C语言中定义与声明的本质区别：有无内存分配

变量在定义时被分配内存，并且变量可以指定一个初始化的值。变量只能在这个程序中定义一次。声明在该程序中指定了变量的类型和名称。定义也是一种声明：当我们定义一个变量时，我们声明了它的名字和类型。我们也可以通过使用extern关键字来声明一个变量的名字而不用定义。

　　一个声明可以是加extern前缀的并包含了目标的类型和名称，如：

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　　extern int i; //声明了i但是并未定义

　　int i;

　　加extern关键字的声明不是定义并且不分配内存(实际上声明是不分配内存的)。实际上，它只是宣称了已经在其它文件中定义的变量的名称和类型。一个变量可以声明多次，但是只能定义一次。声明只有在他同样是定义时才能初始化，因为只有定义才会分配内存。

　　如果一个声明初始化，那么它就被当做定义，即便已经被extern标记，例如

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　　extern double pi = 3.1416; //定义

　　尽管使用了extern,它还是定义了pi.该变量分配了内存并初始化。只有出现在函数的extern声明才可能初始化。因为被当做定义，所以后续的任何对该变量的定义都是错误：

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　　extern double pi = 3.1416; // 定义

　　double pi; // 错误：重定义pi

　　我对定义和声明的认识是：

　　定义给变量分配了内存，并且只能定义一次，如 int i = 0;

　　声明没有给变量分配内存，可以声明多次，最常用的是函数参数的声明如 void main(int a, int b);这里面的a和b都未分配内存，只是说明了变量的名称和数据类型。

　　注：《C Primer Plus》上的原文资料

　　A definition of a variable allocates storage for the variable and may also specify an initial value for the variable. There must be one and only one definition of a variable in a program.A declaration makes known the type and name of the variable to the program. A definition is also a declaration: When we define a variable, we declare its name and type. We can declare a name without defining it by using the extern keyword. A declaration that is not also a definition consists of the object's name and its type preceded by the keyword extern:

　　extern int i; // declares but does not define i

　　int i; // declares and defines i

　　An extern declaration is not a definition and does not allocate storage. In effect, it claims that a definition of the variable exists elsewhere in the program. A variable can be declared multiple times in a program, but it must be defined only once.

　　A declaration may have an initializer only if it is also a definition because only a definition allocates storage. The initializer must have storage to initialize. If an initializer is present, the declaration is treated as a definition even if the declaration is labeled extern:

　　extern double pi = 3.1416; // definition

　　Despite the use of extern, this statement defines pi. Storage is allocated and initialized. An extern declaration may include an initializer only if it appears outside a function.

　　Because an extern that is initialized is treated as a definition, any subseqent definition of that variable is an error:

　　extern double pi = 3.1416; // definition

　　double pi; // error: redefinition of pi