Pointers in C, Part II: CV-Qualifiers

“A teacher is never a giver of truth; he is a guide, a pointer to the truth that each student must find for himself.”
— Bruce Lee

In part I of this series, I explained what pointers are in general, how they are similar to arrays, and — more importantly — where, when, and why they are different to arrays. Today, I’ll shed some light on the so-called ‘cv qualifiers’ which are frequently encountered in pointer contexts.

CV-QUALIFIER BASICS

CV-qualifiers allow you to supplement a type declaration with the keywords ‘const’ or ‘volatile’ in order to give a type (or rather an object of a certain type) special treatment. Take ‘const’, for instance:

‘const’ is a guarantee that a value isn’t (inadvertently) changed by a developer. On top of that, it gives the compiler some leeway to perform certain optimizations, like placing ‘const’ objects in ROM/non-volatile memory instead of (expensive) RAM, or even not storing the object at all and instead ‘inline’ the literal value whenever it’s needed.

‘volatile’, on the other hand, prevents optimizations. It’s a hint to the compiler that the value of an object can change in ways not known by the compiler and thus the value must never be cached in a processor register (or inlined) but instead always loaded from memory. Apart from this ‘don’t optimize’ behavior, there’s little that ‘volatile’ guarantees. In particular — and contrary to common belief — it’s no cure for typical race condition problems — It’s mostly used in signal handlers and to access memory-mapped hardware devices.

Even if it sounds silly at first, it’s possible to combine ‘const’ and ‘volatile’. The following code declares a constant that shall not be inlined/optimized:

Using both ‘const’ and ‘volatile’ together makes sense when you want to ensure that developers can’t change the value of a constant and at the same time retain the possibility to update the value through some other means, later. In such a setting, you would place ‘MAX_SENSORS’ in a dedicated non-volatile memory section (ie. flash or EEPROM) that is independent of the code, eg. a section that only hosts configuration values*. By combining ‘const’ and ‘volatile’ you ensure that the latest configuration values are used and that these configuration values cannot be altered by the programmer (ie. from within the software).

To sum it up, ‘const’ means “not modifiable by the programmer” whereas ‘volatile’ denotes “modifiable in unforeseeable ways”.

CV-QUALIFIERS COMBINED WITH POINTERS

Like I stated in the intro, cv-qualifiers often appear in pointer declarations. However, this poses a problem because we have to differentiate between cv-qualifying the pointer and cv-qualifying the pointed-to object. There are “pointers to ‘const'” and “‘const’ pointers”, two terms that are often confused. Here’s code involving a pointer to a constant value:

Since the pointer is declared as pointing to ‘const’, no changes through this pointer are possible, even if it points to a mutable object in reality.

Constant pointers, on the other hand, behave differently. Have a look at this example:

The takeaway is this: if the ‘const’ keyword appears to the left of the ‘*’, the pointed-to value is ‘const’ and hence we are dealing with a pointer to ‘const’; if the ‘const’ keyword is to the right of the ‘*’, the pointer itself is ‘const’. Of course, it’s possible to have the ‘const’ qualifier on both sides at the same time:

The same goes for multi-level pointers:

Here, ‘v’ is a regular (non-‘const’) pointer to ‘const’ pointer to a pointer to a ‘const’ integer.

Yuck! Sometimes, I really wish the inventors of C had used ‘<-‘ instead of ‘*’ for pointer declarations — the resulting code would have been easier on the eyes! Consider:

versus

So

would read from right to left as “v is a POINTER TO const POINTER TO const int”. Life would be some much simpler… but let’s face reality and stop day-dreaming!

Everything I said about ‘const’ equally applies to pointers to ‘volatile’ and ‘volatile’ pointers: pointers to ‘volatile’ ensure that the pointed-to value is always loaded from memory whenever a pointer is dereferenced; with ‘volatile’ pointers, the pointer itself is always loaded from memory (and never kept in registers).

Things really get complicated when there is a free mix of ‘volatile’ and ‘const’ keywords with pointers involving more than two levels of indirection:

Let’s better not go there! If you are in multi-level pointer trouble, remember that there’s a little tool called ‘cdecl‘ which I showcased in the previous episode. But now let’s move on to the topic of how and when cv-qualified pointers can be assigned to each other.

ASSIGNMENT COMPATIBILITY I

Pointers are assignable if the pointer on the left hand side of the ‘=’ sign is not more capable than the pointer on the right hand side. In other words: you can assign a less constrained pointer to a more constrained pointer, but not vice versa. If you could, the promise made by the constrained pointer would be broken:

If the previous statement was legal, a programmer could suddenly get write access to a read-only variable:

Again, the same restrictions hold for pointers to ‘volatile’. In general, pointers to cv-qualified objects are more constrained than their non-qualified counterparts and hence may not appear on the right hand side of an assignment expression. By the same token, this is not legal:

ASSIGNMENT COMPATIBILITY II

The rule which requires that the right hand side must not be more constrained than the left hand side might lead you to the conclusion that the following code is perfectly kosher:

However, it’s not, and for good reason, as I will explain shortly. But it’s far from obvious and it’s a conundrum to most — even seasoned — C developers. Why is it possible to assign a pointer to non-const to a pointer to ‘const’:

but not a pointer to a pointer to non-const to a pointer to a pointer to ‘const’?

Here is why. Imagine this example:

Graphically, our situation is this. ‘ppc’ points to ‘p’ which in turn points to some random memory location, as it hasn’t been initialized yet:

Now, when we dereference ‘ppc’ one time, we get to our pointer ‘p’. Let’s point it to ‘VALUE’:

It shouldn’t surprise you that this assignment is valid: the right hand side (pointer to const int) is not less constrained than the left hand side (also pointer to const int). The resulting picture is this:

Everything looks safe. If we attempt to update ‘VALUE’, we won’t succeed:

But we are far from safe. Remember that we also (indirectly) updated ‘p’ which was declared as pointing to a non-const int and ‘p’ was declared as pointing to non-const? The compiler would happily accept the following assignment:

which leads to undefined behavior, as the C language standard calls it.

This example should have convinced you that it’s a good thing that the compiler rejects the assignment from ‘int**’ to ‘const int**’: it would open-up a backdoor for granting write access to more constrained objects. Finding the corresponding words in the C language standard is not so easy, however and requires some digging. If you feel “qualified” enough (sorry for the pun), look at chapter “6.5.16.1 Simple assignment”, which states the rules of objects assignability. You probably also need to have a look at “6.7.5.1 Pointer declarators” which details pointer type compatibility as well as “6.7.3 Type qualifiers” which specifies compatibility of qualified types. Putting this all into a cohesive picture is left as an exercise to the diligent reader.

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*) Separating code from configuration values is generally a good idea in embedded context as it allows you to replace either of them independently.

Comments (1)

  1. 14:28, 31-07-2017Andy "Brexit" Dingle  / Reply

    Thanks Ralf, for updateability the const volatile is very poweful, but I also think People should read the Dan Saks article for the use of const also. http://www.dansaks.com/articles/1999-02%20const%20T%20vs%20T%20const.pdf

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