Variadic function

tip

man 3 stdarg

Declaration

To declare a function with a variable number of arguments, use the ... notation:

#include <stdarg.h>

void my_function(int n, ...);

stdarg.h declares a type va_list and defines three macros for stepping through a list of arguments.

va_start

void va_start(va_list ap, last);

It initializes the va_list ap. last is the last argument of which the calling function knows the type.

va_end

void va_end(va_list ap);

This must be called after va_start() and your treatment.

va_arg

type va_arg(va_list ap, type);

Each call to va_arg() modifies ap so that the next call returns the next argument.

The argument will have the same type as the given type.

Number of arguments

• It is not possible to know how many arguments have been passed to the function.
• If va_arg is called after the last argument, this will lead to an undefined behavior.

Example

#include <stdarg.h>
#include <stdio.h>

void print_unsigned(int nb, ...)
{
    unsigned int i;
    va_list      ap;

    va_start(ap, nb); // Initialization.

    for (int n = 0; n < nb; ++n)
    {
        i = va_arg(ap, unsigned); // Get an unsigned int.
        printf("%u ", i);
    }

    va_end(ap); // Set ap to undefine.
}

int main(void)
{
    print_unsigned(4, 1, 2, 3, 4);
    putchar('\n');
    print_unsigned(6, -1, 2, 3, 4, 5, 6);
    return 0;
}