how to use elipsis (...) [duplicate] - c

Question

This question already has answers here:
How to pass variable number of arguments to printf/sprintf
(7 answers)
Closed 7 years ago.
i want to write a function in c which uses the elipsis (...) argument, but i have no idea how it works.
i want to do something like this:
void error(const char* fmt, ...);
void error(const char* fmt, ...) {
// fprintf(stderr, fmt, ...); << didnt work!
fprintf(stderr, fmt, /* ??? */);
}
i want to use it like a "normal" printf() call.
error("bla");
error("nr: %d", 42);
error("pi: %f", 3.1415);
how can i access the elipsis as hole thing and pass it to the next function?

Answer 1

The ellipsis does not constitute a "pack" in any way that you can handle or forward directly. The only way you can manage function arguments that don't match any function parameters is via the <stdarg.h> features.
This means that for every variable function foo you should also always have a corresponding function vfoo that consumes a va_list. For example:
#include <stdarg.h>
void foo(const char * fmt, ...);
void vfoo(const char * va_list ap);
The former is typically implemented in terms of the latter:
void foo(const char * fmt, ...)
{
va_list ap;
va_start(ap, fmt);
vfoo(fmt, ap);
va_end(ap);
}
Luckily, printf family of functions follows these rules. So when you want to farm out to a printf function, you actually use the corresponding underlying vprintf version:
void error(const char * fmt, ...)
{
do_stuff();
va_list ap;
va_start(ap, fmt);
vfprintf(stderr, fmt, ap);
va_end(ap);
}
Of course following the same rules you should write a verror function first and make that one call vprintf:
void verror(const har * fmt, va_list ap)
{
do_stuff();
vfprintf(stderr, fmt, ap);
}

Answer 2

If I understand your question properly, what you're looking for is information on variable arguments or a Variadic Function. Under C you will want to research the 'stdargs.h' header and associated manual page. Here is a simple example that takes an arbitrary number of integers and returns the average.
#include <stdarg.h>
float average(int v, ...)
{
va_list args;
int i = 0;
int num = 0;
va_start(args, v);
for (; v; v--){
i += va_arg(args, int);
num++;
}
va_end(args);
return (float)i/num;
}

Answer 3

See stdarg.h for dealing with functions with variable arguments.
For the specific case of simply passing the variable number of arguments to fprintf, there is vfprintf for that, e.g.,
void error (const char *fmt, ...) {
va_list args;
va_start(args, fmt);
vfprintf(stderr, fmt, args);
va_end(args);
}
For such a simple case you may also consider variadic macros (introduced in C99), e.g.,
#define error(fmt, ...) fprintf(stderr, "Error: " fmt, __VA_ARGS__)