Recently it showed that the C standard seems to be ambiguous on how to interpret the controlling expression of
_Generic, the one that determines the choice. Compiler implementors have given different answers to this question; we will see below that there is code that is interpreted quite differently by different existing compilers. None of them is “wrong” at a first view, so this tells us that we must be careful when we use
_Generic. In this post I will try to explain the problem and to give you some work around for common cases.
May 11, 2015
Recently it showed that the C standard seems to be ambiguous on how to interpret the controlling expression of
April 25, 2015
In discussions about C or other standards (e.g POSIX) you may often have come across the term undefined behavior, UB for short. Unfortunately this term is sometimes mystified and even used to scare people unnecessarily, claiming that arbitrary things even outside the scope of your program can happen if a program “encounters undefined behavior”. (And I probably contributed to this at some point.)
First, this is just crappy language. How can something “encounter” behavior? It can’t. What is simple meant by such slang is that such a program has no defined behavior, or stated yet otherwise, the standard doesn’t define what to do if such a program reaches the state in question. The C standard defines undefined behavior
behavior, upon use of a nonportable or erroneous program construct or of erroneous data,
for which this International Standard imposes no requirements
Simple examples of undefined behavior in C are out-of-bounds access to arrays or overflow of signed integer types. More complicated cases arise when violating aliasing rules or when access to data races between different threads.
Now we often hear statements like “UB may format your hard drive” or “UB may steal all your money from your bank account” implying somehow that a program that is completely unrelated to my disk administration or to my online banking, could by some mysterious force have these evil effects. This is (almost) complete nonsense.
If UB in a simple program of yours formats your hard drive, you shouldn’t blame your program. No simple application run by an unprivileged user should have such devastating consequences, this would be completely inappropriate. If such things happen, it is your system which is at fault, get you another one.
As an analogy from every-day life, take the idea of locking your house at night, which seems to be the rule in some societies. Sure, if you don’t do that, you make it easier for somebody to sneak into your house and shoot you. But still, that person would be a murderer, to be punished for that by the law, no sane legal system would acquit such a person or see this as mitigating circumstances.
Now things are in fact different when there is a direct relation between the object of your negligence and the ill-effect that it causes. If you leave your purse on the table in the pub when going to the bathroom, you should take a share in responsibility if it is stolen. Or to come back to the programming context, if you are programming a security sensible application (e.g handling passwords or bank credentials) then you should be extremely careful and stay on well-defined grounds.
When programming in C there are different kinds of constructs or data for which the program behavior then is undefined.
- Behavior can be explicitly declared undefined or implicitly left undefined.
- The error can be detectable or undetectable at compile time.
- The error can be detectable or undetectable during execution.
- Behavior of certain constructs can be undefined by a specific standard to allow extensions.
Unfortunately, people often use UB as an excuse to evacuate questions about certain behavior of their code (or compiler). This is just rude behavior by itself, and you usually shouldn’t accept this. An implementation should have reasons and policies how it deals with UB, otherwise it is just bad, and you should switch to something more friendly, if you may.
There is no reason not to be friendly, and there are many to be.
That is, in our context, once a (your!) code detects that it has a problem it must handle it. Possible strategies are
- abort the compilation or the running program
- return an error code
- report the problem
- define and document the behavior in your own terms
For the first you should always have in mind that the program should be debugable, so you really should use
static_assert for compile time errors and
abort for run time errors. Also, willingly aborting the program is not the same as having the program crash, see below.
Obviously, the second is only possible if the function in question has an established error return convention and if you can expect users of the function to check for that convention. POSIX has many such cases where the documentation says something like “may” return a certain error code. A C (or POSIX) library implementation that detects such a “may” case and doesn’t react accordingly, is of bad quality.
Reporting detected errors is an important alternative and some compiler implementors have chosen this as their default answer to problems. Perhaps you already have seen gcc’s famous “diagnostic” message
dereferencing pointer ‘bla’ does break strict-aliasing rules
This message supposes that you know what aliasing rules are, and that you also know why it came to that. Observe that it says “does” so the compiler claims to have proven that the aliasing rules are violated, and that the behavior is thus undefined. What the message doesn’t tell, and that is bad, is how it resolves the problem.
The last variant, defining your own stuff, should be handled with extreme care. Not only that what you define must be sensible, you also make a commitment in doing so. You promise your clients that you will follow these new rules in the future and that they may suppose that you will take care of the underlying problem. In most cases, you should leave the definition of extensions to the big players, platform designers or other standards. E.g POSIX defines a lot of cases that are UB for C as such.
