如何与声明一起定义外部变量？ [英] How to define extern variable along with declaration?

问题描述

维基说：

  

的的extern 关键词的意思是没有确定申报。换言之，这是显式声明的变量，或强制的声明没有定义的一种方法。的另外，也可以明确地定义一个变量，即，迫使一个定义。它被分配一个初始值给一个变量完成。

该初始化变量

这意味着，的的的extern 声明作为该变量的定义。因此，

  / *只是用于测试目的只* /
＃包括LT＆;＆stdio.h中GT;
EXTERN INT Y = 0;
诠释主（）{
    的printf（％d个\\ N，Y）;
    返回0;
}
 

应该是有效的（在C ++ 11 编译）。但与选项编译 -Wall -pedantic -Wextra -std = C99 在GCC 4.7.2，产生一个警告：

 【警告】'y'的初始​​化，并宣布的extern[默认启用]
 

这不应该。据我所知，

 的extern INT Y = 0;
 

是有效相同

  INT I = 0;
 

这是怎么回事错在这里？

解决方案

标准的三个版本 - ISO / IEC 9899：1990，ISO / IEC 9899：1999和ISO / IEC 9899：2011 - 包含在一个例子同题部分的外部对象定义（C90的§6.7.2和C99和C11的§6.9.2）表示：

  

例1

  INT I1 = 1; //定义，外部链接
静态INT I2 = 2; //定义，内部链接
EXTERN INT I3 = 3; //定义，外部链接
INT I4; //暂定定义，外部链接
静态INT I5; //暂定定义，内部链接
 

这个例子继续，但的extern INT I3 = 3; 行清楚地表明，该标准表明它应该被允许。但是请注意，在标准的例子是技术上不规范（见标准的前言）;他们是不是是什么，是不允许一个明确的说法。

这是说，大部分人大部分时间不使用的extern 和初始化。

Wiki says:

The extern keyword means "declare without defining". In other words, it is a way to explicitly declare a variable, or to force a declaration without a definition. It is also possible to explicitly define a variable, i.e. to force a definition. It is done by assigning an initialization value to a variable.

That means, an extern declaration that initializes the variable serves as a definition for that variable. So,

/* Just for testing purpose only */ 
#include <stdio.h>
extern int y = 0;
int main(){
    printf("%d\n", y);
    return 0;
}

should be valid (compiled in C++11). But when compiled with options -Wall -Wextra -pedantic -std=c99 in GCC 4.7.2, produces a warning:

[Warning] 'y' initialized and declared 'extern' [enabled by default]

which should not. AFAIK,

extern int y = 0;  

is effectively the same as

int i = 0;  

What's going wrong here ?

解决方案

All three versions of the standard — ISO/IEC 9899:1990, ISO/IEC 9899:1999 and ISO/IEC 9899:2011 — contain an example in the section with the title External object definitions (§6.7.2 of C90, and §6.9.2 of C99 and C11) which shows:

EXAMPLE 1

int i1 = 1;        // definition, external linkage
static int i2 = 2; // definition, internal linkage
extern int i3 = 3; // definition, external linkage
int i4;            // tentative definition, external linkage
static int i5;     // tentative definition, internal linkage

The example continues, but the extern int i3 = 3; line clearly shows that the standard indicates that it should be allowed. Note, however, that examples in the standard are technically not 'normative' (see the foreword in the standard); they are not a definitive statement of what is and is not allowed.

That said, most people most of the time do not use extern and an initializer.

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