Operator precedence while type casting in C

Question

The following piece of code gives the correct result for the multiplication

int var0 = 245895;
int var1 = 478565

long long val = 0;

val = (long long) var0 * var1;

but this piece gives the incorrect result:

int var0 = 245895;
int var1 = 478565
long long val = 0;

val = (long long) (var0 * var1);

Could anybody help me with why?

Answer 1

(long long) var0 * var1
~~~~~~~~~~~~~~~~
       1
~~~~~~~~~~~~~~~~~~~~~~~
           2

In the above code, first var0 casts to long long, after that, the result of multiplication will be calculated as long long with no overflow. In fact compiler promotes the type of var1 from int to long long implicitly.

(long long) (var0 * var1)
            ~~~~~~~~~~~~~
                  1
~~~~~~~~~~~~~~~~~~~~~~~~~
           2

In the second code, first multiplication occurs and the result doesn't fit in a long type, so the cast after that doesn’t help anymore. It casts the number that is overflow-ed before.

Therefore, the first one is better than second one to avoid overflows.

Answer 2

It's all a matter of how the mid calculation result is stored:

val = (long long) (var0 * var1); is interpreted as:

int midResult = (int)var0 * (int)var1;
val = (long long)midResult;

While, val = (long long) var0 * var1 is interpreted as:

long long midResult = (long long) var0 * (long long)var1;