C语言不定参数的实现
C语言在定义函数参数时，允许参数的使用数量可变，这在C语言中称为 可变参数函数（variadic function） 当然在C标准库中不乏可变参数函数的使用，例如：
C标准函数 printf 的声明方式为：int printf(const char *fmt, …);
C标准函数 scanf 的声明方式为：int scanf(const char * fmt, …);
它们有共同的特点：
- 函数都有一个强制参数
- 后面跟着的参数的个数和类型是可变的（用三个点“…”做参数占位符）
在使用C语言不定参数时需要包含C标准库 <stdarg.h> ，该标准库文件提供了实现可变参数功能的函数和宏，其中主要使用的几个函数和宏定义如下：
-
va_list：用来保存宏va_start1、va_arg和va_end所需信息的一种类型。为了访问变长参数列表中的参数，必须声明 va_list 类型的一个对象。
-
va_start: 访问变长参数列表中的参数之前使用的宏，它初始化用 va_list 声明的对象，初始化结果供宏 va_arg 和 va_end 使用；
-
va_arg: 展开成一个表达式的宏，该表达式具有变长参数列表中下一个参数的值和类型。每次调用 va_arg 都会修改用 va_list 声明的对象，从而使该对象指向参数列表中的下一个参数；
-
va_end: 该宏使程序能够从变长参数列表用宏 va_start 引用的函数中正常返回。
C标准库 <stdarg.h>
可使用不同的编译器打开C标准库 <stdarg.h> 进行查看，可能不同平台以及不同版本存在些许差异，但效果是一样的。
在Visual Studio 2019打开标准库 <stdarg.h> 后代码如下：
//
// stdarg.h
//
// Copyright (c) Microsoft Corporation. All rights reserved.
//
// The C Standard Library <stdarg.h> header.
//
#pragma once
#define _INC_STDARG
#include <vcruntime.h>
#pragma warning(push)
#pragma warning(disable: _VCRUNTIME_DISABLED_WARNINGS)
_CRT_BEGIN_C_HEADER
#define va_start __crt_va_start
#define va_arg __crt_va_arg
#define va_end __crt_va_end
#define va_copy(destination, source) ((destination) = (source))
_CRT_END_C_HEADER
#pragma warning(pop) // _VCRUNTIME_DISABLED_WARNINGS
在DevC++中打开如下：
/* Copyright (C) 1989, 1997, 1998, 1999, 2000 Free Software Foundation, Inc.
This file is part of GCC.
GCC is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2, or (at your option)
any later version.
GCC is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with GCC; see the file COPYING. If not, write to
the Free Software Foundation, 51 Franklin Street, Fifth Floor,
Boston, MA 02110-1301, USA. */
/* As a special exception, if you include this header file into source
files compiled by GCC, this header file does not by itself cause
the resulting executable to be covered by the GNU General Public
License. This exception does not however invalidate any other
reasons why the executable file might be covered by the GNU General
Public License. */
/*
* ISO C Standard: 7.15 Variable arguments <stdarg.h>
*/
#if defined(__GNUC__)
#ifndef _STDARG_H
#ifndef _ANSI_STDARG_H_
#ifndef __need___va_list
#define _STDARG_H
#define _ANSI_STDARG_H_
#endif /* not __need___va_list */
#undef __need___va_list
/* Define __gnuc_va_list. */
#ifndef __GNUC_VA_LIST
#define __GNUC_VA_LIST
typedef __builtin_va_list __gnuc_va_list;
#endif
/* Define the standard macros for the user,
if this invocation was from the user program. */
#ifdef _STDARG_H
#define va_start(v,l) __builtin_va_start(v,l)
#define va_end(v) __builtin_va_end(v)
#define va_arg(v,l) __builtin_va_arg(v,l)
#if !defined(__STRICT_ANSI__) || __STDC_VERSION__ + 0 >= 199900L || defined(__GXX_EXPERIMENTAL_CXX0X__)
#define va_copy(d,s) __builtin_va_copy(d,s)
#endif
#define __va_copy(d,s) __builtin_va_copy(d,s)
/* Define va_list, if desired, from __gnuc_va_list. */
/* We deliberately do not define va_list when called from
stdio.h, because ANSI C says that stdio.h is not supposed to define
va_list. stdio.h needs to have access to that data type,
but must not use that name. It should use the name __gnuc_va_list,
which is safe because it is reserved for the implementation. */
#ifdef _HIDDEN_VA_LIST /* On OSF1, this means varargs.h is "half-loaded". */
#undef _VA_LIST
#endif
#ifdef _BSD_VA_LIST
#undef _BSD_VA_LIST
#endif
#if defined(__svr4__) || (defined(_SCO_DS) && !defined(__VA_LIST))
/* SVR4.2 uses _VA_LIST for an internal alias for va_list,
so we must avoid testing it and setting it here.
