11.1 进程间通信的基本概念

相关信息

进程间通信（Inter Process Communication - IPC）意味着两个不同的进程间可以交换数据，为了完成这一点，操作系统中应提供两个进程可以同时访问的内存空间。

• 同时访问：一个进程在修改内存时其余进程可以访问吗？
• 系统提供：进程具有完全独立的内存结构

通过管道实现进程间通信

       #include <unistd.h>

       int pipe(int pipefd[2]);

参数	值	用途	说明
pipefd	pipefd[0] refers to the read end of the pipe. pipefd[1] refers to the write end of the pipe	create a pipe	Data written to the write end of the pipe is buffered by the kernel until it is read from the read end of the pipe.

1. 管道中的数据只能从write端流向read端；
2. write后的数据由内核（系统）缓存，直到read读取

管道的使用
实现：
先创建管道，然后创建子进程

int main(int argc, char* argv[])
{
    char buf[BUF_SIZE] = {0};
    int pipefd[2];
    if(pipe(pipefd) == -1) {
        error_handling("pipe() error");
    }

    pid_t pid = fork();
    if(pid == -1) { 
        error_handling("fork() error");
    }
    else if(pid == 0) { // Child process
        write(pipefd[1], "Hello from child", 16);
    }
    else{
        read(pipefd[0], buf, BUF_SIZE);
        printf("Parent received: %s\n", buf);
        sleep(1); // Ensure child process has time to finish
    }
    
    return 0;
}

行号	功能	说明
4-7	创建管道
14	子进程向管道写入数据
17-19	父进程读取管道中的数据

效果：

ming@ubuntu:/media/sf_share/Network/build$ ./Network
Parent received: Hello from child

测试管道的性质

1. 单向流通
   实现：
   向出口写入，从入口读取

        write(pipefd[0], "Hello from child", 16);
        
		read(pipefd[1], buf, BUF_SIZE);

效果：

ming@ubuntu:/media/sf_share/Network/build$ ./Network
Parent received:
ming@ubuntu:/media/sf_share/Network/build$

没有读取到数据

2. buffered by kernel
   实现：
   等待多次写入，然后一次性读取

    else if(pid == 0) { // Child process
        write(pipefd[1], "Hello from child", 16);
        write(pipefd[1], "Another message", 16);
    }
    else{
        sleep(3);
        read(pipefd[0], buf, BUF_SIZE);
        printf("Parent received: %s\n", buf);
        sleep(1); // Ensure child process has time to finish
    }

效果：

ming@ubuntu:/media/sf_share/Network/build$ ./Network
Parent received: Hello from childAnother message

或者 每次读取少量数据

    else if(pid == 0) { // Child process
        write(pipefd[1], "Hello from child", 16);
    }
    else{
        while(read(pipefd[0], buf, 1))
        {
            printf("Parent received: %s\n", buf);
        }
    }

效果：

ming@ubuntu:/media/sf_share/Network/build$ ./Network
Parent received: H
Parent received: e
Parent received: l
Parent received: l
Parent received: o
Parent received:
Parent received: f
Parent received: r
Parent received: o
Parent received: m
Parent received:
Parent received: c
Parent received: h
Parent received: i
Parent received: l
Parent received: d

通过管道进行进程间双向通信

由于管道的单向流通以及内核缓存的特性，如果父子进程间需要双向通信，只使用一个管道，那么在没有完全读取 或者 读取过程中有新的数据写入，比如父进程先读，读了5字节数据，以为读取完毕，实际又来了3个，父进程开始写入4字节。然后子进程读取。这样子进程就把自己发送给父进程的数据取出来了。

书中的示例程序是子进程 先写入，但是写入后立即进行读取，导致管道中没有数据，那么先读取的父进程就会一直等待。

解决办法：创建两个管道，每个管道各自负责不同的数据流动