《K&R》(第二版)中有一些非常明确的定义;这有助于把它们放在一个地方，并作为一个整体来阅读:

"Definition" refers to the place where the variable is created or assigned storage; "declaration" refers to the places where the nature of the variable is stated but no storage is allocated. [p. 33] ... It is important to distinguish between the declaration of an external variable and its definition. A declaration announces the properties of a variable (primarily its type); a definition also causes storage to be set aside. If the lines int sp; double val[MAXVAL] appear outside of any function, they define the external variables sp and val, cause storage to be set aside, and also serve as the declaration for the rest of that source file. On the other hand, the lines extern int sp; extern double val[]; declare for the rest of the source file that sp is an int and that val is a double array (whose size is determined elsewhere), but they do not create the variables or reserve storage for them. There must be only one definition of an external variable among all the files that make up the source program. ... Array sizes must be specified with the definition, but are optional with an extern declaration. [pp. 80-81] ... Declarations specify the interpretation given to each identifier; they do not necessarily reserve storage associated with the identifier. Declarations that reserve storage are called definitions. [p. 210]

为了理解声明和定义之间的区别，我们需要查看程序集代码:

uint8_t   ui8 = 5;  |   movb    $0x5,-0x45(%rbp)
int         i = 5;  |   movl    $0x5,-0x3c(%rbp)
uint32_t ui32 = 5;  |   movl    $0x5,-0x38(%rbp)
uint64_t ui64 = 5;  |   movq    $0x5,-0x10(%rbp)
double   doub = 5;  |   movsd   0x328(%rip),%xmm0        # 0x400a20
                        movsd   %xmm0,-0x8(%rbp)

这只是定义:

ui8 = 5;   |   movb    $0x5,-0x45(%rbp)
i = 5;     |   movl    $0x5,-0x3c(%rbp)
ui32 = 5;  |   movl    $0x5,-0x38(%rbp)
ui64 = 5;  |   movq    $0x5,-0x10(%rbp)
doub = 5;  |   movsd   0x328(%rip),%xmm0        # 0x400a20
               movsd   %xmm0,-0x8(%rbp)

正如你所看到的，没有任何变化。

声明与定义不同，因为声明提供的信息仅供编译器使用。例如uint8_t告诉编译器使用asm函数movb。

看到:

uint def;                  |  no instructions
printf("some stuff...");   |  [...] callq   0x400450 <printf@plt>
def=5;                     |  movb    $0x5,-0x45(%rbp)

声明没有等价的指令，因为它不是要执行的东西。

此外，声明告诉编译器变量的作用域。

我们可以说，声明是编译器用来确定正确使用变量的信息，以及某些内存属于某个变量多长时间。

宣言

声明告诉编译器a 程序元素或名称存在。一个 声明引入一个或多个 命名到程序中。声明可以 在一个程序中出现一次以上。 因此，类，结构， 枚举类型等 用户定义的类型可以声明为 每个编译单元。

定义

定义指定哪些代码或数据 名称描述。名称必须为 在使用之前声明。

声明:“在某处，存在一个foo。”

定义:“…就是这儿!”

声明向编译器提供了一个符号名。定义是为符号分配空间的声明。

int f(int x); // function declaration (I know f exists)

int f(int x) { return 2*x; } // declaration and definition

定义意味着实际编写的函数，声明意味着简单的声明函数 如。

void  myfunction(); //this is simple declaration

and

void myfunction()
{
 some statement;    
}

这是函数myfunction的定义