SVR4 uses _VA_LIST as a flag in stdarg.h, but we should
have no conflict with that. */
#ifndef _VA_LIST_
#define _VA_LIST_
#ifdef __i860__
#ifndef _VA_LIST
#define _VA_LIST va_list
#endif
#endif /* __i860__ */
typedef __gnuc_va_list va_list;
#ifdef _SCO_DS
#define __VA_LIST
#endif
#endif /* _VA_LIST_ */
#else /* not __svr4__ || _SCO_DS */
/* The macro _VA_LIST_ is the same thing used by this file in Ultrix.
But on BSD NET2 we must not test or define or undef it.
(Note that the comments in NET 2's ansi.h
are incorrect for _VA_LIST_--see stdio.h!) */
#if !defined (_VA_LIST_) || defined (__BSD_NET2__) || defined (____386BSD____) || defined (__bsdi__) || defined (__sequent__) || defined (__FreeBSD__) || defined(WINNT)
/* The macro _VA_LIST_DEFINED is used in Windows NT 3.5 */
#ifndef _VA_LIST_DEFINED
/* The macro _VA_LIST is used in SCO Unix 3.2. */
#ifndef _VA_LIST
/* The macro _VA_LIST_T_H is used in the Bull dpx2 */
#ifndef _VA_LIST_T_H
/* The macro __va_list__ is used by BeOS. */
#ifndef __va_list__
typedef __gnuc_va_list va_list;
#endif /* not __va_list__ */
#endif /* not _VA_LIST_T_H */
#endif /* not _VA_LIST */
#endif /* not _VA_LIST_DEFINED */
#if !(defined (__BSD_NET2__) || defined (____386BSD____) || defined (__bsdi__) || defined (__sequent__) || defined (__FreeBSD__))
#define _VA_LIST_
#endif
#ifndef _VA_LIST
#define _VA_LIST
#endif
#ifndef _VA_LIST_DEFINED
#define _VA_LIST_DEFINED
#endif
#ifndef _VA_LIST_T_H
#define _VA_LIST_T_H
#endif
#ifndef __va_list__
#define __va_list__
#endif
#endif /* not _VA_LIST_, except on certain systems */
#endif /* not __svr4__ */
#endif /* _STDARG_H */
#endif /* not _ANSI_STDARG_H_ */
#endif /* not _STDARG_H */
#endif /*__GNUC__ */
/* include mingw stuff */
#include <_mingw_stdarg.h>
下面给出不定参数的一个小例子
void _vaTest(char* fmt, ...)
{
va_list va_args;
int d = 0;
double f = 0.0;
char c = '\0';
char* s = NULL;
va_start(va_args, fmt);
while (*fmt != '\0')
{
switch (*fmt)
{
case '%':
++fmt;
switch (*fmt)
{
case 'd':
d = va_arg(va_args, int);
break;
case 'f':
f = va_arg(va_args, double);
break;
case 'c':
c = va_arg(va_args, char);
break;
case 's':
s = va_arg(va_args, char*);
break;
}
++fmt;
break;
default:
putchar(*fmt);
break;
}
}
va_end(va_args);
}
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