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ANSI C provides macros to support for accessing variable arguments of a function. These macros are va_start, va_arg, va_end.

Which include file to use:
stdarg.h

APIs:
void va_start(va_list ap, last);
type va_arg(va_list ap, type);
void va_end(va_list ap);
void va_copy(va_list dest, va_list src);

Here is the man page of stdarg

How to declare a variable arguments function:
int varargsfunction(int numargs, …);
“…” three dots at the end signifies variable arguments.

How to call variable argument functions:
here are some examples..

varargsfunction(3, “aa”, “bb”, “cc”);

varargsfunction(”%s%d%s”, “aa”, 22, “cc”);

varargsfunction(”aa”, “bb”, “cc”, NULL);

How to recognize number of arguments, variable types in function while accessing them:
For accessing and processing variables we require basically
          How many arguments passed
          Types of arguments

Here are the methods to identify number of arguments and type of arguments
1)if you know the variable types you are passing then mentioning number of arguments while calling variable argument function would be sufficient. Consider an example where you are passing a string then all integers

4ff2466d3b04ca34348b4d495d4cbfbc000

You can call this function as something like
   stdargs_function(4, “ccc”, 23,43,45);

2)if you know the types of variables you are passing then to identify number of arguments passing, terminate the argument list with NULL.

int stdargs_function(int msg, ...)
{
  char *msg;
  va_list ap;
 
  va_start(ap, numargs);
  for (; msg != NULL;)
  {
    msg = va_arg(ap, char *);
  }
  va_end(ap);
  return 0;
}

You can call this function as something like
stdargs_function(”ccc”, “dd”, NULL);

3)make available number of arguments and type arguments available in first argument (ex. printf function)

int stdargs_function(int format_string, ...)
{
  char *msg;
  va_list ap;
 
  numargs = GetNumArgs(format_string);
  va_start(ap, format_string);
  for (i=0;i<numargs; i++)
  {
    type = GetNextFormatSpecifier(format_string);
    switch(type)
    {
      case 'd':
        msg = va_arg(ap, char *);
        break;
      case 's':
        msg = va_arg(ap, int);
        break;
    }
  }
  va_end(ap);
}

You can call this function as something like
stdargs_function(”%d%s”, 23, “dd”);

Is there any other implementation other than stdarg.h:
Yes, varargs.h was used before ANSI C. varargs.h is obsolete now. Varargs usage is slightly different
Than stdargs usage. Stdargs require at least one argument need to be mentioned before using variable args, varargs doesn’t mandate this.

An Example Program:

#include <stdio.h>
#include <stdarg.h>
 
int print(int max, ...);
int main(void)
{
  print(2, "OpenAsthra", "formula pattern");
  print(1, "we are done with this program");
  return 0;
}
 
int print(int numargs, ...)
{
  va_list ap;
  int args = 0;
  char *strings[10];
 
  va_start(ap, numargs);
 
  while(args < numargs)
  {
    strings[args] = va_arg(arg_ptr, char *);
    printf("%s\n";, strings[args++]);
  }
  va_end(ap);
}

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This entry was written by ponnada and posted on January 22, 2007 at 2:09 pm and filed under C Tidbits, C/C++ Programming Concepts. Bookmark the permalink. Follow any comments here with the RSS feed for this post. Post a comment or leave a trackback: Trackback URL.