Saturday, January 19, 2019

The type system

 btrc: compile2 : the type system

The type system is a simple yet powerful way to ensure that the code knows enough about the data it is manipulating. It is used to declare conditions at interfaces, which are then enforced at each invocation.
The compiler is very good to check types. The condition is trivial to enforce, and doesn’t cost much compilation resource, so it’s a powerful combination.

typedef and weak types

C is sometimes labelled a “weakly typed” language, presumably because it is associated to the behavior of one of its keywords typedef.
The keyword itself implies that typedef DEFines a new TYPE, but it’s unfortunately a misnomer.

As an example, typedef can be used this way :

typedef int meters;
typedef int kilograms;

This defines 2 new “types”, meters and kilograms, which can be used to declare variables.

meters m;
kilograms k;

One could logically expect that, from now on, it’s no longer allowed to mix meters and kilograms, since they represent different types, hence should not be compatible.

Unfortunately, that’s not the case : meters and kilograms are still considered as int from the C type system perspective, and mixing them works without a single warning, even when every possible compiler warning is enabled.

As such, typedef must be considered a mere tagging system. It’s still useful from a code reviewer perspective, since it has documenting value, and that may help notice discrepancies, such as this example. But the compiler won’t be able to provide any signal.

Strong types in C

To ensure that two types cannot be accidentally mixed, it’s necessary to strongly separate them. And that’s actually possible.
C has one thing called a struct (and its remote relative called union).
Two struct defined independently are considered completely foreign, even if they contain exactly the same members.
They can’t be mixed unintentionally.

This gives us a basic tool to strongly segregate types.


Using struct comes with severe limitations. To begin with, the set of default operations is much more restricted. It’s possible to allocate struct on stack, and make it part of a larger struct, it’s possible to assign with = or memcpy(), but that’s pretty much it. No simple operation like + - * /, no comparison like < <= => >, etc.

Users may also access members directly, and manipulate them. But it breaks the abstraction.
When structures are used as a kind of “bag of variables”, to simplify transport, and enforce naming for clarity, it’s fine to let users access members directly. Compared to a function with a ton of parameters, an equivalent function with a structure as input will help readability tremendously, just because it enforces naming parameters.
But in the present case, when structures are used to enforce abstractions, users should be clearly discouraged from accessing members directly. Which means, all operations must be achieved at struct level directly.

To comply with these limitations, it’s now necessary to create all allowed operations one by one, giving a uniquely named symbol to each one. So if meters and kilograms can be added, both operations need their own function signature, such as add_meters() and add_kilograms(). This feels like a hindrance, and indeed, if there are many types to populate, it can require a lot of glue code.

But on the plus side, only what’s allowed is now possible. For example, multiplying meters with meters shouldn’t produce some meters, but rather a square_meters surface, which is a different concept. Allowing additions, but not multiplications, is an impossible subtlety for basic typedef.


There is no “intermediate” situation, where a type would be “compatible” with another type, yet different. In the mechanisms explained so far, types are either compatible and identical, using typedef, or completely incompatible, using a new definition and a new name.

In contrast, in Object Oriented languages, a cat can also be an animal, thanks to inheritance, so it’s possible to use cat to invoke animal methods, or use functions with animal parameter(s).

struct strongly leans towards composition. A struct cat can include a struct animal, which makes it possible to invoke animal related functions, though it’s not transparent : it’s necessary to explicitly spell the substructure (cat.animal) as a parameter or return value of the animal related function.

Note that even Object Oriented languages generally approve the composition over inheritance guiding principle. The guiding principle states that, if there isn’t a very good reason to employ inheritance, composition must always be preferred, because it generally fares better as the code evolves and becomes more complex (multiple inheritances quickly translate into a nightmare).

struct can be made more complex, with tables of virtual function pointers, achieving something similar to inheritance and polymorphism. But this is a whole different level of complexity. I will rather avoid this route for the time being. The current goal is merely to separate types in a way which can be checked by the compiler. Enforcing a unified interface on top of different types is a more complex topic, better left for a future article.

Opaque types

struct are fine as strong types, but publishing their definition implies that their members are public, meaning any user can access and modify them.
When it’s the goal, it’s totally fine.

But sometimes, one could wish that, in order to protect users from unintentional mis-usage, it would be better to make structure members unreachable. This is called an opaque type. An additional benefit is that whichever is inaccessible cannot be relied upon, hence may be changed in the future without breaking user code.

Object oriented language have the private tag, which allows exactly that : some members might be published, but they are nonetheless unreachable from the user (well, in theory…).

A “poor man” equivalent solution in C is to comment the code, clearly indicating which members are public, and which ones are private. No guarantee can be enforced by the compiler, but it’s still a good indication for users.
Another step is to give private members terrible names, such as never_ever_access_me, which provides a pretty serious hint, and is less easy to forget than a code comment.

Yet, sometimes, one wishes to rely on stronger compiler-backed guarantee, to ensure that no user will access private structure members. C doesn’t have private, but can do something equivalent.
It relies on the principles of incomplete type.

