Passing va_list to other functions

Question

I have been trying to pass variable arguments to other function in C but it is producing inconsistent result in different runtime environment as well as in different runs in same environment:

int main() 
{ 
    int result = myprintf("Something \n %d", 9); 
    return result;
}

int myprintf(const char *format, ...){
    printf("Something \n %d", 9); 
    printf("\n");
    va_list args;        
    va_start(args, format);    
    int result = printf(format,args);
    printf("\n");
    va_end(args);
    return result;
} 

And the result produced is:

WWW.FIRMCODES.COM 
 9
WWW.FIRMCODES.COM 
 438656664

I could not find the reason for "438656664".

Answer 1

You cannot pass the variadic arguments to a variadic function. Instead, you must call a function that takes a va_list as argument. The standard library provides variants of printf and scanf that take a va_list; their names have the prefix v.

Your example should look like:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <stdarg.h>

int printfln(const char *format, ...)
{
    int result;
    va_list args;

    va_start(args, format);
    result = vprintf(format, args);
    printf("\n");
    va_end(args);

    return result;
}

int main()
{
    int result = printfln("Something \n %d", 9);

    printf("(%d)\n", result);

    return 0;
}

There are some gotchas, for example when you want to call two v... function for printing to the screen and a log file: The v... function may exhaust the va_list, so you must pass in a fresh one to each call if your code should be portable.

Answer 2

For the C++ fellow also reading this. You can actually do it using pack expansion without using vprintf. This trick is quite handy when you need to wrap a method that takes the ellipsis (...) and not a va_list.

For instance:

template <class ... Args>
void foo(const char *format, Args ... args)
{
    printf(format, args...);
}

Here class ... Args is template parameter pack, Args ... args is function parameter pack, and args... is function parameter pack expansion.