child and parent process id

Question

Just got confused with parent pid value in child process block. My program is given below:

 int main(int argc, char *argv[])
  {
    pid_t pid;
    pid=fork();
    if(pid==-1){
            perror("fork failure");
            exit(EXIT_FAILURE);
    }
    else if(pid==0){
            printf("pid in child=%d and parent=%d\n",getpid(),getppid()); 
    }
    else{
            printf("pid in parent=%d and childid=%d\n",getpid(),pid);
    }
    exit(EXIT_SUCCESS);
  }

Output: pid in parent=2642 and childid=2643

pid in child=2643 and parent=1

In "Advanced Unix programming" it says that child process can get parent process id using getppid() function. But here I am getting "1" which is "init" process id.

How can I get the parent pid value in the child process block.. Please help me in getting output.

I executed in "Linux Mint OS" but in "WindRiver" OS I am not getting this problem. Does this program change behaviour according to OS?

Solution

That's because the father can / will exit before the son. If a father exists without having requested the return value of it's child, the child will get owned by the process with pid=1. What is on classic UNIX or GNU systems SystemV init.

The solution is to use waitpid() in father:

int main(int argc, char *argv[])
{
    pid_t pid;
    pid=fork();
    if(pid==-1){
        perror("fork failure");
        exit(EXIT_FAILURE);
    }
    else if(pid==0){
        printf("pid in child=%d and parent=%d\n",getpid(),getppid()); 
    }
    else{
        printf("pid in parent=%d and childid=%d\n",getpid(),pid);
    }

    int status = -1;
    waitpid(pid, &status, WEXITED);

    printf("The child exited with return code %d\n", status);
    exit(EXIT_SUCCESS);
}

Comments

[melpomene]

Possible duplicate of Why getppid() from the child return 1

[Santosh Sahu]

I put "sleep(2)" at the end of program and is working fine and your answer is correct.

[Anantha Raju C]

provide an explanation to your answer

[Rishabh Kumar]

This is basic c program for calculate the child id and parent id , including the sleep function.