It is of course unsigned (for the aforementioned reasons) and it is as large as it needs to be, for any count of "things you can have in memory".

The problem then arises when a signed-integer-loving developer mixes his int-using code with the better size_t-using code. Good compilers will complain about pitfalls in signed/unsigned arithmetic and the developer is forced to respond with an equal measure of type conversions or casts.

The result will usually be unnecessarily verbose code that is less capable of handling large amounts of data (since the sign bit can no longer be used for actually counting). Granted, the last bit is more a problem on 32-bit systems than it is on 64-bit systems, at least at the time of this writing - but you never know then someone decides to use your code on a 32-bit system. If your code is any good (and often even if it isn't), it will end up on all sorts of systems without your knowledge anyway...

Let's take a look at an example I came across today: This is an extract of a piece of production code that actually works, it's just unnecessarily ugly because the developer had an unhealthy affinity towards signed integers:

int sent_bytes = 0; while (sent_bytes != static_cast<int>(data.size())) { int res = SSL_write(m_ssl, data.data() + sent_bytes, int(data.size()) - sent_bytes); int err = SSL_get_error(m_ssl, res); switch (err) { case SSL_ERROR_NONE: sent_bytes += res; notifyObservers(event_t::conn_tx, res); break;

So we have a variable sent_bytes with a nice readable descriptive name, so far so good. It escapes me how that can be a signed integer - how exactly do we send a negative number of bytes? Right, that is difficult indeed. By making it int rather than size_t, we have either 63 bits instead of 64 bits for the counter (which is probably fine) or 31 bits instead of 32 (which may be a problem if we use large buffers). But regardless of whether the more limited positive range is a problem or not, we don't get anything in return. Nothing is gained by opening up for negative numbers; there is no plausible scenario in which we could ever have sent a negative number of bytes. So we get nothing for something and that's never a good trade. Now I would buy the argument for making this a signed integer if it somehow made the rest of the code more readable - let's proceed with the analysis then.

So in our loop we compare the number of sent bytes to the actual number of bytes we wish to send. Notice how a static_cast<int> is used to stop the compiler from complaining that it is dangerous to compare the two types (as they don't have the same range a conversion is made - and that is not necessarily what you want). Now consider for a moment what this cast accomplishes... It casts the value so that the compiler stops complaining - it does not actually solve the problem the compiler is complaining about. This is like shutting off the fire alarm instead of putting out the fire - it stops the noise but it doesn't really solve the problem. The problem of course would (as of this writing) most likely only occur on 32 bit systems where we would have a 2.5G buffer and after sending the first 2G our sent_bytes would wrap and depending on the rest of the logic we may have more or less luck with our algorithm when sent_bytes is -1.5G (or some other number - depending on the good will of the compiler; read on)

And this is actually what makes it even more difficult for me to understand: Unsigned arithmetic in C++ is valid and well defined - it is integer arithmetic modulo 2^n where n is the bit-size of your integer. In other words, when you add one to your maximum integer value, you're back to zero. There's nothing magical about that, but it is extremely useful. In contrast, the standard does not define behaviour of signed overflow - if you rely on signed integers and they overflow, the behaviour is undefined by the standard. Why would anyone tend towards a type that has less standardized behaviour if it does not provide a benefit? The mind boggles.

We move on to the SSL_write line. Here we must subtract the number of sent bytes from the total size of our buffer so as to compensate for adding the number of sent bytes to the start offset of our data buffer. The developer has chosen an alternative means of getting around the compiler warning this time: Construct a temporary signed integer from the unsigned size of the buffer, then subtract the signed sent_bytes from there. Now, if we were to attempt to send 2.5G of data on a 32-bit platform this code would explode, but the compiler does not tell us about this because we skillfully mislead it by inserting valid constructs that hide our real crimes from the compiler.

Ultimately, we add our signed integer res to our number of sent bytes. Naturally, we expect that res cannot be negative when we use it in this manner - no assertion or otherwise is inserted for this, but that is something one could have done. Or we can trust the library to honour its contract and never return success and set res to a negative value. This concludes the use of sent_bytes, and in no situation do we gain anything from having it be a signed value. How about we consider a fixed version of the code:

size_t sent_bytes = 0; while (sent_bytes != data.size()) { int res = SSL_write(m_ssl, data.data() + sent_bytes, data.size() - sent_bytes); int err = SSL_get_error(m_ssl, res); switch (err) { case SSL_ERROR_NONE: sent_bytes += res; notifyObservers(event_t::conn_tx, res); break;

Note the distinct lack of casts and explicit construction of temporaries to work around type mismatches. Other than that, the only functional difference between the two versions are, that this shorter and more readable version will successfully send 2.5G of data on a 32-bit platform, rather than blowing up unpredictably.

I fear that the affinity for signed integers comes from the old "magic return value" where we use non-negative values for the real return value and assign special meaning to negative magic numbers that callers must remember to check for (like SSL_write in the example earlier). That, of course, is the worst excuse; especially when working in C++ that actually has means to deal with exceptional circumstances where less fortunate C developers might feel compelled to return negative magic numbers.

Old habits die hard I guess. But it troubles me that I see this from young developers - they did not code C back in the '90s (in fact they were born in the '90s). I don't have all the answers - this one is definitely a mystery to me.

Useful return from short-circuit or


This could have been a bootnote on the next post but time flew and now it will have to be a post on its own. Consider two solutions to the same problem:

// C++ // a(), b() and c() return std::optional std::string pick() { return a().value_or(b().value_or(c().value_or("-default-"))); } ;; LISP ;; (a), (b) and (c) return nil or string (defun pick () (or (a) (b) (c) "-default-"))

My point earlier was that having or return not a boolean, but the actual first non-false argument led to some simple beautiful constructs. I stand by that, but there's another important difference between the two solutions. Consider first the actual sequence of evaluations in the C++ example:

// First, as evaluate the arguments to the value_or functions, we // must call all three functions tmp0 = a(); tmp1 = b(); tmp2 = c(); tmp3 = std::string("-default-"); // Now, with the arguments evaluated, we can "fold" back to get // the result tmp4 = tmp2.value_or(tmp3); tmp5 = tmp1.value_or(tmp4); tmp6 = tmp0.value_or(tmp5); // Our result is in tmp6

Since value_or is a function, it's argument must be evaluated before the function can be called. With this follows that it is necessary to evaluate all three functions before we can decide which result to use - even though all we want is the first value. Compare that to a short-circuit or:

(let ((tmp0 (a))) (if tmp0 tmp0 (let ((tmp1 (b))) (if tmp1 tmp1 (let ((tmp2 (c))) (if tmp2 tmp2 "-default-"))))))

The important point here is, that if (a) evaluates to non-nil, it is returned and nothing else is evaluated. Only if it is nil, do we evaluate (b), and so forth.

So in conclusion; not only is a short-circuit or operator that actually returns the first non-false value useful for elegant programming constructs, it is also the most efficient solution for the solution of this simple problem.

In programming, if you find that your solution to a simple problem is not simple, you are in trouble. The short-circuit or is indeed a simple solution to a simple problem.