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21.3 restrict-Qualified Pointers

You can declare a pointer as “restricted” using the restrict type qualifier, like this:

int *restrict p = x;

This enables better optimization of code that uses the pointer.

If p is declared with restrict, and then the code references the object that p points to (using *p or p[i]), the restrict declaration promises that the code will not access that object in any other way—only through p.

For instance, it means the code must not use another pointer to access the same space, as shown here:

int *restrict p = whatever;
int *q = p;
foo (*p, *q);

That contradicts the restrict promise by accessing the object that p points to using q, which bypasses p. Likewise, it must not do this:

int *restrict p = whatever;
struct { int *a, *b; } s;
s.a = p;
foo (*p, *s.a);

This example uses a structure field instead of the variable q to hold the other pointer, and that contradicts the promise just the same.

The keyword restrict also promises that p won’t point to the allocated space of any automatic or static variable. So the code must not do this:

int a;
int *restrict p = &a;
foo (*p, a);

because that does direct access to the object (a) that p points to, which bypasses p.

If the code makes such promises with restrict then breaks them, execution is unpredictable.