The following example demonstrates how vlfmt() could be used to write an errlog() routine. The va_alist() macro is used as the parameter list in a function definition. The va_start(ap, ...) call, where ap is of type va_list, must be invoked before any attempt to traverse and access unnamed arguments. Calls to va_arg(ap, atype) traverse the argument list. Each execution of va_arg() expands to an expression with the value and type of the next argument in the list ap, which is the same object initialized by va_start(). The atype argument is the type that the returned argument is expected to be. The va_end(ap) macro must be invoked when all desired arguments have been accessed. The argument list in ap can be traversed again if va_start() is called again after va_end().) In the example below, va_arg() is executed first to retrieve the format string passed to errlog(). The remaining errlog() arguments (arg1, arg2, ...) are passed to vlfmt() in the argument ap.

#include <pfmt.h> #include <stdarg.h> /* * errlog should be called like * errlog(log_info, format, arg1, ...); */ void errlog(long log_info, ...) { va_list ap; char *format; va_start(ap, ); format = va_arg(ap, char *); (void) vlfmt(stderr, log_info|MM_ERROR, format, ap); va_end(ap); (void) abort(); }