va_arg( , double) return each time an invalid value [closed]

Answer 1

Well, 3 and 1 are integers and not doubles. Their binary representation of these numbers is different than that of 3.0 and 1.0.

var(1,3.9);
var(1,1.2);
var(1,5.7);


$ ./a 
a.c,17,Sep 25 2012
func: 18
3.900000
1.200000
5.700000

Answer 2

Variadic functions (that is, functions which take a variable number of arguments indicated by the ellipsis (...) parameter) have weak typing in C. Except in the case of certain special functions like printf and scanf, the compiler makes no effort to verify that you're passing the correct types of arguments to them.

In your case, the function is expecting a double parameter, but you're trying to pass in an int. The compiler does not do any promotion here from int to double, so undefined behavior results. You need to always pass in a double value here, either as an explicit double constant value such as 1.0, or perform the conversion using a typecast.