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Dynamic argument list



Written by:shab
Published by:Nightscript
Published on:2004-01-05 00:31:04
Topic:C
Search OSI about C.More articles by shab.
 viewed 17166 times send this article printer friendly

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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.

va_list
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 */


va_start
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.

va_arg
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.

va_end
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.

Example
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 *))
  puts(p);
 va_end(va);

 return; }


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

Did you like this article? There are hundreds more.

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