c中具有不同类型参数的可变参数函数

Question

I was wondering if it's possible in C to create a variadic function that takes different types of arguments. ie

void fillDatabase(char* name, int age){
  writeToDatabase(name, age);
}

int main(){
  fillDatabase("Paul", 19);
  fillDatabase("Herbert");
}

Here I am trying to fill up a database with names and ages. But it's also possible to fill it up with only a name and no age. So I am wondering if I could use the same function for that or if I have to write two different ones?

Online I could only find examples for variadic functions with the same type of arguments.

Thanks in advance!

Answer 1

In C99, variadic functions use stdarg(3) operations (generally implemented as macros expanding to compiler-specific magical stuff, eg GCC builtins). The first arguments should have some fixed known type, and generally determinates how the other arguments are fetched; in several today's ABIs, variadic function argument passing don't use registers so is less efficient than fixed arity function calls.

In your case, you'll better have several functions . For inspiration sqlite has several open functions .

Notice that POSIX defines some function with one last optional argument, in particular open(2) . This is probably implemented as having a function whose last argument can be not supplied.

You could read the ABI specification and the calling conventions specific to your implementation.

BTW, lack of function overloading (like in C++) can be understood as an advantage of C (the function name -which is the only thing the linker care about, at least on Linux & Unix- determines its signature). Hence C++ practically needs name mangling . Read also about dynamic linking and dlopen

Answer 2

Check the man page for va_list, va_start, etc.

You declare your function prototype using the ... sequence to denote variable args:

void myfunction(FILE *, *fmt, ...);

For example, we declatre a struct of type va_list (assuming you included stdarg):

struct va_list l;

Initialise the list using the parameter BEFORE the ... - se we'd pass the *fmt pointer to va_start:

va_start(l, fmt);

Now, we can call va_arg to get a value...if we're processing a format string we might have a loop something like this:

while (*fmt) {
    /* Look for % format specifier... */
    if (*fmt == '%') {
        /* Following character can be c for char, i for int or f for float. */
        // Opps - advance to char following opening %...
        fmt++;

        if (*fmt == 'c')
            fprintf(stream, "%c", (char) va_arg(l, int));
        else if (*fmt == 'i')
            fprintf(stream, "%i", (int) va_arg(l, int));
        else if (*fmt == 'f')
            fprintf(stream, "%f", (float) va_arg(l, double));
        ... and so on...

You're relying on the character following the % to tell you what data type to expect in the corresponding parameter. Suggest you read the man pages for va_list.