Question

所以我试图在第一个孩子身上创建2个子进程和一个孙子进程。我正在尝试运行第一个子进程，然后是孙子，然后是第二个子进程，父进程在进程完成时调用pid。我完全陷入困境 - 我有许多代码版本的代码，但无法按正确的顺序触发它们。任何帮助，建议或链接将不胜感激。

所需的输出将是

- 我是第一个孩子

----我是孙子

我是父母，第一个孩子有pid 21505

- 我是第一个孩子，孙子有21506 pid

- 再试一次

- 我是第二个孩子

- - ＆gt;第二个子节点：传输了1317个字节。

我是父母，第二个孩子有21507 pid

父母 - 儿童21507完成

- 再试一次

****应该在这里看到前20行lab4.c文件

父母 - 其他孩子21505已完成

int main(int* argc, char* args[])
{
pid_t child1, child2, gchild; //process ids
child1 = fork();           //Create the two child processes
if (child1 == 0)
{ // parent
    child2 = fork();
    if (child2 == 0)
    { //parent
        printf("P: first child has pid %d\n", child1);
        printf("P: second child has pid %d\n", child2);

        pid_t wpid; // process id that is finished
        int status; //status code returned by process  
                    //Wait for all child processes to complete
        while ((wpid = wait(&status)) > 0)
        {
            if (wpid > 0)
            {
                printf("P: Child %d is done \n", wpid);
            }
        };

        exit(0); //exit main parent process
    }
    else
    {
        //Second process's code

        printf("SC: I am the second child.\n");
        int totalBytes = 0; //total number of bytes in the file we are going to read in
        char buffer[2056]; //file contents size 2056 bytes
        ssize_t read_bytes = 0; //Bytes from each chunk get placed here temporarily

        ///Read in file contents: home/common/lab_sourcefile
        int fd_in = open("/home/COIS/3380/lab4_sourcefile", O_RDONLY);//open the file
        do
        {
            //read the file chunk by chunk to the buffer
            read_bytes = read(fd_in, buffer, (size_t)2056);

            // End of file or error.
            if (read_bytes <= 0)
            {
                break; //done reading file
            }

            //Save the total number of bytes
            totalBytes = totalBytes + read_bytes;
        } while (1); //infinite loop until we are done reading the file

        //Write out contents to: lab4_file_copy
        char filepath[1024]; //temp path
        getcwd(filepath, sizeof(filepath)); //Get the current working directory
        strcat(filepath, "/lab4_file_copy"); //Tack on the filename
        int fd = open(filepath, O_RDWR | O_CREAT, S_IRUSR | S_IRGRP | S_IROTH); //open the file. O_RDWR = read and write flag, O_CREAT = create if doesnt exist, S_* flags are permission flags from fstat 
        write(fd, buffer, strlen(buffer)); //write to the file

        close(fd_in);//close file we were reading
        close(fd); //close copy file

        // print out the number of bytes the file
        printf("SC: --> Second child: %d bytes transferred.\n", totalBytes);

        exit(0); //done kill second child process
    }
}
else
{
    //First child process

    printf("FC: I'm the first child.\n");

    //Create the granchild process
    gchild = fork();
    if (gchild == 0)
    {
        printf("GC: I am the grandchild.\n");

        sleep(3); //sleep for 3 seconds

        printf("GC: #### Output start ####\n");
        execlp("head", "head", "-n", "20", "l.c", NULL); //this should exit automatically
        printf("ERROR: execlp command failed\n");//This will only run if the execlp process fails
        exit(1); //FAILED!
    }
    else
    {
        //Output the granchild pid
        printf("FC: I am the first child, grandchild has pid %d\n", gchild);
        int grandchildStatus;
        //Wait for grandchild process to be done. Polling...
        while ((waitpid(gchild, &grandchildStatus, WNOHANG)) != gchild)
        {
            printf("FC: Try again\n");
            sleep(1);
        };
        printf("GC: #### Output end ####\n");
        //End of first child. Grandchild must have completed.

        exit(0); //Done. kill first child process
    }
}

}

Answer 1

你的程序基本上有两个错误：

最后else部分：

您致电fork，以便fork功能之后的所有内容都会在两个流程中执行。

两者父母和第二个孩子将打印：＆＃34; 我是父母，第一个孩子有pid ...... ＆＃34;

要解决此问题，您必须添加if(child2 == 0)（在else区块内）...
指示顺序：

当您启动子流程时，子流程真正与父流程同时运行：

如果一个进程打印出＆＃34; Hello world。＆＃34;另一个打印出来＆＃34; 这是一个测试。＆＃34;您可能会在屏幕上看到以下文字：＆＃34; 此 Hello 是 世界。 测试＆＃34; （或类似的）因为两个进程同时写入屏幕。

您可以使用子进程父进程中的waitpid函数等待子进程完成，以确保子进程在父进程继续之前完成（打印出文本）。

如果您希望首先在父进程中打印文本，则必须找到一种方法来停止子进程并等待父进程完成打印。

例如，您可以使用raise(SIGSTOP);来停止当前正在运行的进程。在另一个流程中，使用kill(pid, SIGCONT);继续正在等待的其他流程pid：

child = fork(); if(child == 0) { raise(SIGSTOP); /* Wait for the parent */ /* Do something */ } else { /* Do something */ kill(child, SIGCONT); /* Tell the child to continue */ }

...但是：正如PSkocik刚刚提到的那样，即使在这种情况下你的程序也可能会挂起 - 所以这并不容易！

尝试创建按顺序运行的2个子进程和1个孙子

1 个答案: