Forward an invocation of a variadic function in C

Question

In C, is it possible to forward the invocation of a variadic function? As in,

int my_printf(char *fmt, ...) {
    fprintf(stderr, \"Calling printf with fmt %         


        

Answer 1

Not directly, however it is common (and you will find almost universally the case in the standard library) for variadic functions to come in pairs with a varargs style alternative function. e.g. printf/vprintf

The v... functions take a va_list parameter, the implementation of which is often done with compiler specific 'macro magic', but you are guaranteed that calling the v... style function from a variadic function like this will work:

#include 

int m_printf(char *fmt, ...)
{
    int ret;

    /* Declare a va_list type variable */
    va_list myargs;

    /* Initialise the va_list variable with the ... after fmt */

    va_start(myargs, fmt);

    /* Forward the '...' to vprintf */
    ret = vprintf(fmt, myargs);

    /* Clean up the va_list */
    va_end(myargs);

    return ret;
}

This should give you the effect that you are looking for.

If you are considering writing a variadic library function you should also consider making a va_list style companion available as part of the library. As you can see from your question, it can be prove useful for your users.