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Dynamic argument list

Written by:shab
Published by:Nightscript
Published on:2004-01-05 00:31:04
Search OSI about C.More articles by shab.
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If your reletivly new to C/C++ you might not have noticed it, but im rather sure you have used it. Useful when your writing functions like for example printf.

This is not a very big subject really, it's give things we will look @ in this article. I will list them below, with an description below.

  • ...
  • va_list
  • va_start
  • va_arg
  • va_end

This is what you put in your function declaration, if you have seen printf it looks like this:
 int printf(const char *, ...);

The ... show where the dynamic arguments begins, it can never be first and it can never be static arguments after it, you will understand why later.

The control handle for the arguments, on most systems it's a void *, but some tend to still hold on to char * for some reason.

control handle is not correct acually, it is the memory address that the argument is in, I will explain this more in va_start, you make one like this(rather obvious).
va_list va; /* Replace va with your name */

This in a macro, so no prototype can be shown. But this is how you could use it for example:
 void a(char b, ...) {
 va_list va;

 va_start(va, b);

What va_start really does it that is takes the memory address of b(argument 2) and place it in va_list, hence it's a void *. That means the second argument should be the argument before the dynamic arguments begins.

Just like va_start this is a macro, this one is a simple one just now, I really want to show you a prototype, but im afraid that's rather impossible. But here is an code example.
void a(char b, ...) {
 char c;
 va_list va;

 va_start(va, b);
 c = va_arg(va, char); }

The first argument is just like va_start the memory address, the second is how much memory it should move, if you would place int there is would be four, it also "return"(hence it's a macro) that memory.

I havn't acually checked what this acually does, but it probably free the memory, you use it like this:
void a(char b, ...) {
 char c;
 va_list va;

 va_start(va, b);
 c = va_arg(va, char);
 va_end(va); }

One single argument, as simple as anything.

Here is little function that I made up for example, I just coded it out of thin air while writing this article so excuse any misbehaviour.
#include <stdarg.h> /* I don't think I said it but here is it declared. */
#include <stdio.h>

void print(char *one, ...) {
 va_list va;
 char *p;

 for(p = one; p; p = va_arg(va, char *))

 return; }

I hope that all works ok, otherwise it could be very embarassing.

Did you like this article? There are hundreds more.

2006-04-21 20:33:52
wtf?? Mindless illiterate ramblings?
2006-04-21 23:41:05
lol - it does look a bit like that doesnt it!
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