了解 fork()、sleep() 和进程通量

一直在练习这些系统调用，但我坚持使用这段代码：

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

main()
{
    pid_t pid;
    switch(pid = fork())
    {
        case -1:
            printf("fork failed");
            break;
        case 0:  //first child
            printf("\ni'm the first child, my pid is %d", getpid());
            fflush(stdout);
            break;
        default:    //parent
            sleep(5); /** sleep is generating problems **/
            printf("\ni'm the parent process, my pid is %d", getpid());
            printf("\ngenerating a new child");
            fflush(stdout);
            switch(pid = fork())
            {
                case -1:
                    printf("fork failed");
                    break;
                case 0: //second child
                    printf("\nhere i am, the second child, my pid is %d", getpid());
                    break;
                default:  //parent
                    wait((int *)0);
                    printf("\nback to parent, my pid is %d", getpid());
            }
    }

    return 0;
}

我得到的输出是：

i'm the first child, my pid is 6203
i'm the parent process, my pid is 6202
generating a new child
back to parent, my pid is 6202
Process returned 0 (0x0)   execution time: 5.004 s
Press ENTER to continue

here i am, the second child, my pid is 6204  

我正在尝试的是这些消息的简单打印，通过管理时间sleep()。 我不明白为什么程序在打印第二条子消息之前返回。 默认情况（第二个分叉之后的那个）在其子项（第二个）作用于输出之前被打印，就像他忽略了它一样wait()。因此，它的子进程在进程返回后被打印。

我无法弄清楚出了什么问题。我已经标记了 sleep() 函数，因为如果我用它替换它wait((int *)0);流程流量按照其设计的方式运行（无论如何，没有任何时间安排）。 此时，我不再确定进程流量或 sleep() 用法（手册页没有那么有用，说实话太简洁了）。

事实上，你的等待电话有效。它检测第一个子进程的结束并随后继续。如果连续调用两次 wait()，您将得到正确的行为。

更新的测试代码：

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

main()
{
    pid_t pid;
    int status;
    switch(pid = fork())
    {
        case -1:
            printf("fork failed");
            break;
        case 0:  //first child
            printf("\ni'm the first child, my pid is %d", getpid());
            fflush(stdout);
            break;
        default:    //parent
            sleep(5); /** sleep is generating problems **/
            printf("\ni'm the parent process, my pid is %d", getpid());
            printf("\ngenerating a new child");
            fflush(stdout);
            switch(pid = fork())
            {
                case -1:
                    printf("fork failed");
                    break;
                case 0: //second child
                    printf("\nhere i am, the second child, my pid is %d", getpid());
                    break;
                default:  //parent
                    pid = wait(&status);
                    printf("\nParent detects process %d was done", pid);
                    pid = wait(&status);
                    printf("\nParent detects process %d was done", pid);
                    printf("\nback to parent, my pid is %d", getpid());
            }
    }

    return 0;
}

Output :

i'm the first child, my pid is 30897
i'm the parent process, my pid is 30896
generating a new child

here i am, the second child, my pid is 30940
Parent detects process 30897 was done
Parent detects process 30940 was done
back to parent, my pid is 30896