Variable argument lists in C

So I've been plagued by a concept for a while, and finally decided to take the time to solve the issue. The problem was that I had a function which took a variable argument list, and I wanted to pass that list along to another function which took a variable argument list.

For instance, I wanted to write a function that would take a variable list, use sprintf to print the list into a buffer, and then call OutputDebugString to print it out. The trouble is -- how do you pass the variable argument list on?

For starters, let's peek at the first attempt of our C function

static void DebugLog( const char *msg, ... )

{

	char buffer[1024] = { 0 };

	sprintf_s( buffer, _countof( buffer ), msg, ???? );


	::OutputDebugStringA( buffer );

}

For those of you who need a refresher course, the ... in the function declaration is C's way of specifying a variable argument list (or ParamArray, in RB). The question is: what do you put in place of the ???? to make the call to sprintf_s work properly?

For starters, you cannot simply pass ... in there -- it simply will not compile. Which makes sense as ... is simply a magical place holder for a va_list, of sorts.

So that got me thinking -- perhaps I could iterate over the list of arguments (as a va_list) and somehow rebuild the list. However, that leaves the same problem: what do you pass? Then I realized that ... was just a special form of va_list, so perhaps I could simply declare the function as taking a va_list, like this:

static void DebugLog( const char *msg, va_list list )

and then pass list along to sprintf_s. The code would compile by itself, but when I attempted to actually *use* the va_list by calling DebugLog, it would no longer compile. That also makes sense because the method's signature only has two parameters, so anything beyond two parameters is not legal.

Ho hum. Then it dawned on me -- I simply needed to convert the ... into a va_list, then I could use the variadic version of sprintf_s! You see, there is another form of sprintf_s, called vsprintf_s, whose last parameter receives a va_list. And it never dawned on me before that this was how to pass along a variable arguments list in C. So now it was just a simple matter of turning ... into a va_list, which is something trivial (using va_start). So now the function looks like this:

static void DebugLog( const char *msg, ... )

{

	va_list va;

	va_start( va, msg );


	char buffer[1024] = { 0 };

	vsprintf_s( buffer, _countof( buffer ), msg, va );

	va_end( va );

	::OutputDebugStringA( buffer );

}

And it works like a charm. So that's how you pass along a variable arguments list -- declare the second function as receiving a va_list and you can use this paradigm to pass the variable arguments along.

Neat, eh?