为什么使用“ extern”创建变量是声明而不是定义？ [英] Why is creating a variable using 'extern' a declaration and not a definition?

问题描述

在阅读时遇到了以下问题...只是无法理解其背后的逻辑。

  auto int c; 
 static int c; 
 register int c; 
 extern int c; 
  

假设前三个是定义，最后一个是声明..为什么？ p>

解决方案

最后一个带有 extern 的存储库未定义<$ c $的存储c> c 。它仅表示 c 在某处存在，并且链接程序应该能够将其解析为在其他位置定义的某些全局 c 。 / p>

如果编译并链接了一个.c文件，并尝试使用最后一个 c ，则将具有链接器错误。对于前3个 c ，您将不会因为它们在当前编译单元中具有实质内容（已被定义）。

如果您想了解有关 extern 和声明与定义的更多信息，请使用关于该主题的好文章。引用该文章：

声明变量/函数只是声明变量/函数存在于程序中的某个位置，但内存是没有为他们分配

I came across the following problem while reading ...just cant get the logic behind this.

auto int c;
static int c;
register int c;
extern int c;

It is given that the first three are definition and last one is declaration ..how come ?

解决方案

The last one with extern does not define storage for c. It merely indicates that c exists somewhere and the linker should be able to resolve it to some global c defined elsewhere.

If you compiled and linked a single .c file and tried to use the last c you'd have a linker error. With the the first 3 cs you would not as they have substance (they've been defined) in the current compilation unit.

If you'd like to learn more about extern and declaration vs definition here's a good article on the topic. To quote from that article:

Declaration of a variable/function simply declares that the variable/function exists somewhere in the program but the memory is not allocated for them

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