static extern int memcmp(byte[] b1, byte[] b2, UIntPtr count);

When calling memcmp via pinvoke there is no need to use unsafe code or pinning because the framework will automatically pin the arrays for you.

   private static extern int memcmp(byte[] b1, byte[] b2, UIntPtr count);

      return memcmp(b1, b2, new UIntPtr((uint)b1.Length)) == 0;