Using Variable-Length Argument Lists (stdarg.h)

This lesson covers variable-length argument lists in C using stdarg.h, so you can write a function that accepts any number of arguments. It's for anyone searching "C variable arguments stdarg.h usage."

stdarg.h lets you build a function with a variable-length argument list — one that accepts any number of arguments. Think of it like "a lucky bag that fits any number of items." printf() itself is built on this same mechanism.

The example uses a sequence of macros — va_list, va_start, va_arg, and va_end — for the sum() function to pull out and total the given number of arguments in order. Notice the argument count is passed separately as count.

A common early mistake is not realizing the program has no mechanism to know the type or count of the variable arguments on its own. C's variable arguments aren't type-safe, so mismatching what the caller actually passes can lead to a serious bug.

In real projects, this mechanism is used for something like a logging function, where you want to handle values of varying type and count together.

📖 Reference code

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

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

int main(void) {
    printf("%d\n", sum(3, 1, 2, 3));
    printf("%d\n", sum(0));
    return 0;
}

✍️ Your code

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🧪 This site can't compile or run C directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).