As a last alternative, when
- the error is undetectable
- the detection of the error would be too expensive
you should simply let the program crash. The best strategy I know of is to always initialize all variables and always treat 0 as a special value. This may, under rare circumstance deal a tiny bit of performance against security, because on some rare occasions your compiler might not be able to optimize an unused initialization. If you do this, most errors that you will see are dereferences of null pointers.
Dereferencing a null pointer has UB. But modern architecture no how to handle this: they raise a “segmentation fault” error and terminate the program. This is the nicest failure path that you can offer to your clients, failing anytime, anywhere.
February 7, 2015
I am pleased to announce the feature completion of Level 2 of my book
It deals with most principal concepts and features of the C
programming language, such as control structures, data types,
operators and functions. Its knowledge should be sufficient for an
introductory course to Algorithms, with the noticeable particularity
that pointers aren’t fully introduced here, yet.
As before, the current version of the book can be found at my homepage
and also as before, constructive feedback is highly welcome. Many
thanks to those that already gave such valuable feedback for previous
October 14, 2014
Today, Rich Felker has published the next release of musl the lightweight, standard conforming C library. He says:
September 8, 2014
In two ancient posts I have talked about arrays in modern C, “don’t be afraid of variably modified types” and “VLA as function arguments”. Still there seem to be a lot of people that, perhaps just by bad habit, that prefer to use “fake matrices” instead of real matrices in C. Unfortunately among these people are a lot of university teachers that preach that bad parole to their students. I just try to make a list of the advantages of real matrices, here.
April 2, 2014
I recently reviewed some document on security recommendations where I was baffled by the fact that the code examples were sprinkled with casts all over the place. I had thought that people that are concerned with software security in mind would adhere to one of the most important rules in C programming:
Casts are harmful and evil.
The “evil” here is to be read as reference to black magic. Most uses of cast are merely done in the spirit of “casting a spell” by people that try to quieten their compiler. The sorcerer’s apprentice approach: if I don’t see the evil, it isn’t there.
For me it is evident that every cast punches a hole in C’s type system. So, concerned with code security, we should avoid them as much as possible. But this evidence doesn’t yet seem shared (meaning that it is not so evident :) and I decided to explain things here in more detail.
Casts (explicit conversions) in C come with three different flavors, depending on the cast-to and cast-from type
- pointer to pointer
- pointer to integer or vice versa
- integer to integer
December 18, 2013
As I showed int this post, using > as right angle brackets was not a particularly good idea, but trying to patch this misdesign even makes it worth. After a bit of experimenting I found an expression that is in fact valid for both, C++98 and
C++11, but that has a different interpretation in both languages:
fon< fun< 1 >>::three >::two >::one
So if you have to maintain a large code base with templates that depend on integers that are perhaps produced automatically by some tools, be happy, you will not be out of work for a while: changing your compiler to
C++11 might change the semantics of your code.
December 15, 2013
It is long time that I didn’t look into C++, I have to admit. By coincidence I recently unearthed a hilarious example that I had once written that shows the difficulty of parsing some C++ code, as well as for compilers as for us poor humans. It all starts with the
>> operator that (supposedly until C++11) could cause problems as in the following:
toto< tutu< 3 >> A;
Here the >> is (was) interpreted as `right shift’ operator and thus this code would create a compile time error. C++11 changed this by introducing the possibility that in that case the right-shift-operator-token closes the two template angle brackets. The argument is that shift operators in template arguments are rare (which is probably true) and so this sacrifices some valid uses of that operator for the sake of causing less brain damage to C++ newbies.
October 28, 2013
Let’s take the occasion of the change back from DST here in Europe, not in the US, yet, to look how times are handled in C.
The C standard proposes a large variety of types for representing times:
double and textual representations as
char. It is a bit complicated to find out what the proper type for a particular purpose is, so let me try to explain this.
The first class of “times” can be classified as calendar times, times with a granularity and range as it would typically appear in a human calendar, as for appointments, birthdays and so on. Some of the functions that manipulate these in C99 are a bit dangerous, they operate on global state. Let us have a look how these interact:
August 22, 2013
Sometimes in C it is useful to distinguish if an expression is an “integral constant expression” or a “null pointer constant”. E.g for an object that is allocated statically, only such expressions are valid initializers. Usually we are able to determine that directly when writing an initializer, but if we want to initialize a more complicated
struct with a function like initializer macro, with earlier versions of C we have the choice:
- Use a compiler extension such as gcc’s
- We’d have to write two different versions of such a macro, one for static allocation and one for automatic.
In the following I will explain how to achieve such a goal with C11’s
_Generic feature. I am not aware of a C++ feature that provides the same possibilities. Also, this uses the ternary operator (notably different in C and C++), so readers that merely come from that community should read the following with precaution.