The restrict type qualifier
This type qualifier is introduced in the C99 standard.
A pointer is the address of a location in memory. More than one pointer can access the same chunk of memory and modify it during the course of a program. The restrict (or __restrict or __restrict__)1 type qualifier can be applied to a pointer type to form a restrict-qualified pointer. During the execution of the block that is associated with the declaration of an object that provides a way to designate a restrict-qualified pointer, the memory addressed via the restrict-qualified pointer cannot be modified or can be accessed only via this pointer if the pointer does not point to a const-qualified type. The compiler may choose to optimize code involving restrict-qualified pointers in a way that might otherwise result in incorrect behavior. It is the responsibility of the programmer to ensure that restrict -qualified pointers are used as they were intended to be used. Otherwise, undefined behavior may result.
void foo(int n, int * restrict a, int * restrict b, int * restrict c)
{
int i;
for (i = 0; i < n; i++)
a[i] = b[i] + c[i];
}
{
int * restrict x;
int * restrict y;
x = y; // undefined
{
int * restrict x1 = x; // okay
int * restrict y1 = y; // okay
x = y1; // undefined
}
}
In nested blocks containing restricted pointers, only
assignments of restricted pointers from outer to inner blocks are
allowed. The exception is when the block in which the restricted pointer
is declared finishes execution. At that point in the program, the
value of the restricted pointer can be carried out of the block in
which it was declared.- The restrict qualifier is represented by the
following keywords (all have the same semantics):
- The restrict keyword is recognized under compilation with xlc or c99 or with the -qlanglvl=stdc99 or -qlanglvl=extc99 options or -qkeyword=restrict. The __restrict and __restrict__ keywords are recognized at all language levels.
- Using the -qrestrict option is equivalent to adding the restrict keyword to the pointer parameters within the specified functions.