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Type Qualifiers


Type qualifiers are the keywords which describe additional semantics about a type. They are an integral part of type signatures. They can appear both at the topmost level of a declaration (directly affecting the identifier) or at sub-levels (relevant to pointers only, affecting the pointed-to values):

constPrevents the mutation of the declared object (by appearing at the topmost level) or prevents the mutation of the pointed-to value (by appearing next to a pointer subtype).
volatileInforms the compiler that the declared object (at topmost level) or the pointed-to value (in pointer subtypes) may change its value as a result of external conditions, not only as a result of program control flow.
restrictAn optimization hint, relevant to pointers only. Declares intent that for the lifetime of the pointer, no other pointers will be used to access the same pointed-to object.

The ordering of type qualifiers with respect to storage class specifiers (static, extern, auto, register), type modifiers (signed, unsigned, short, long) and type specifiers (int, char, double, etc.) is not enforced, but the good practice is to put them in the aforementioned order:

static const volatile unsigned long int a = 5; /* good practice */
unsigned volatile long static int const b = 5; /* bad practice */

Top-level qualifications

/* "a" cannot be mutated by the program but can change as a result of external conditions */
const volatile int a = 5;

/* the const applies to array elements, i.e. "a[0]" cannot be mutated */    
const int arr[] = { 1, 2, 3 };

/* for the lifetime of "ptr", no other pointer could point to the same "int" object */
int *restrict ptr;

Pointer subtype qualifications

/* "s1" can be mutated, but "*s1" cannot */
const char *s1 = "Hello";

/* neither "s2" (because of top-level const) nor "*s2" can be mutated */
const char *const s2 = "World";

/* "*p" may change its value as a result of external conditions, "**p" and "p" cannot */
char *volatile *p;

/* "q", "*q" and "**q" may change their values as a result of external conditions */
volatile char *volatile *volatile q;

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