Like the restrict __declspec modifier, the __restrict keyword indicates that a symbol is not aliased in the current scope. The __restrict keyword differs from the restrict __declspec modifier in the following ways:

  • The __restrict keyword is valid only on variables, and __declspec(restrict) is only valid on function declarations and definitions.

  • When __restrict is used, the compiler will not propagate the no-alias property of a variable. That is, if you assign a __restrict variable to a non-__restrict variable, the compiler will not imply that the non-__restrict variable is not aliased.

Generally, if you affect the behavior of an entire function, it is better to use the __declspec than the keyword.

__restrict is similar to restrict from the C99 spec, but __restrict can be used in C++ or C programs.

There is no support for __restrict on C++ references. 


When used on a variable that also has the volatile (C++) keyword, volatile will take precedence.

// __restrict_keyword.c
// compile with: /LD
// In the following function, declare a and b as disjoint arrays
// but do not have same assurance for c and d.
void sum2(int n, int * __restrict a, int * __restrict b, 
          int * c, int * d) {
   int i;
   for (i = 0; i < n; i++) {
      a[i] = b[i] + c[i];
      c[i] = b[i] + d[i];

// By marking union members as __restrict, tell compiler that
// only z.x or z.y will be accessed in any given scope.
union z {
   int * __restrict x;
   double * __restrict y;