Variadic arguments (stdarg.h)

Include stdarg.h to read arguments passed through an ellipsis parameter list.

Types and Macros

Name	Meaning
va_list	Variadic argument cursor that walks the caller's stack frame.
va_start	Start reading variadic arguments after the named parameter last.
va_arg	Fetch the next variadic argument as type.
va_end	Finish reading variadic arguments.

Runtime model

va_list is a char * cursor walking the caller's stack frame. Start it with va_start, fetch each argument with va_arg, and finish with va_end.

The DCC C Compiler uses 16-bit int and pointer arguments, 32-bit long and float arguments, and no double. Pass the exact promoted type to va_arg: use int for narrow integer types after default promotions, long for long values, and float for floating values on this target.

Variadic functions

The last named parameter passed to va_start must be the parameter immediately before the ellipsis:

#include <stdarg.h>

int sum(int count, ...)
{
    va_list ap;
    int total = 0;
    int i;

    va_start(ap, count);
    for (i = 0; i < count; ++i)
        total += va_arg(ap, int);
    va_end(ap);
    return total;
}

A va_list can be forwarded straight to vprintf, vfprintf, or vsprintf to build printf-style wrappers without re-parsing the arguments. See Console and file I/O for the supported formatted-output subset and the compiler options required for long or float formatting.

#include <stdarg.h>
#include <stdio.h>

void log_msg(const char *fmt, ...)
{
    va_list ap;

    va_start(ap, fmt);
    vprintf(fmt, ap);
    va_end(ap);
}