C - passing variable arguments

Question

I'm writing essentially an implementation of printf. I want it so you can pass many strings within strings, e.g.:

kprintf("Hello %s", "Goodbye %s", "Farewell\n");

Don't ask why, it may very well be insane. Anyway, it recursively calls itself after looking for how many % characters are in the string it is passing; so if more than one % is in it, it should pass many arguments. However, I've been unable to achieve this with any elegance, see the below one:

case 1:
    kprintf(str, argus[0]);
break;
case 2:
    kprintf(str, argus[0], argus[1]);
break;
case 3:
    kprintf(str, argus[0], argus[1], argus[2]);
break;
case 4:
    kprintf(str, argus[0], argus[1], argus[2], argus[3]);
break;
case 5:
    kprintf(str, argus[0], argus[1], argus[2], argus[3], argus[4]);
break;

With argus as an array of pointers. This works, but I hate it. Is there a way to pass variable arguments to a function ?

What about kprintf("%d %s", 47, "%u", 11);, should that work like Python's "%d %s" % (47, "%u" % 11) and print 47 11? If so, good luck! :) — unwind
– unwind, Commented Feb 20, 2013 at 11:13
What if you write a version that takes in a va_arg pointer (like vprintf does)? — slugonamission
– slugonamission, Commented Feb 20, 2013 at 11:17

bash.d · Accepted Answer · 2013-02-20 11:18:14Z

3

There is a type to do your work. It is called "va_list" and you can feed it with arguments (4096 at most, I guess). In order to use it, include starg.h-header and you'll need to use ellipses ... in your function declaration.

int kprintf(int count, ...){
va_list vl;
va_start(vl,count);
for (i=0;i<n;i++)
{
val=va_arg(vl,double);
printf (" [%.2f]",val);
}
va_end(vl);
}

See here for an example.

answered Feb 20, 2013 at 11:18

bash.d

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2 Comments

Losiowaty Over a year ago

Bear in mind, that you have to know argument type when you are retrieveing it.

bash.d Over a year ago

@Losiowaty yep, absolutely true

888 · Accepted Answer · 2013-02-20 11:17:14Z

0

If you can use C++, here you can find an article about variable argument functions, with downloadable source code, which should solve your problem.

answered Feb 20, 2013 at 11:17

888

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Comments

Mats Petersson · Accepted Answer · 2013-02-20 11:41:23Z

0

While it would be possible to write something that parses %<something> inside the strings passed to %s, it is generally not how it is done in C/C++.

If you want to put a formatted string into a printf statement, you use sprintf() to format the first string (in a temporary buffer), then pass that string to the actual printf function. Yes, that involves an extra step, but it makes the code much easier - if you are parsing strings passed into printf, you pretty much would have to make printf recursive [or have some sort of stack inside the printf, to track what you are doing].

answered Feb 20, 2013 at 11:41

Mats Petersson

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