在设置子收割器时，有没有一些简短的使用prctl()的例子？

Question

我目前正在尝试学习如何使用Linux prctl(PR_SET_CHILD_SUBREAPER)和prctl(PR_GET_CHILD_SUBREAPER)。

不幸的是，当我使用这些函数时，我似乎不知道发生了什么。有人能帮我找出我理解中的一个疏漏吗？

我已经将主进程设置为subreaper。然后，我尝试使用fork()创建子进程，并再次尝试创建子进程。然后我杀死了子进程，看看孙子进程发生了什么，但我无法检查它。

int main(void) {
    int p = fork();
    prctl(PR_SET_CHILD_SUBREAPER, 1);
    if(p < 0)
        return EXIT_FAILURE;
    if(p > 0){
        //Main process
        printf("I am the MainProcess, My PID: %d and PPID: %d\n", getpid(), getppid());
    }
    else{
        printf("I am the Child, My PID: %d and PPID: %d\n", getpid(), getppid());

        int p2 = fork();
        if(p2 < 0)
            return EXIT_FAILURE;
        if(p2 > 0){
            //still Child process
        }
        else{
            int *reaper = NULL;
            prctl(PR_GET_CHILD_SUBREAPER, reaper);
            printf("I am the Grandchild, My PID: %d and PPID: %d\n", getpid(), getppid());
            printf("Reaper ID: %d\n", *reaper);
            kill(getppid(), SIGKILL);
            printf("I am the Grandchild, My PID: %d and PPID: %d\n", getpid(), getppid());
            prctl(PR_GET_CHILD_SUBREAPER, reaper);
            printf("Reaper ID: %d\n", *reaper);
        }
        return EXIT_SUCCESS;
    }
  return EXIT_SUCCESS;
}

输出：

I am the MainProcess, My PID: 9088 and PPID: 23010
I am the Child, My PID: 9089 and PPID: 9088
I am the Grandchild, My PID: 9090 and PPID: 9089

令我惊讶的是，一些printf()实例(在代码的granchild部分)在运行时没有被调用。原因是什么？

Answer 1

下面是一个演示PR_SET_CHILD_SUBREAPER工作原理的小程序

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

int main(void)
{
    int *status;
    int i=0;

    prctl(PR_SET_CHILD_SUBREAPER, 1, 0, 0, 0);
    perror("PARENT:Set");
    printf("PARENT: %d :  my dad : %d\n", getpid(), getppid());
    if(fork() != 0)
    {
        while(1)
        {
            wait(status);
            if(++i == 2)
            {
                break;
            }
        }
        int p = 1;
        prctl(PR_GET_CHILD_SUBREAPER, &p);
        printf("PARENT : %d\n",p);
        printf("PARENT Exiting\n");
    }
    else
    {
        printf("Before CHILD: %d: my dad  %d\n",getpid(), getppid());
        if(fork() == 0)
        {
            int p = 1;
            printf("Before grandchild: %d: my dad %d\n",getpid(), getppid());
            sleep(2);
            printf("After grandchild: %d: my dad %d\n",getpid(), getppid());
            prctl(PR_GET_CHILD_SUBREAPER, &p);
            printf("After grandchild : %d\n",p);
            printf("Grandchild Exiting\n");
            exit(0);
        }
        else
        {
            int p = 1;
            prctl(PR_GET_CHILD_SUBREAPER, &p);
            printf("After CHILD : %d\n",p);
            printf("After CHILD: %d: my dad  %d\n",getpid(), getppid());
            printf("CHILD Exiting\n");
            exit(1);
        }
    }   
    return 0;
}

输出：

PARENT:Set: Success
PARENT: 4002 :  my dad : 2222
Before CHILD: 4003: my dad  4002
After CHILD : 0
After CHILD: 4003: my dad  4002
CHILD Exiting
Before grandchild: 4004: my dad 4003
After grandchild: 4004: my dad 4002
After grandchild : 0
Grandchild Exiting
PARENT : 1
PARENT Exiting

观察：

通过设置PR_SET_CHILD_SUBREAPER，父进程(4002)已成为子收割器

父进程(4002)已经派生并创建子进程(4003)

子进程(4003)已经派生并创建了孙进程(4004)

子进程(4003)尝试使用PR_GET_CHILD_SUBREAPER并接收0。因为prctl()只有一个实例，它不会保留在派生的进程中。Does the CHILD_SUBREAPER bit persist across fork()?

