24.1 Explicit Type Conversion

You can do explicit conversions using the unary cast operator, which is written as a type designator (see Type Designators) in parentheses. For example, (int) is the operator to cast to type int. Here’s an example of using it:

{
  double d = 5.5;

  printf ("Floating point value: %f\n", d);
  printf ("Rounded to integer: %d\n", (int) d);
}

Using (int) d passes an int value as argument to printf, so you can print it with ‘%d’. Using just d without the cast would pass the value as double. That won’t work at all with ‘%d’; the results would be gibberish.

To divide one integer by another without rounding, cast either of the integers to double first:

(double) dividend / divisor
dividend / (double) divisor

It is enough to cast one of them, because that forces the common type to double so the other will be converted automatically.

The valid cast conversions are:

• One numerical type to another.
• One pointer type to another. (Converting between pointers that point to functions and pointers that point to data is not standard C.)
• A pointer type to an integer type.
• An integer type to a pointer type.
• To a union type, from the type of any alternative in the union (see Unions). (This is a GNU extension.)
• Anything, to void.