参数和指针

传值和传址：

看下面的代码（传值）：

#include<stdio.h>
void swap (int x, int y);

void swap (int x, int y)
{
    int t;
    
    printf("In swap,互换前：x = %d, y = %d\n", x, y);//x = 3,y = 5;
    
    t = x;
    x = y;
    y = t;
    
    printf("In swap,互换后：x = %d, y = %d\n", x, y);//x = 5,y = 3;
    
}
int main ()
{
    int x = 3, y = 5;
    
    printf("In main,互换前：x = %d, y = %d\n", x, y);//x = 3, y = 5;
    swap(x, y);
    printf("In main,互换后：x = %d, y = %d\n", x, y);//x = 3, y = 5;
    
    return 0; 
}

运用指针之后（传址）：

#include<stdio.h>
void swap (int *x, int *y);

void swap (int *x, int *y)
{
    int t;
    
    printf("In swap,互换前：x = %d, y = %d\n", *x, *y);//x =  3,y = 5;
    
    t = *x;
    *x = *y;
    *y = t;

    printf("In swap,互换后：x = %d, y = %d\n", *x, *y);//x = 5,y = 3;

int main ()
{
    int x = 3, y = 5;
    
    printf("In main,互换前：x = %d, y = %d\n", x, y);//x = 3,y = 5;
    swap(&x, &y);//注意这里是交换地址 
    printf("In main,互换后：x = %d, y = %d\n", x, y);//x = 5, y = 3;
    
    return 0; 
 } 

函数无法改变实参的值不懂参考百度知道(实参和形参)

可变参数

#include<stdarg.h>
- va list
- va start
- va arg
- va end

代码：

#include<stdio.h>
#include<stdarg.h>////有很多宏，可以处理可变参数的函数 

int sum (int n,...);//"..."代表占位符，未知数量 ，至少一个 

int sum (int n,...)
{
    int i, sum = 0;
    
    va_list vap;//定义参数列表 
    
    va_start(vap, n);//初始化参数列表 
    for(i = 0;i < n;i++)
    {
        sum += va_arg(vap, int);//获取参数的值 ,参数的类型写清楚 
    }
    va_end(vap);//关闭参数列表
     
    return sum; 
}
int main ()
{
    int r;

    r = sum(3, 1, 2, 3);
    printf("r = %d\n", r);

    return 0; 
 }