Forward an invocation of a variadic function in C

If you don't have a function analogous to vfprintf that takes a va_list instead of a variable number of arguments, you can't do it. See http://c-faq.com/varargs/handoff.html.

Example:

void myfun(const char *fmt, va_list argp) {    vfprintf(stderr, fmt, argp);}

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 <stdarg.h>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.

C99 supports macros with variadic arguments; depending on your compiler, you might be able to declare a macro that does what you want:

#define my_printf(format, ...) \    do { \        fprintf(stderr, "Calling printf with fmt %s\n", format); \        some_other_variadac_function(format, ##__VA_ARGS__); \    } while(0)

In general, though, the best solution is to use the va_list form of the function you're trying to wrap, should one exist.