What is a fully promoted type?

Question

I came across this in va_copy(3):

/* need a cast here since va_arg only
 * takes fully promoted types */
c = (char) va_arg(ap, int);

What is a fully promoted type?

Answer 1

This is referring to the rules of integer promotion. Anytime an integer value with a type smaller than int (i.e. char, short) is used in a context where an int can be used, the value is promoted to an int.

In the case of a variadic function, the type of the arguments to a function are not known at compile time, so this promotion applies.

For example, suppose you had the following functions:

void f1(char c);
void f2(int count, ...);

They are called like this:

char x = 1;
f1(x);       // x is passed as char
f2(1, x);    // x is passed as int

This behavior is documented in section 6.3.1.1p2 of the C standard:

The following may be used in an expression wherever an int or unsigned int may be used:

• An object or expression with an integer type (other than int or unsigned int ) whose integer conversion rank is less than or equal to the rank of int and unsigned int .
• A bit-field of type _Bool , int , signed int ,or unsigned int .

If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int ; otherwise, it is converted to an unsigned int . These are called the integer promotions . All other types are unchanged by the integer promotions.