无法使用 getppid() 获取预期的 id

Question

我正在做多进程的实践。我只是想查看父进程id，它看起来和我想象的不一样。这是我的代码:

#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/ipc.h>
#include <string.h>
#include <sys/sem.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

void child1()
{
    //printf("we are child1..\n");
    printf("my pid is:%d\n",getpid());
    printf("I am child1..\n");
}

void child2()
{
    //printf("we are child2..\n");
    printf("my pid is:%d\n",getpid());
    printf("I am child2.., my parent is:%d\n",getppid());
}

int main()
{
    pid_t childpid[4];
    int i;
    printf("main pid is:%d\n",getpid());
    for (i=0;i<2;i++)
    {
        childpid[i] = fork();
        if (childpid[i]<0)
            printf("fork fail..\n");
        else if (childpid[i]==0)
        {
            childpid[i+2] = fork();
            if (childpid[i+2] <0)
                printf("fork inside fail..\n");
            else if (childpid[i+2]==0)
                child2();
            else // this is parent process..
                child1();   
        }
    }
}

这是执行结果:

my pid is:3951
I am child1..
my pid is:3950
I am child1..
my pid is:3952
I am child2.., my parent is:1118
my pid is:3954
I am child1..
my pid is:3953
I am child2.., my parent is:1118
my pid is:3956
I am child1..
my pid is:3955
I am child2.., my parent is:1118
my pid is:3957
I am child2.., my parent is:1118

由于我的 child2 进程是在 process1 内部 fork 的，因此我预计父进程 pid 可能类似于:3950。为什么我的结果是:1118？

感谢您的帮助!

Answer 1

当我逐字运行您的代码时(好吧，我向每个空参数列表添加了 void 并将子函数设置为 static)，然后我得到了示例输出:

main pid is:46761
my pid is:46762
I am child1..
my pid is:46763
my pid is:46764
I am child1..
I am child2.., my parent is:46762
my pid is:46765
I am child2.., my parent is:1
my pid is:46766
I am child1..
my pid is:46768
my pid is:46767
I am child2.., my parent is:46766
I am child1..
my pid is:46769
I am child2.., my parent is:1

为了娱乐值(value)，我将输出运行到管道并得到:

main pid is:46770
main pid is:46770
my pid is:46773
I am child1..
main pid is:46770
my pid is:46772
I am child1..
main pid is:46770
my pid is:46775
I am child2.., my parent is:1
main pid is:46770
my pid is:46774
I am child2.., my parent is:46772
main pid is:46770
my pid is:46772
I am child1..
my pid is:46776
I am child1..
main pid is:46770
my pid is:46772
I am child1..
my pid is:46778
I am child2.., my parent is:1
main pid is:46770
my pid is:46774
I am child2.., my parent is:46772
my pid is:46777
I am child1..
main pid is:46770
my pid is:46774
I am child2.., my parent is:46772
my pid is:46779
I am child2.., my parent is:46777

参见printf() anomaly after fork()以获得对该行为的解释。

然后我对您的代码进行了稍微不同的检测，但核心逻辑没有改变:

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

static void child1(void)
{
    printf("Child1: PID = %d, PPID = %d\n", (int)getpid(), (int)getppid());
}

static void child2(void)
{
    printf("Child2: PID = %d, PPID = %d\n", (int)getpid(), (int)getppid());
}

int main(void)
{
    pid_t childpid[4];
    int i;
    printf("main pid is:%d\n", getpid());
    for (i = 0; i < 2; i++)
    {
        fflush(0);
        childpid[i] = fork();
        if (childpid[i] < 0)
            printf("fork fail in PID %d\n", getpid());
        else if (childpid[i] == 0)
        {
            childpid[i + 2] = fork();
            if (childpid[i + 2] < 0)
                printf("fork fail in PID %d\n", getpid());
            else if (childpid[i + 2] == 0)
                child2();
            else
            {
                printf("child pid %d forked child %d\n", (int)getpid(), (int)childpid[i+2]);
                child1();
            }
        }
        else
            printf("main pid %d forked child %d\n", (int)getpid(), (int)childpid[i]);
    }
    int status;
    int corpse;
    while ((corpse = wait(&status)) != -1)
        printf("PID %d: child %d died with status %.4X\n", (int)getpid(), corpse, status);
    printf("PID %d: finished\n", (int)getpid());
    return getpid() % 256;
}

我确实添加了代码来等待子进程死亡，并报告进程何时完成，并为大多数进程添加了非零退出状态。该代码还在 fork 之前刷新标准输出以保持干净(即使输出将发送到管道或文件)。

在我的 Mac OS X 10.11.1 El Capitan 机器上，我得到了，例如:

main pid is:46730
main pid 46730 forked child 46731
main pid 46730 forked child 46732
child pid 46731 forked child 46733
Child1: PID = 46731, PPID = 46730
child pid 46732 forked child 46734
main pid 46731 forked child 46735
Child1: PID = 46732, PPID = 46730
Child2: PID = 46733, PPID = 46731
Child2: PID = 46734, PPID = 46732
PID 46734: finished
main pid 46733 forked child 46736
PID 46732: child 46734 died with status 8E00
PID 46732: finished
child pid 46735 forked child 46737
Child1: PID = 46735, PPID = 46731
PID 46730: child 46732 died with status 8C00
Child2: PID = 46737, PPID = 46735
child pid 46736 forked child 46738
Child1: PID = 46736, PPID = 46733
PID 46737: finished
Child2: PID = 46738, PPID = 46736
PID 46735: child 46737 died with status 9100
PID 46735: finished
PID 46738: finished
PID 46736: child 46738 died with status 9200
PID 46731: child 46735 died with status 8F00
PID 46736: finished
PID 46733: child 46736 died with status 9000
PID 46733: finished
PID 46731: child 46733 died with status 8D00
PID 46731: finished
PID 46730: child 46731 died with status 8B00
PID 46730: finished

当您研究该输出中的排序时，会发现一些有趣的执行交错​​。

这些都不能解释为什么你得到 1118；在很多方面，这是无法解释的，因为没有人可以按照你的机器上的方式对事物进行完全排序。然而，我所展示的是看似合理的行为，并且它不会重现“所有进程都有一个 PID 作为 PPID”。您应该能够对代码进行增量监控改进，并演示哪些更改会改变您计算机上的行为。