Promotion of Float Argument to Double in Variadic Functions

In a previous discussion, it was noted that printf() promotes a float argument to a double before accepting it. This raises the question: do all variadic functions perform this promotion?

Variadic Argument Promotion

Yes, according to both the C99 and C standards, float arguments passed to variadic functions are promoted to double.

Standard Specifications

• C99 (section 6.5.2.2): "arguments that have type float are promoted to double."
• C (section 5.2.2): "a floating point type... is converted to the promoted type before the call."
• C (section 4.6): "A prvalue of type float can be converted to a prvalue of type double. The value is unchanged."

cppreference on Variadic Function Conversions

cppreference provides a summary of the default conversions for variadic functions in C :

• std::nullptr_t to void*
• Float arguments to double
• Integer promotion for bool, char, short, and enumerations

Origin of Promotion

This promotion was introduced in C for compatibility with the K&R C standard, despite its potential drawbacks. As the C Rationale states:

"For compatibility with past practice, all argument promotions occur as described in K&R in the absence of a prototype declaration, including the not always desirable promotion of float to double."