C/C++: Passing variable number of arguments around

Question

Say I have a C function which takes a variable number of arguments: How can I call another function which expects a variable number of arguments from inside of it, passing all the arguments that got into the first function?

Example:

void format_string(char *fmt, ...);

void debug_print(int dbg_lvl, char *fmt, ...) {
    format_string(fmt, /* how do I pass all the arguments from '...'? */);
    fprintf(stdout, fmt);
 }

Answer 1

See

• http://www.cplusplus.com/reference/clibrary/cstdarg/va_list.html
• http://bobobobo.wordpress.com/2008/01/28/how-to-use-variable-argument-lists-va_list
• http://en.wikipedia.org/wiki/Stdarg.h
• http://en.wikipedia.org/wiki/Stdarg.h#.3Cvarargs.h.3E
• http://www.delorie.com/gnu/docs/glibc/libc_675.html

Answer 2

To pass the ellipses on, you have to convert them to a va_list and use that va_list in your second function. Specifically;

void format_string(char *fmt,va_list argptr );


void debug_print(int dbg_lvl, char *fmt, ...) 
{    
 va_list argptr;
 va_start(argptr,fmt);
 format_string(fmt, argptr);
 va_end(argptr);
 fprintf(stdout, fmt);
}

Answer 3

There's no way of calling (eg) printf without knowing how many arguments you're passing to it, unless you want to get into naughty and non-portable tricks.

The generally used solution is to always provide an alternate form of vararg functions, so printf has vprintf which takes a va_list in place of the .... The ... versions are just wrappers around the va_list versions.