When describing data, the C language offers data types and data qualifiers. The data type is well known to any C programmer, defining the kind of data stored: char, int, float, and so on. The qualifier describes additional aspects of the data, such as how it’s used or whether the compiler should optimize the data’s storage.

Here are the data qualifier keywords in the C language:

const
restrict
volatile
_Atomic

These differ from storage class specifiers: auto, register, static, extern, and typedef. In a way, these declarations tell the compiler how to store the data. The data qualifiers tell the compiler how the data is to be treated and suggest optimizations. My descriptions are general and subject to ire of sophomore computer science students, so I can be accused of over-simplification here.

I cover the const keyword in my books and online courses. It ensures that the data declared isn’t modified in the code. Its scope is within the function where the declaration is made. Because the constant value can’t be modified, it’s assigned a value in its declaration:

const int full = 100;

Above, integer full is set to 100 and made constant. This variable is used throughout the function, but never altered. If an attempt is made, the compiler shrieks in horror.

The const data qualifier differs from the defined constant in scope. The precompiler DEFINE directive creates a constant visible throughout the source code. But a defined constant value is replaced as the program is built. On the other hand, a const value is assigned storage like any other variable and used that way in the program.

Beyond const, my bet is that only a few C programmers use the restrict, volatile, and _Atomic qualifiers. From my research, these specifiers are (no surprise) quite specific in how they modify data and when their use is necessary.

You find the restrict specifier used in various function declarations in the man pages:

int printf(const char * restrict format, ...);
char *fgets(char * restrict str, int size, FILE * restrict stream);
FILE *fopen(const char * restrict path, const char * restrict mode);

In each declaration, see how restrict is associated with a pointer? In a way, think of a restrict specifier as a promise made to the compiler that no other pointer is ever used to access the same data chunk. This restriction (get it?) also implies that each string passed to a function is unique.

The restrict specification is made primarily for compiler optimization. By ensuring that only one pointer references a chunk of data, the compiler is able to reduce overhead when accessing the data. I’m unsure of the mechanics, and though sample code is available to show restrict in action, it’s difficult to visually demonstrate how this optimization works or how it’s effective.

Next week I ruminate on the volatile keyword.