Variadic functions

C

Reference manual

@see printf function

Variadic functions are a powerful feature in the C programming language that allows you to define functions that can accept a variable number of arguments. They are commonly used for functions like printf and scanf, which can handle different types and numbers of arguments.

Usage

Variadic functions in C are typically implemented using the <stdarg.h> header, which provides a set of macros and types to work with variable-length argument lists.

To create a variadic function, you define a function with a fixed number of parameters followed by an ellipsis (…) These fixed parameters are typically used to specify information about the variable arguments, such as the number or type of arguments.

#include <stdarg.h>

int myVariadicFunction(int fixedArg1, int fixedArg2, ...)
{
    // Function implementation
}

Inside the variadic function, you typically use the following macros and functions provided by <stdarg.h> to access the variable arguments:

• va_list: Type that represents the argument list, which is an internal data structure used to access the arguments passed to a variadic function.

• va_start: A macro that initializes the va_list for variable argument processing. It takes two arguments: the va_list itself and the name of the last known parameter in the function. The last known parameter serves as a reference point for accessing the variable arguments.

• va_arg: A macro that retrieves the next argument from the va_list with a specified data type.

• va_end: A macro that is used to clean up and release resources associated with the va_list. It should be called once you’re done processing variable arguments.

Example

In the following example average is a variadic function that receives a regular parameter named num which specifies how many parameters follow. The function asumes the argument list elements are of type double and uses them to calculate and return the average number.

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

double average(int num, ...)
{
    va_list args;
    double sum;

    sum = 0;
    va_start(args, num);
    for (int i = 0; i < num; i++)
    {
        sum += va_arg(args, double);
    }
    va_end(args);
    return (sum / num);
}

int main(void)
{
    printf("Average: %.2f\n", average(4, 2.5, 3.0, 4.5, 5.0));
    return (0);
}