How to Pass Variable Arguments Between C Functions with Variable Argument Lists?

How to Pass Variable Arguments between Functions with Variable Argument Lists

Problem:

You have two functions, example() and exampleB(), with similar variable argument lists:

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

You need to call example() from within exampleB() without modifying the latter's variable argument list, as it is used elsewhere.

Solution:

Unfortunately, there is no direct method to pass variable arguments in C. To achieve this, you must define a helper function that takes a va_list parameter. Here's an example:

#include <stdarg.h>

static void exampleV(int b, va_list args);

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);
}

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

static void exampleV(int b, va_list args)
{
    ...whatever you planned to have exampleB do...
    ...except it calls neither va_start nor va_end...
}

In this setup:

• exampleV() is the helper function that takes a va_list argument.
• do_something(a) is an example of using the a argument within exampleA().
• exampleV() is called from both exampleA() and exampleB() with the corresponding variable argument list.
• exampleV() performs the operations originally intended for exampleB() but does not call va_start() or va_end().

By adding exampleV() as an intermediary, you can pass the variable arguments to exampleB() without modifying its original implementation.

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