关于并发程序设计的-红联Linux系统门户

#include
#include
#include
#include
int main()
{
int r,p1,p2,fd[2],i;
char buf[50];//缓冲区
pipe(fd);//定义管道
for(i=0;i<2;i++)
{ //循环体怎么就执行一遍呢？
while((p1=fork())==-1);//父进程创建子进程，当失败时反复创建，直到成功跳出while
if(p1==0)
{//从子进程p1返回,执行子进程p1
lockf(fd[1],1,0);//对管道加锁，实现两子进程互斥
sprintf(buf,"the messge is from process 1\n");//向缓冲区写入数据
printf("process 1 is running!the id is %d\n",getpid());
write(fd[1],buf,60);//将buf中内容写入管道中
sleep(3);//睡眠3秒，让父进程读，控制父子进程之间的同步
lockf(fd[1],0,0);//释放管道写入端
exit(0);//关闭子进程p1
}//if
else//从父进程返回,执行父进程
{
while((p2=fork())==-1); //创建子进程p2,失败时循环
if(p2==0)
{ //从子进程p2返回，执行子进程p2
lockf(fd[1],1,0);//锁定写入端，控制子进程间互斥
sprintf(buf,"the message is from process 2\n");
printf("process 2 is running!the id is %d\n",getpid());
write(fd[1],buf,60); //将buf中内容写入管道
sleep(3); //睡眠3秒，让父进程读管道中的消息
lockf(fd[1],0,0); //释放管道写入端
exit(0); //关闭子进程p2
}
wait(0); //父进程被阻塞，进入等待队列，子进程结束时，返回原来程序继续执行，控制父子进程之间的同步
if(r=read(fd[0],buf,60)==-1) //从管道中读数据并存入buf中
printf("can't read the pipe\n");
else
printf("father process is reading message:%s\n",buf);
//显示管道中的消息
wait(0); //父进程被阻塞，进入等待队列，子进程结束时，返回原来程序继续执行，控制父子进程之间的同步
if(r=read(fd[0],buf,60)==-1) //从管道中读数据并存入buf中
printf("can't read the pipe\n");
else
printf("father processs is reading message:%s\n",buf);
exit(0); //关闭父进程
} //else
}//for
}

运行结果怎么有两种情况呢？详细情况如下：
一、
process 1 is running ! the id is 3005
process 2 is running ! the id is 3006
father process is reading message : the message is from process 1

father process is reading message : the message is from process 2

二、
process 1 is running ! the id is 3031
father process is reading message : the message is from process 1
process 2 is running ! the id is 3032

father process is reading message : the message is from process 2

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