Variable-Length Parameter Lists

Standard libarary header stdarg.h let you create variable length parameter lists.

Functions with variable-length parameter lists can be declared like this:

void foo(char *fmt, ...);

• The variable-length parameter list ... must be at the end of parameter list
• A named parameter must appear before the variable-length parameter list

Some macros are defined in the header to get the list:

#include <stdarg.h>
void foo (char *fmt, ...)
{
  va_list args;
  va_start (args, fmt);
}

• args is declared as a va_list variable that will be pointed to the start of list
• va_start(va_list args, last_fixed_parameter) points the args to the next parameter of last_fixed_parameter

To get the next argument in the list, use the va_arg():

type_t va_arg (va_list args, type_t);

#include <stdarg.h>
#include <stdio.h>
void foo (char *fmt, ...)
{
  va_list args;
  va_start (args, fmt);
  char *str;
  while (*fmt)
    {
      switch (*fmt++)
        {
          case '%':
            {
              if (*fmt == 's')
                {
	              str = va_arg(args, char *);
                  put_string(str);
                  fmt++;
                }
              break;
            }
          default: put(*fmt);
        }
    }
    va_end(args);
}

• va_args will
  • get the value of the current argument in the list being pointed to
  • increment the args and point it to the next value
• Undefined behaviour occurs when:
  • type_t mismatch with the argument being pointed to
  • Trying to access when there're no argument left
• So, it would be better if you pass in the total number of arguments, or know when to terminate (in the case of fmt_string)

In the end, use va_end(va_list args) to free the usage of va_list