[Inter-process communication]: pipe communication/named/unnamed

进程间通信的方式：

• 管道 (半双工（Can only receive or send,不能同时） ,全双工（同步进行,同发同收）
• 信号量
• 共享内存
• 消息队列
• 套接字
• 管道： 有名管道和无名管道
  有名管道：可以在任意进程间通信
  无名管道：Only communicate between parent and child processes
• 定义：
  Link data flow from one process to another
  说人话：The output of one process is passed through some medium as the input of another process
  This medium is the pipe.This is also why pipes are needed.
  命令的写法：cmd1 | cmd2
  比如shell Have both standard input and standard output piped to the terminal screen：
  cmd1 standard input command（键盘）
  cmd1 Standard output is passed to cmd2 ,作为cmd2的标准输入
  cmd2 标准输出到屏幕
  如下图：
• 管道通信：Program implementation if required,需要两个函数（popen()和pclose())to transfer data between two processes.
  command :The name of the program to run and the corresponding parameters
  open_mode:r / w 权限
  r : The exported program can be used by the exporting program,Output program usepopen返回的FILE*文件流指针,使用freadto read the output of the exported program.
  w: The output program passesfwrite Send data to the output program.Instead, the output program reads data from standard input.【It doesn't know that the meal has just been spit out of someone else's mouth.放在盘子里.】
  【接盘侠,It can be understood as getting a second-hand product that someone else spit out from a matchmaking agency】

#include<stdio.h>
FILE *popen(const char* command,const char* open_mode);
int pclose(FILE *stream_to_close);

popen() ：Allows a program to start another program as a new process.
That is, the output command of one program is used as the input of another program
pclose() ： 只在popenReturns when the started process ends.The return value is the exit code of the process in which the closed file stream resides.If the parent process obtains the exit code of the process in advance,Then the process and its resources are reclaimed.其返回值为-1或error.
也就是pcloseMust execute before the parent process gets the process exit code.

例如：ps -ef 输出 | 输入grep “main”

            出现一下问题：
             1.  Pipes must read and write processes at the same time open,否则会阻塞
             2. 如果管道没有数据,那么read ,阻塞
             3.  管道的写端关闭,读read返回值为0
             4. 管道的读端关闭,Writing will generate an exception（发送SIGPIPE)
              (Girlfriend hangs up,What's the use of you keep talking？）

#include<signal.h>
void fun(int sig)
{
    
        printf("sig =%d",sig);
}
int main()
{
    
    signal(SIGPIPE,fun);

• 创建有名管道
  mkfifo FIFO (fifo 可以随便起）

touch a.c b.c
特殊之处：Open the filepipe file,会在内存中开辟一片空间,The written data will be written to memory.No matter the named pipe or the unnamed pipe

• a.c文件

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<fcntl.h>
int main()
{
    
	int fdw = open("fifo", O_WRONLY);
	if (fdw == -1)
	{
    
		exit(-1);
	}
	char buff[128] = {
     0 };
	fgets(buff, 128, stdin);
	write(fdw, buff, strlen(buff) - 1);
	close(fdw);
	exit(0);
}

• b.c文件

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

int main()
{
	int fdr = open("fifo", O_RDONLY);
	if (fdr == -1)
	{
		exit(1);
	}
	printf("fdr =%d\n", fdr);
	char buff[128] = { 0 };
	int num = read(fdr, buff, 127);
	printf("buff =%s\n", buff);
	close(fdr);
	exit(0);
 }

• 改进：（连续发送数据,和接收数据）
  No data in the pipe will block（Just don't close the pipe）

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<fcntl.h>
int main()
{
    
	int fdw = open("fifo", O_WRONLY);
	if (fdw == -1)
	{
    
		exit(-1);
	}
	while(1){
    
	char buff[128] = {
     0 };
	fgets(buff, 128, stdin);
    if(strncmp(buff,"end",3)==0 )
    {
    
     break;
     }
	write(fdw, buff, strlen(buff) - 1);
	}
	close(fdw);
	exit(0);
}

• b.c文件

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

int main()
{
	int fdr = open("fifo", O_RDONLY);
	if (fdr == -1)
	{
		exit(1);
	}
	printf("fdr =%d\n", fdr);
	while(1){
	char buff[128] = { 0 };
	int num = read(fdr, buff, 127);
	if(num == 0)  //读到的数据为0
	{
	break;
	}
	printf("buff =%s\n", buff);
	}
	close(fdr);
	exit(0);
 }

无名管道创建
man pipe
通过pipe打开管道文件,Return the file descriptor of the pipe directly
会用fork()产生一个子进程,（So the parent process is responsible for writing,The child process is responsible for reading）
Both parent and child processes must be closed,The file is closedca.（The last person left in the classroom,才算真的关闭）

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<fcntl.h>
int main()
{
    
	int fd[2];
	if (pipe(fd) == -1)  //fd[0] 读,fd[1]写
	{
    
		exit(1);
	}
	printf("fd[0] = %d,fd[1] = %d\n", fd[0], fd[1]);

    pid_t pid =fork();
    if(pid == -1)
    {
    
            exit(0);
    }
    if(pid == 0)
    {
    
      close(fd[1]);
      char buff[128] ={
    0};
      read(fd[0],buff,127);
      printf("child read :%s\n",buff);
      close(fd[0]);
    }
    else
    {
    
            close(fd[0]);
            write(fd[1],"abc",3);
            close(fd[1]);

    }
	exit(0);
 }

• 面试会问：
  
  dup(3,1) 将3file descriptor to copy to1的位置
  ls>a.test 文件重定向 （append and overwrite）
  Because standard input and output have already been useda.txt 覆盖,So there is no standard output and standard input.因此hello打印再a.txt中

#include<stdlib.h>
#include<unistd.h>
#include<fcntl.h>
int main()
{
    
	int fd = open("a.txt", O_WRONLY | O_CREAT, 0600)
	{
    
		if (fd == -1)
		{
    
			printf("open a.txt filed\n");
			exit(1);
		}
		dup2(fd, 1);
		dup2(fd, 2);
	}
	printf("hello\n"); //Because standard input and output have already been useda.txt 覆盖,So there is no standard output and standard input.因此hello打印再a.txt中
	exit(0);
}

管道的实现：