ethernet.c

模拟Ethernet帧的发送过程 父进程创建两个子进程和一个共享内存 这个共享内存模拟介质总线 这个两个进程分别从这个内存发送信并进行冲突检测

#include<unistd.h>
#include<stdio.h>
#include<stdlib.h>
#include<sys/wait.h>
#include<sys/shm.h>
#include<sys/types.h>
#include<error.h>
#define ERROR_SHMGET -1
#define ERROR_SHMAT -2
#define ERROR_WAIT -3 
#define ERROR_FORK -4
#define SLOT_TIME 51.2

static unsigned long get_backoff(const int n)//截止二进制指数后退算法的实现
{
   double exp_rand_slot;
   unsigned long backoff;
   int k = n;
   k = 16-k;
   if(k>10)
     k = 10; 
   exp_rand_slot = (1<<k)*random()*SLOT_TIME;
   backoff = ((unsigned long)exp_rand_slot)%1024;
   return backoff;
}

//发送数据流程
static void send_msg(const void *addr,const pid_t pid)
{
  pid_t *shm_value = (pid_t*)addr;
  int i;
  int collision;
  for(i = 0; i<10;i++ )
  {
    collision = 16;
    do 
    {
    while(*shm_value!=0);//总线不空闲,循环等待
    usleep(SLOT_TIME);
    *shm_value |= pid;//发送数据
    usleep(SLOT_TIME);
    if(*shm_value != pid)//判断是否冲突
    {
       *shm_value = 0;//复原总线为0
       collision--;
       if(collision>0)
       {
         printf("%d send collision.\n",pid);
         usleep(get_backoff(collision));//随机延时
       }
       else
       {
            printf("%d send failure.\n",pid);//发送失败
            break;
       }
    }
    else
    {
      *shm_value = 0;
      break;
    }
  }while(1);
  if(collision>0)
  {
    printf("%d send success.\n",pid);
  }
  usleep(random()%1024);
 }
}


int main(int arg,char *argv[])
{
   pid_t pid = 0;
   //int status = 0;
   int shmid = 0;
   void *addr = 0;
   shmid = shmget(IPC_PRIVATE,sizeof(pid_t),SHM_R|SHM_W);//获取共享内存
   if(shmid == -1)
   {
     perror("shmget error.");
     return ERROR_SHMGET;
   }
   pid = fork();//创建子进程
   if(pid == 0)
   {
      pid = getpid();//获取子进程ID
      addr = shmat(shmid,0,0);//初始化共享内存为/0
      if(addr == (void*)-1)
      {
         perror("shmat error");
         if(shmctl(shmid,IPC_RMID,0) == -1)
         {
            perror("shmctl error");
         }
         return ERROR_SHMAT;
       }
       srandom(pid);
       send_msg(addr,pid);//发送数据
   }
   else if(pid>0)
   {
     pid = fork();
     if(pid == 0)
      {
        pid = getpid();
        addr = shmat(shmid,0,0);
	  if(addr == (void*)-1)
  	  {
  		perror("shmat error");
     		if(shmctl(shmid,IPC_RMID,0) == -1)
 		   perror("shctl error.");
                return ERROR_SHMAT;
	  }
        srandom(pid);
     	  send_msg(addr,pid);
      }
     else if(pid>0)
      {
  	if(wait(NULL) == -1)
     	{
	  perror("wait error");
	   return ERROR_WAIT;
   	}
        else
  		return 0;
      }
     else
      {
	perror("fork error");
	return ERROR_FORK;
       }
   }
   else
    {
     perror("fork error");
     return ERROR_FORK;
     }
   return 0; 
}