Appendix A printf, varargs and stdarg A.3 stdarg.h ANSI version of varargs.h

A.3 stdargs.h: ANSI version of varargs.h
Functions with variadic argument lists whose first argument's type is virtually unchanged from call to call.The main difference between varargs.h and stdargs.h comes from this fact.A function like printf can determine the type of its second argument by examining its first argument.However, we cannot find any information from the parameter list that allows us to determine the type of the first parameter.Therefore, functions using stdarg.h must have at least one argument of fixed type, which can be followed by an unknown number of arguments of unknown type.
void error(char *, ...);
Use the function of stdarg.h to directly declare its fixed parameters, and use the last fixed parameter as the parameter of the va_start macro, that is, use the fixed parameter as the basis for the variable parameter.
The definition of error is as follows:
#include
#include

void error(char *format, ...) {
va_list ap;
va_start(ap, format);
fprintf(stderr, "error: ");
vfprintf(stderr, format, ap);
va_end(ap);
fprintf(stderr, "\n");
exit(1);
}
In this example, there is no need to use the va_arg macro, so the format string here belongs toFixed part of the parameter list.
#include
int printf(char *format, ...) {
va_list ap;
int n;
va_start(ap, format);
n = vprintf(format, ap);
va_end(ap);
Return n;
}