__restrict (keyword)

[John E]

I've just been sent some code which uses the __restrict keyword. I looked it up at MSDN which gives the following example:

// __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];
    }
}

Maybe it's just me, but that example doesn't really tell me what __restrict is doing. What exactly does it allow (or disallow) ?

[2kaud]

__restrict is basically a promise to the compiler that for the scope of the pointer, the target of the pointer will only be accessed through that pointer (and pointers copied from it).

See http://stackoverflow.com/questions/7...word-mean-in-c

This gives various references to papers etc with explanations and examples. Have fun!

[John E]

Thanks 2kaud, it makes a bit more sense now!

BTW, the original code used __restrict__ (with extra underscores at the end). I'm guessing that's essentially the same thing but for a different compiler (gcc?)

[2kaud]

Yeah - restrict is not a part of c++ yet (its from c) so its implementation is compiler specific - hence the beginning __. It might even do nothing!