子进程(4003)终止使孙进程(4004)成为孤立进程

由于父进程(4002)被设置为SUBREAPER孙进程(4004)成为父进程(4002)的子进程，并且孙进程(4004)的退出状态被父进程(4002)接收，子进程已经终止

Answer 2

这里有一个更长的PR_SET_CHILD_SUBREAPER工作原理的例子(基于Sunil Kumar的简短回答)：

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

// Make a grandchild process, which sleep(2)s and then exits
int doGrandchild(void) {
    int pid;
    if ((pid = fork()) != 0) {
        return pid;
    }
    
    printf("    GRANDCHILD(%d): before sleep, parent is %d\n", getpid(), getppid());
    sleep(2);   // Wait for CHILD to report and exit
    printf("    GRANDCHILD(%d): after sleep, parent is %d\n", getpid(), getppid());
    
    printf("    GRANDCHILD(%d): exiting\n", getpid());
    exit(0);
    // Will never return
}

// Make a child process, which makes a grandchild process, sleep(1)s, and then exits
int doChild(void) {
    int pid;
    if ((pid = fork()) != 0) {
        return pid;
    }
    sleep(1);   // Wait for PARENT to report fork
    printf("  CHILD(%d): parent is %d\n", getpid(), getppid());
    
    pid = doGrandchild();
    printf("  CHILD(%d): forked grandchild %d\n", getpid(), pid);
    sleep(1);   // Wait for GRANDCHILD to report
    
    printf("  CHILD(%d): exiting\n", getpid());
    exit(0);    // Exit before GRANDCHILD exits
    // Will never return
}

// Wait for all child descendents to exit
void waitDescendents(void) {
    printf("PARENT(%d): waiting for descendents to exit\n", getpid());
    while(1) {
        // Wait for any descendant process to exit
        int pid = wait(NULL);
        if(pid == -1) {
            printf("PARENT(%d): no more descendants\n", getpid());
            break;
        } else {
            printf("PARENT(%d): pid %d exited\n", getpid(), pid);
        }
    }
}

// Run the test
int main(void) {
    int pid;

    printf("PARENT(%d): parent is %d\n", getpid(), getppid());
    
    prctl(PR_SET_CHILD_SUBREAPER, 1, 0, 0, 0);
    printf("PARENT(%d): ===== Run test with CHILD_SUBREAPER set to 1 =====\n", getpid());
    
    pid = doChild();
    printf("PARENT(%d): forked child %d\n", getpid(), pid);
    waitDescendents();
    
    prctl(PR_SET_CHILD_SUBREAPER, 0, 0, 0, 0);
    printf("PARENT(%d): ===== Run test with CHILD_SUBREAPER set to 0 =====\n", getpid());
    
    pid = doChild();
    printf("PARENT(%d): forked child %d\n", getpid(), pid);
    waitDescendents();
    
    printf("PARENT(%d): ===== Exiting =====\n", getpid());
    return 0;
}

编译和运行：

griscom@nob:~/tmp$ cc -Wall subreaper_test.c -o subreaper_test
griscom@nob:~/tmp$ echo "My PID is $$"
My PID is 1001
griscom@nob:~/tmp$ ./subreaper_test 
PARENT(1002): parent is 1001
PARENT(1002): ===== Run test with CHILD_SUBREAPER set to 1 =====
PARENT(1002): forked child 1003
PARENT(1002): waiting for descendents to exit
  CHILD(1003): parent is 1002
  CHILD(1003): forked grandchild 1004
    GRANDCHILD(1004): before sleep, parent is 1003
  CHILD(1003): exiting
PARENT(1002): pid 1003 exited
    GRANDCHILD(1004): after sleep, parent is 1002
    GRANDCHILD(1004): exiting
PARENT(1002): pid 1004 exited
PARENT(1002): no more descendants
PARENT(1002): ===== Run test with CHILD_SUBREAPER set to 0 =====
PARENT(1002): forked child 1005
PARENT(1002): waiting for descendents to exit
  CHILD(1005): parent is 1002
  CHILD(1005): forked grandchild 1006
GRANDCHILD(1006): before sleep, parent is 1005
  CHILD(1005): exiting
PARENT(1002): pid 1005 exited
PARENT(1002): no more descendants
PARENT(1002): ===== Exiting =====
griscom@nob:~/tmp$     GRANDCHILD(1006): after sleep, parent is 1
    GRANDCHILD(1006): exiting

顺序：

• 父代将其CHILD_SUBREAPER设置为1
• 父代派生一个子代，子代作为父代
• 子代退出，留下孙子作为孤儿，父代作为父代
• 孙辈退出，并通知父代其最后一个子代已退出
• 父代将其CHILD_SUBREAPER设置为0
• 父代派生一个子代，以子代为父代
• 子代退出，留下孙子作为孤儿进程1( init进程)作为父代
• 父代看到其最后一个子代已退出和本身退出
• 在主程序返回

之后，孙子进程退出。