进程通信

管道：基于文件级别的单向通信

创建父子进程，使得进程的struct file*fd_array[]的文件描述符指向同一个struct file文件，这个文件是内存级文件。

父进程关写端，子进程再关闭读端。实现单向通信

子进程写入，父进程读取。

如果进程不是父子关系，则无法利用管道，因此管道应用于父子或者兄弟进程

以上的管道叫做匿名管道。

创建管道:pipe

输出型参数

pipefd[0]读下标

pipefd[1]写下标

代码示例

#include <iostream>
#include<string>
#include<cstdlib>
#include<unistd.h>
#include<string.h>
#include<sys/types.h>
#include<sys/wait.h>

using namespace std;
#define N 2
#define NUM 1024
void Writer(int wfd)
{
    string s="hello I am child";
    pid_t self=getpid();
    int number=0;

    char buffer[NUM];
    while(true)
    {
        buffer[0]=0; //字符串清空
        //将字符串的内容放入buffer缓冲区中
        snprintf(buffer,sizeof(buffer),"%s-%d-%d",s.c_str(),self,number++);
        cout<<buffer<<endl;
        // 发送给父进程
        write(wfd,buffer,strlen(buffer));
        sleep(1);

    }
}
void Reader(int rfd)
{
    char buffer[NUM];
    while(true)
    {
        buffer[0]=0;
        // n表示实际读到字节的大小
        ssize_t n=read(rfd,buffer,sizeof(buffer));
        if(n>0)
        {
            buffer[n]=0; //当成字符串加入"\0"
            cout<<"father get a message["<<getpid()<<"]#"<<buffer<<endl;
        }
    }
}
int main()
{
    int pipefd[]={0};
    int n=pipe(pipefd);
    if(n<0) return 1;
    cout<<"pipefd[0]:"<<pipefd[0]<<", pipefd[1]: "<<pipefd[1]<<endl;
    pid_t id=fork();
    if(id<0) return 2;
    if(id==0) 
    {
        close(pipefd[0]);
        //IPC code
        Writer(pipefd[1]);
        close(pipefd[1]);
        exit(0);
    }
    close(pipefd[1]);
    Reader(pipefd[0]);
    pid_t rid=waitpid(id,nullptr,0);
    if(rid<0) return 3;
    close(pipefd[0]);

    return 0;
}

然而多执行流会会出现访问冲突的问题--父进程访问的数据到一半时，旧数据被写端覆盖。

父子进程协同，保护管道文件数据安全

读写端正常，如果管道为空，读端阻塞

管道文件有大小，写满写端阻塞

读端正常读，写端关闭，写进程变成僵尸进程，读端就会读到0，表明读到文件结尾，而且不会阻塞

写端正常写，读端关闭，操作系统通过信号杀掉写入的进程。

ulimit查看pipe size大小，但是不同内核可能有差别

管道面向字节流，一次性读完，有多少读多少，且将分割符看成一个普通字符，管道规范可以解决这个问题

管道是基于文件的，文件的生命周期是随进程的

管道的应用场景

使用管道实现简易版本的进程池

 Task.hpp

#pragma once
#include<cstdlib>
#include<iostream>
#include<vector>
#include<unistd.h>
#include<assert.h>
#include<sys/types.h>
#include<sys/wait.h>
#include<string>
typedef void (*task_t)();
std::vector<task_t> tasks;

//任务是四个函数
void task1()
{
    std::cout<<"task 1 call"<<std::endl;
}
void task2()
{
    std::cout<<"task 2 call"<<std::endl;
}
void task3()
{
    std::cout<<"task 3 call"<<std::endl;
}
void task4()
{
    std::cout<<"task 4 call"<<std::endl;
}
// const & 输入 ->向函数内部输入
// * 输出
// & 输入 输出
void LoadTask(std::vector<task_t> *p_tasks)
{
    p_tasks->push_back(&task1);
    p_tasks->push_back(&task2);
    p_tasks->push_back(&task3);
    p_tasks->push_back(&task4);
}

