全局变量VS局部变量的重新声明 [英] Redeclaration of global variable vs local variable

问题描述

当我编译如下

#include<stdio.h>

int main()
{
  int a;
  int a = 10;
  printf("a is %d \n",a);
  return 0;
}

我得到一个错误：

I get an error:

test3.c: In function ‘main’:
test3.c:6:5: error: redeclaration of ‘a’ with no linkage
test3.c:5:5: note: previous declaration of ‘a’ was here

但是，如果我把这些变量全球然后正常工作。

But if I make the variable global then it works fine.

#include<stdio.h>

int a;
int a = 10;
int main()
{
  printf("a is %d \n",a);
  return 0;
}

为什么声明相同的全局变量两次不是错误，但这样做，对于一个局部变量是一个错误？

Why is declaring the same global variable twice not an error, but doing that for a local variable is an error?

推荐答案

在C，语句 int类型的; 时，在文件范围内制成，是一个声明和一个暂定定义的。你可以有很多的暂定定义，只要你想，只要​​他们都相互匹配。

In C, the statement int a; when made at file scope, is a declaration and a tentative definition. You can have as many tentative definitions as you want, as long as they all match each other.

如果定义（与初始化）翻译单元结束前出现，该变量将被初始化到该值。拥有多个初始值是一个编译器错误。

If a definition (with an initializer) appears before the end of the translation unit, the variable will be initialized to that value. Having multiple initialization values is a compiler error.

如果达到翻译单元的端部，并没有非暂定定义被发现，该变量将是零初始化

If the end of the translation unit is reached, and no non-tentative definition was found, the variable will be zero initialized.

以上不适用于局部变量。这里声明兼作定义，并且具有多于一个导致一个错误。

The above does not apply for local variables. Here a declaration also serves as a definition, and having more than one leads to an error.

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