My own preference is to declare an incomplete type by pairing it with typedef :

typedef struct house_s house;
typedef struct car_s car;

Notice that we have not published anything about the internals of struct house_s. This is intentional. Since nothing is published, nothing can be accessed, hence nothing can be misused.

Fine, but what can we do about such a thing ? To begin with, we can’t even allocate it, since its size is not known.
That’s right, the only thing that can be declared at this stage is a pointer to the incomplete type, like this :

house* my_house;
car* my_car;

And now ?
Well, only functions with house* or car* as parameter or return type can actually do something with it.
These functions must access struct house_s and struct car_s internal definitions. These definitions are therefore published in a relevant unit *.c file, rather than the header *.h. Being not part of the public interface, the structure’s internal remains effectively private.

The first functions required are allocators and destructors.
For example, I’m used to the following name convention :

thing* PREFIX_createThing();
void PREFIX_freeThing(thing* t);

Now, it’s possible to allocate space for thing*, and eventually do something with it (with additional functions).
A good convention is that functions which accept thing* as mutable argument should have thing* as first parameter, like in this example :

int PREFIX_pushElement(thing* t, element e);
element PREFIX_pullElement(thing* t);

Notice that we are getting pretty close to object oriented programming with this construction. Functions and data members, while not declared in an encompassing “object”, must nonetheless be defined together: the need to know the structure content to do anything about it forces function definitions to be grouped into the unit that declares the structure content. It’s fairly close.

Compared with a direct struct, a few differences stand out :

  • Members are private
  • Allocation is implemented by a function, it can only be invoked
    • no way to allocate on stack
    • no way to include a thing into another struct
      • but it’s possible to include a pointer thing*
    • Initialization can be enforced directly in the constructor
      • removes risks of garbage content due to lack of initialization.
  • The caller is in charge of invoking the destructor.
    • The pattern is exactly identical to malloc() / free() (see future article on Resource Control)

The responsibility to invoke the destructor after usage is very important.
It’s no different than invoking free() after a malloc(),
but that’s still an additional detail to take care of, with the corresponding risk to forget or mismanage it.

To bypass this responsibility, and take control of the allocation process, it can be preferable to consider opaque types with static allocation. That’s the topic of the next article.


This closes this first chapter on the type system. We have seen that it’s possible to create strong types, and we can use this property to ensure users can’t mix up different types accidentally. We have seen that it’s possible to create opaque types, and ensure users can only invoke allowed operations, or can’t rely on secret internal details, clearing the path of future evolution. These properties are compiler-checked, so they are always automatically enforced.

That’s not bad. Just using these properties will seriously improve code resistance to potential mis-usages.

Yet, there is more the compiler can do to detect potential bugs in our code. To be continued…


  1. Using a union is a interesting approach. It never occurred to me - I'd always created an array of structs that is internal to the module and then passed back a pointer to an unused struct. This is much more flexible. Definitely plan on trying it out.

    Look forward to seeing more.

  2. Nice approach using a union that had not occurred to me! Is the union in the listing meant to be named thingBody_u? Otherwise, it looks like this declares an anonymous (unusable) union type.

    1. Yes, you are right Matthew, the union must be named. to fix.

  3. Although the Standard allows implementations to exploit N1570 6.5p7 (often called the "Strict Aliasing Rule") in any fashion they see fit, it includes a footnote stating that its purpose is to indicate when things may alias. While the footnote is not normative, it does provide guidance for how *quality* implementations should interpret the rule. The authors of the Standard have acknowledged that it would be possible to contrive an implementation that is conforming but succeeds at being useless, and the Standard thus relies upon compiler writers trying to make useful implementations without regard for whether or not the Standard would require them to do so.

    According to a strict but allowable reading of the Standard, given "struct foo {int x;} s;" the code "s.x = 2;" invokes Undefined Behavior because it uses an lvalue of type "int" to access an object of type "struct foo", but the only types of lvalue that can be used to access a "struct foo" are "struct foo" or qualfied versions thereof, structure types that contain such types, or character types. The type "int" has none of those qualities, and may thus not be used to access an object of type "struct foo".

    To be sure, a quality implementation should have no difficulty recognizing that "s.x = 2;" might access an object of type "struct foo", but nothing in the Standard would requires it to do so. Instead, the authors of the Standard expected implementations to make a reasonable effort to support constructs beyond those mandated by the Standard when necessary to avoid "prevent[ing] the programmer from doing what needs to be done" [wording taken from the published Rationale document]. The fact that some poor quality implementations are incompatible with a program does not imply that the program is in any way defective. The Standard, after all, doesn't require implementations to be capable of running *any* useful programs.

    Trying to comply with 6.5p7 is a pointless exercise since doing so would require that one jump through totally absurd hoops. One might try to comply with 6.5p7 as extended by some particular implementations, but tolerating willfully stupid behavior from compilers will do nothing to make programming safer. Instead, it will encourage compiler writers to push the boundaries of stupid-but-conforming behavior so as to force programmers to jump through even more hoops.

  4. Since the last part, featuring static allocation and strict aliasing, was attracting most comments, I investigated a bit more on this topic, and transferred its content into its own blog entry, since its content was becoming quite large.