SPRUI04F july 2015 – april 2023
To help the compiler determine memory dependencies, you can qualify a pointer, reference, or array with the restrict keyword. The restrict keyword is a type qualifier that can be applied to pointers, references, and arrays. Its use represents a guarantee by you, the programmer, that within the scope of the pointer declaration the object pointed to can be accessed only by that pointer. Any violation of this guarantee renders the program undefined. This practice helps the compiler optimize certain sections of code because aliasing information can be more easily determined.
The "restrict" keyword is a C99 keyword, and cannot be accepted in strict ANSI C89 mode. Use the "__restrict" keyword if the strict ANSI C89 mode must be used. See Section 7.13.
In the following example, the restrict keyword is used to tell the compiler that the function func1 is never called with the pointers a and b pointing to objects that overlap in memory. You are promising that accesses through a and b will never conflict; therefore, a write through one pointer cannot affect a read from any other pointers. The precise semantics of the restrict keyword are described in the 1999 version of the ANSI/ISO C Standard.
void func1(int * restrict a, int * restrict b)
{
/* func1's code here */
}The following example uses the restrict keyword when passing arrays to a function. Here, the arrays c and d must not overlap, nor may c and d point to the same array.
void func2(int c[restrict], int d[restrict])
{
int i;
for(i = 0; i < 64; i++)
{
c[i] += d[i];
d[i] += 1;
}
}