1. 平时进行函数声明的时候，无参数的函数应该声明成f(void)

C99 P119:

10 The special case of an unnamed parameter of type void as the only item in the list

specifies that the function has no parameters  

----------------------------------------------

16： The declaration

int f(void), *fip(), (*pfi)();

declares a function f with no parameters returning an int, a function fip with no parameter specification

returning a pointer to an int, and a pointer pfi to a function with no parameter specification returning an

int.

-----------------------------------------------

不写void是K&R C标准中的, 但是现在，在任何一本书中都推荐使用void的写法 

C99: 6.7.5.3

Empty parameter lists 

C distinguishes between a function declared with an empty parameter list and

a function declared with a parameter list consisting of only void.

The former is an unprototyped function taking an unspecified number of arguments,

while the latter is a prototyped function taking no arguments. 

// C code

extern int  foo();          // Unspecified parameters

extern int  bar(void);      // No parameters

void baz()

{

foo(0);         // Valid C, invalid C++

foo(1, 2);      // Valid C, invalid C++

bar();          // Okay in both C and C++

bar(1);         // Error in both C and C++

} 

2. K&R C标准,还可以在函数外定义函数输入参数

void fun(i)

int i;