问题:

I was running the program

#include<stdio.h>
#include <unistd.h>
main()
{
    pid_t pid, ppid;
    printf("Hello World1\n");
    pid=fork();
    if(pid==0)
    {
        printf("I am the child\n");
        printf("The PID of child is %d\n",getpid());
        printf("The PID of parent of child is %d\n",getppid());
    }
    else
    {
        printf("I am the parent\n");
        printf("The PID of parent is %d\n",getpid());
        printf("The PID of parent of parent is %d\n",getppid());        
    }
}

THe output I got was.

$ ./a.out 
Hello World1
I am the parent
The PID of parent is 3071
The PID of parent of parent is 2456
I am the child
The PID of child is 3072
The PID of parent of child is 1

I couldnt understand the line

The PID of parent of child is 1

It should have been 3071?

回答1:

Because parent process is finished by the time the child asks for its parent's pid.

When a process finishes, all its children are reassigned as children of the init process, which pid is 1.

Try using wait() in parent's code to wait for the child to execute. It should then work as you expect.

回答2:

pid 1 is for init process and it looks like the parent process finished before the child could print.

If you edit the else part like this :-

else
    {
        printf("I am the parent\n");
        printf("The PID of parent is %d\n",getpid());
        printf("The PID of parent of parent is %d\n",getppid());   
        while(1);
    }

You should see the right output.