How does printf and co differentiate between float and double

Question

Since it isn't strongly typed I thought it just picked the right memory size and interpreted it based on the type of argument. But float and double both use %f and they are different sizes.

P.S. I can see how promotion via copying the value to a temp and casting(is this right?) might work but how does it work for scanfs/sscanf?

Solution

It doesn't differentiate. It's not possible to receive a float as a vararg: any float argument that you provide is first promoted to double.

6.5.2.2/6 defines "default argument promotions", and /7 states that default argument promotions are applied to "trailing arguments", that is varargs denoted by ....

how does it work for scanfs/sscanf?

The %f format for scanf requires a pointer to float. %lf requires a pointer to double, %Lf requires a pointer to long double.

copying the value to a temp and casting(is this right?)

If you provide a float argument, then the implementation creates a temporary of type double, initializes it with the float value, and passes this as the vararg. Casting by definition is explicit conversion by use of the cast operator -- you can cast if you like in order to make it exactly clear to the reader what's going on, but float f = 3; printf("%f", f); is exactly the same as float f = 3; printf("%f", (double)f);. The default argument promotion has the same meaning as the cast.

Comments

[phuclv]

Why does printf() promote a float to a double?

[Roman A. Taycher]

Thanks, do printfs(standard or extensions) accept that for double as well to keep symmetry?

[Steve Jessop]

Oh, if you mean "can I use %lf and %Lf as formats in printf and co" then yes, you can. %lf is exactly the same as %f, both take a double. %Lf takes a long double.