 ProcessPool.cc

#include "Task.hpp"
//对管道进行描述

#define processnum 10
class channel
{
public:
    channel(int task_id,int pid,std::string processname)
        :_cmdfd(task_id)
        ,_pid(pid)
        ,_processname(processname)
        {}
public:
    int _cmdfd;
    int _pid;
    std::string _processname;
};
std::vector<channel> channels;
void slaver()
{
    while(true)
    {
        int tasknum=0;
        ssize_t num=read(0,&tasknum,sizeof(int));//read block,等待输入
        
        // tasknum=tasknum%tasks.size();
        // (*tasks[tasknum])();
        //std::cout<<"i and pid"<<i<<pid<<std::endl;
        if (!num)break;
        else
        {
            std::cout<<"child process "<<getpid()<<"pid: "<<"receive task_id: "<<tasknum<<std::endl;
            (*tasks[tasknum])();
        }
    }
}
void InitProcessPool(std::vector<channel>* pchannels)
{
    std::vector<int> oldfds;
    for (int i=0;i<processnum;i++){
        //create pipe
        int pipefd[2];
        int n=pipe(pipefd);
        assert(!n); // n=0 success
        
        pid_t pid=fork();
        assert(pid!=-1); //pid =-1 fail
        //child
        //std::cout<<"i = "<<i;
        if(pid==0)
        {   std::cout<<"child process:"<<getpid()<<"have otherfds: ";
            //only one write fd
            for(auto oldfd:oldfds)
            {
                std::cout<<oldfd<<" ";
                close(oldfd);
            }
            std::cout<<std::endl;
            //build relationship
            close(pipefd[1]);
            // pipe read from fd=0 not fd=3;
            dup2(pipefd[0],0);
            close(pipefd[0]);
            // 
            slaver();
            exit(0);
        }
        close(pipefd[0]);
        int status=0;
        // ensure one by one ,block until child process finish
        // pid_t result=waitpid(pid,&status,0);
        // assert(result!=-1);
        pchannels->push_back(channel(pipefd[1],pid,"process "+std::to_string(i)));
        oldfds.push_back(pipefd[1]);
        sleep(1);
    }
    
}
void menu()
{
    std::cout<<"*********************"<<std::endl;
    std::cout<<"******1.task one*****"<<std::endl;
    std::cout<<"******2.task two*****"<<std::endl;
    std::cout<<"******3.task three***"<<std::endl;
    std::cout<<"******4.task four****"<<std::endl;
    std::cout<<"******0.quit*********"<<std::endl;
    std::cout<<"*********************"<<std::endl;
}
void ctrlSlaver()
{
    int which=0;
    while(true)
    {
        menu();
        int enter=0;
        std::cout<<"enter number:";
        std::cin>>enter;
        std::cout<<std::endl;
        
        if (enter==0){
            std::cout<<"quit software"<<std::endl;
            //ssize_t n=write(0,&enter,0); 不用写入，直接退出就好了
            //assert(n!=-1);
            break;
        }
        ssize_t n=write(channels[which]._cmdfd,&enter,sizeof(int));
        assert(n!=-1);
        std::cout<<"parent send a task_num "<<enter<<" to process "<<channels[which]._processname<<std::endl;
        which++;
        which=which%processnum;
    }
}
void quitProcess(std::vector<channel>& pchannels)
{
    for(auto channel:channels)
    {
        std::cout<<"close process"<<channel._pid<<std::endl;
        //关闭读端，进程关闭
        close(channel._cmdfd);
        wait(NULL);
    }
    
}
void PrintTask(const std::vector<task_t> tasks)
{

    for(auto task:tasks)
    {
        (*task)();
    }

}
int main()
{
    // load the task
    
    LoadTask(&tasks);
    //PrintTask(tasks);
    InitProcessPool(&channels);
    ctrlSlaver();
    quitProcess(channels);
    return 0;
}