va_arg无法使用双打

Question

I have a function that takes a variable number of arguments, and then passes each argument to a function, depending on other factors. This is working well for most types, regardless of whether or not they are a pointer

func = (fmtfunc_t)dictobj(deflt, tok);
dat = func(va_arg(lst, void *));

where fmtfunc_t is defined as

typedef char * (*fmtfunc_t)(void *);

This method works, for example, with functions like the following

char *examp1(int i) {
    // i points to the correct integer value
}
char *examp2(char *s) {
    // s points to the correct string value
}

However, it does not work when the argument is a double

char *examp3(double d) {
    // d is 0
}

I am aware of issues with va_arg and double promotion, but I don't believe that this is the root of my problem. I call the function like this

func(23.4);

As you can see, the argument is a double literal, so I don't believe I should be concerned with promotion issues.

Why is va_arg returning an incorrect value for double s, but not for any other type? Am I encountering some kind of undefined behavior and getting lucky with types other than double ?

Answer 1

The issue, as pointed out by @RaymondChen, @Olaf, and @FUZxxl in the comments, was that, by calling the functions with arguments that had types that were incompatible with the declarations

char *examp(int i);

fmtfunt_t func = examp;
func(va_arg(lst, void *)); //called with an argument of void*, but the parameter is of type int

I was causing undefined behavior. I solved this by fetching the arguments as their appropriate types

double arg = va_arg(lst, double);

Rather than trying to use void * as a catch-all.