How Can I Pass Variable Arguments from One Function to Another Without Modifying the Receiving Function?

Passing Variable Arguments to Functions with Variable Argument Lists

Consider two functions with similar variable arguments:

void example(int a, int b, ...);
void exampleB(int b, ...);

Suppose example calls exampleB, but without modifying exampleB (which is used elsewhere). How can we pass the variable arguments from example to exampleB?

Solution:

Directly passing arguments is not possible. Instead, we create a wrapper function that takes a va_list:

#include <stdarg.h>
static void exampleV(int b, va_list args);

Modified Function exampleA:

void exampleA(int a, int b, ...)    // Renamed for consistency
{
    va_list args;
    do_something(a);                 // Use argument a somehow
    va_start(args, b);
    exampleV(b, args);
    va_end(args);
}

Unmodified Function exampleB:

void exampleB(int b, ...)
{
    va_list args;
    va_start(args, b);
    exampleV(b, args);
    va_end(args);
}

Wrapper Function exampleV:

static void exampleV(int b, va_list args)
{
    // Whatever `exampleB` was intended to do...
    // Except it doesn't call either `va_start` or `va_end`.
}

This approach allows us to pass the variable arguments from example to exampleB without modifying the latter. The wrapper function exampleV does not call va_start or va_end, which ensures consistency with the existing usage of exampleB.

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