stdarg Macro 关于可变参数

#include<stdio.h>


#include<stdarg.h>




double average(int val,...)




...{


va_list var_arg;


int count;


double sum=0.0;




/** *//**prepare to visit variable arguments**/


va_start(var_arg,val);






/** *//** Add value from variable list**/


for(count=0;count<val;count+=1)




...{


sum+=va_arg(var_arg,int);


}




/** *//** Finish processing variables**/


va_end(var_arg);


return sum/val;


}






int main()




...{


double dval=0.0;


dval=average(6,1,2,3,4,5,6);


printf("%f ",dval);






return 0;


}

NAME

stdarg - variable argument lists

SYNOPSIS

#include <stdarg.h>

void va_start(va_list ap, last);
type va_arg(va_list ap, type);
void va_end(va_list ap);
void va_copy(va_list dest, va_list src);

DESCRIPTION

A  function may be called with a varying number of arguments of varying
types.  The include file stdarg.h declares a type va_list  and  defines
three  macros for stepping through a list of arguments whose number and
types are not known to the called function.

The called function must declare an object of  type  va_list  which  is
used by the macros va_start, va_arg, and va_end.

va_start
The  va_start  macro  initializes  ap  for subsequent use by va_arg and
va_end, and must be called first.

The parameter last is the name of the last parameter before  the  vari-
able argument list, i.e., the last parameter of which the calling func-
tion knows the type.

Because the address of this parameter  may  be  used  in  the  va_start
macro,  it should not be declared as a register variable, or as a func-
tion or an array type.

va_arg
The va_arg macro expands to an expression that has the type  and  value
of  the  next argument in the call.  The parameter ap is the va_list ap
initialized by va_start.  Each call to va_arg modifies ap so  that  the
next call returns the next argument.  The parameter type is a type name
specified so that the type of a pointer to an object that has the spec-
ified type can be obtained simply by adding a * to type.

The  first  use  of  the  va_arg macro after that of the va_start macro
returns the argument after last.   Successive  invocations  return  the
values of the remaining arguments.

If  there  is  no  next argument, or if type is not compatible with the
type of the actual next argument (as promoted according to the  default
argument promotions), random errors will occur.

If  ap is passed to a function that uses va_arg(ap,type) then the value
of ap is undefined after the return of that function.

va_end
Each invocation of va_start must be matched by a corresponding  invoca-
tion  of  va_end  in  the  same function. After the call va_end(ap) the
variable ap is undefined.  Multiple  transversals  of  the  list,  each
(of length 1), and there one needs
va_list aq;
*aq = *ap;
Finally, on systems where parameters are passed in registers, it may be
necessary for va_start to allocate memory, store the parameters  there,
and  also  an indication of which parameter is next, so that va_arg can
step through the list. Now va_end can free the allocated memory  again.
To  accommodate  this  situation, C99 adds a macro va_copy, so that the
above assignment can be replaced by
va_list aq;
va_copy(aq, ap);
...
va_end(aq);
Each invocation of va_copy must be matched by a  corresponding  invoca-
tion  of  va_end in the same function.  Some systems that do not supply
va_copy have __va_copy instead, since that was the  name  used  in  the
draft proposal.

EXAMPLES

The function foo takes a string of format characters and prints out the
argument associated with each format character based on the type.
#include <stdio.h>
#include <stdarg.h>

void foo(char *fmt, ...) {
va_list ap;
int d;
char c, *p, *s;

va_start(ap, fmt);
while (*fmt)
switch(*fmt++) {
case 's':           /* string */
s = va_arg(ap, char *);
printf("string %s/n", s);
break;
case 'd':           /* int */
d = va_arg(ap, int);
printf("int %d/n", d);
break;
case 'c':           /* char */
/* need a cast here since va_arg only
takes fully promoted types */
c = (char) va_arg(ap, int);
printf("char %c/n", c);
break;
}
va_end(ap);
}

CONFORMING TO

The va_start, va_arg, and va_end macros  conform  to  ANSI  X3.159-1989
(``C89'').  C99 defines the va_copy macro.

COMPATIBILITY

These  macros are not compatible with the historic macros they replace.
A backward  compatible  version  can  be  found  in  the  include  file
va_start(ap);
while(...) {
...
x = va_arg(ap, type);
...
}
va_end(ap);
}
On  some  systems,  va_end  contains  a  closing  '}' matching a '{' in
va_start, so that both macros must occur in the same function, and in a
way that allows this.

BUGS

Unlike  the varargs macros, the stdarg macros do not permit programmers
to code a function with no fixed  arguments.   This  problem  generates
work  mainly  when  converting varargs code to stdarg code, but it also
creates difficulties for variadic functions that wish to  pass  all  of
their arguments on to a function that takes a va_list argument, such as
vfprintf(3).

from: http://ccrma.stanford.edu/planetccrma/man/man3/stdarg.3.html