How to Use Variable Arguments in C: Syntax and Example

In C, you use variable arguments by including <stdarg.h> and defining a function with an ellipsis ... in its parameters. Use va_start, va_arg, and va_end macros to access the extra arguments inside the function.

Syntax

To define a function that accepts variable arguments, use an ellipsis ... after fixed parameters. Inside the function, use va_list to declare a variable for argument access. Then use va_start to initialize it, va_arg to retrieve each argument, and va_end to clean up.

Example syntax:

void functionName(type fixedArg, ...) {
    va_list args;
    va_start(args, fixedArg);
    // Access arguments with va_arg
    va_end(args);
}

c

void functionName(type fixedArg, ...) {
    va_list args;
    va_start(args, fixedArg);
    // Access arguments with va_arg
    va_end(args);
}

Example

This example shows a function sum that takes a count of numbers followed by that many integers. It adds all the integers and returns the total.

c

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

int sum(int count, ...) {
    va_list args;
    int total = 0;
    va_start(args, count);
    for (int i = 0; i < count; i++) {
        total += va_arg(args, int);
    }
    va_end(args);
    return total;
}

int main() {
    int result = sum(4, 10, 20, 30, 40);
    printf("Sum is %d\n", result);
    return 0;
}

Output

Sum is 100

Common Pitfalls

• Not calling va_end after finishing argument access can cause undefined behavior.
• Using va_arg with the wrong type leads to incorrect values or crashes.
• Forgetting to pass the fixed argument before ... in the call makes va_start fail.
• Variable arguments do not have type safety, so be careful to know the types and count.

c

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

// Wrong: missing va_end
int wrong_sum(int count, ...) {
    va_list args;
    int total = 0;
    va_start(args, count);
    for (int i = 0; i < count; i++) {
        total += va_arg(args, int);
    }
    // va_end(args); // Missing cleanup
    return total;
}

// Correct usage
int correct_sum(int count, ...) {
    va_list args;
    int total = 0;
    va_start(args, count);
    for (int i = 0; i < count; i++) {
        total += va_arg(args, int);
    }
    va_end(args);
    return total;
}

Quick Reference

Macro	Purpose
va_list	Type to hold variable argument list
va_start(args, last_fixed_arg)	Initialize args to start after last fixed argument
va_arg(args, type)	Retrieve next argument of given type
va_end(args)	Clean up the argument list

Key Takeaways

Use macros va_start, va_arg, and va_end to handle variable arguments in C.

Always call va_end after finishing to avoid undefined behavior.

Know the number and types of arguments to safely retrieve them with va_arg.

Variable arguments lack type checking, so use carefully with fixed parameters indicating count or types.