C Programming: Forward variable argument list

Question

I\'m trying to write a function that accepts a variable number of parameters like printf, does some stuff, then passes the variable list to printf. I\'m not sure how to do t

Answer 1

Don't pass the results to printf. pass them to vprintf. vprintf specifically exists to handle passing in va_list arguments. From the Linux man page:

#include <stdio.h>

int printf(const char *format, ...);
int fprintf(FILE *stream, const char *format, ...);
int sprintf(char *str, const char *format, ...);
int snprintf(char *str, size_t size, const char *format, ...);

#include <stdarg.h>

int vprintf(const char *format, va_list ap);
int vfprintf(FILE *stream, const char *format, va_list ap);
int vsprintf(char *str, const char *format, va_list ap);
int vsnprintf(char *str, size_t size, const char *format, va_list ap);

Notice how the latter explicitly take va_list arguments such as the ones you declare inside a function taking ... in the parameter list. So your function would be declared like this:

void forward_args( const char *format , ... ){
   va_list arglist;
   va_start( arglist, format );
   vprintf( format, arglist );
   va_end( arglist );
}

Answer 2

I'm not (off the top of my head) familiar with how to implement this. I would suggest looking at an implementation of functions like printf. Several open source implementations exist. glibc, uclibc (not sure what bsd and opensolaris call/use for their libc).

Answer 3

I'm pretty sure you're looking for va_start() / vprintf() / vsnprintf() / va_end(), there's no need to implement these yourself.

Answer 4

You'll be passing along the arglist value to a function designed to consume it. See the stdarg and vprintf documentation for more clues.