程序代码：

#include <stdio.h>
#include <stdlib.h>

/* define a pointer function */
typedef void (*out_fn)(struct ip_addr *ip);

/* struct ip address */
struct ip_addr{
    char* addr;
};

/* struct network interface */
struct netif{
    struct netif *next;
    void * data;
    //void (* out)(struct ip_addr *ip);
    out_fn out;
};

/* a simple function */
void show_ip(struct ip_addr *ip)
{
    printf("ip address:%s \n",ip->addr);
}

/* main function  */
void main()
{
    /* declare and allocation memery */
    struct ip_addr *ip=(struct ip_addr*)malloc(sizeof(struct ip_addr));
    struct netif *net=(struct netif*)malloc(sizeof(struct netif));
    
    /* show the  struct netif size */
    printf("struct netif size :%d \n",sizeof net);          
    
    /* net->data handle */
    if(net->data)
    {
        net->data="this is network interface data.";
        printf("%s\n",net->data);                         
    }
    printf("net->data size :%d \n",sizeof net->data);        

    /* pointer function */
    if(ip)
    {
        ip->addr="192.168.1.110";
        net->out=show_ip;
        net->out(ip);        
    }

}